# ISO discussion, digital SLR compared to film SLR



## TimmyD11 (Aug 6, 2017)

So in film photography you set the ISO for what ISO the film was, 800 film being more sensitive to light than 100 for example...

How does a "fixed" sensor in a digital camera become more or less sensitive to light by just selecting a different ISO? I'd like to know what it's doing so I know how to use it (and use it to my advantage when I can).


----------



## fmw (Aug 6, 2017)

It amplifies the electrical signal.  Something like turning up the volume of your stereo system.  You learn to use it by using it.  As you increase the signal (ISO) you increase noise in the same manner that your stereo increases audio distortion.  The noise is easily visible in your editing software.  Just test and learn how much difference in noise you accept in a given environment.


----------



## Braineack (Aug 6, 2017)

this is hands-down my favorite video for explaining ISO and the effects of it on your image:


----------



## Gary A. (Aug 6, 2017)

What Fred said ... in other words 'Gain'.  As Fred noted, it isn't magical or free ... the trade off for Gain is similar to pushing film, the more you push the greater the grain and contrast.  The more you increase the amplification of the signal in the camera, the greater the noise (looks like grain) and contrast.

My first dSLR was a Canon 20D.  To my tastes ISO 1600 was barely acceptable.  My latest camera, a Fuji XT2, ISO 6400 is quite acceptable to my eye.


----------



## KmH (Aug 6, 2017)

That video is horrible.

The sensitivity to light of the image sensor never changes.

Image sensors and pixels do not work the way he says they do.
Pixels do not detect color at all. Pixels only detect luminosity.

The interference part is also BS.
When he starts spewing the BS about dark current he lost me and I stopped watching the video.

Whatever signal (an analog voltage) a pixel developed during an exposure is amplified (gain) according to the ISO setting. After amplification the voltage is converted to a digital number.
The millions of pixel digital numbers get recorded on the memory card.

Back in the day ISO was standardized  for film and digital.
International Organization for Standardization - Wikipedia
Back in the day we had ASA numbers for film.


> *Film speed* is the measure of a photographic film's sensitivity to light, determined by sensitometry and measured on various numerical scales, the most recent being the ISO system. A closely related ISO system is used to measure the signal gain of digital imaging systems.


*American Standards Association* (*ASA*)


----------



## Peeb (Aug 6, 2017)

Not sure about 'horrible' but if sensors don't work the way the video describes - how do they work (sincere question- not rhetorical)

Your reference to luminosity lost me.

EDIT:  Scott cleared it up for me- I was confused how colors were detected and identified if not by the sensor and I'm now told that they bayer filter does this.  Awesome!


----------



## Overread (Aug 6, 2017)

To know how to use it its simple really. As said earlier the higher the ISO value you select the greater impact the light has on the exposure.

Increase the ISO by one stop  and the exposure increases by one stop. 

The trade off is that higher ISOs generate more digital noise so a very high ISO will be very "nosey" - this impacts detail and clarity. The appearance is somewhat different to grain on film, but essentially its similar in effect.

Note that this "noise" can also be seen if you take a dark photo and then brighten it in editing. As a result an underexposed photo that you brighten will actually show more noise than if you'd used a higher ISO at the time and exposed it correctly. Note that by correctly this refers to the "expose to the right" theory of exposure where by you're aiming to capture as much light data as possible without overexposing the photo. It is an idealist concept and practical real world shooting will often not let you achieve it.

Note also that in recent years there have been some gains in sensor technology and some sensors now offer very little practical degradation of the image at higher ISOs; as a result some cameras are getting to a point where they are far more free with what ISO you want to pick. Nikon and Sony have pushed this technology (well sony really - nikon bought their sensors); which gives them an edge over Canon and many other brands. 




If you really want to learn how sensor work electronically - be careful. It's an area rather like light and physics were the average photographer knows enough to shoot; but hasn't really got much if any proper clue as to what is really going on. They cobble it together from forum posts and websites; where often cutting complex subjects into simple terms adjusts the theory to be more digestible, but not always factually correct.


----------



## SCraig (Aug 6, 2017)

Peeb said:


> "Horrible" seems a bit harsh.  If sensors don't work the way the video describes - how do they work (sincere question- not rhetorical)
> 
> Your reference to luminosity lost me.


Luminosity is the level or quantity of light hitting the sensor, or to simplify how "Bright" it is.  Colors are determined by the Bayer Filter in front of the filter.  The photosites of the sensor convert the quantity of light to an analog voltage.  It is then amplified based on the setting of the ISO.  The ISO (actually a bias voltage chosen based on the ISO setting) controls the amount of amplification provided to the voltage from the sensor; the lower the ISO setting the less the amount of amplification, the higher the ISO setting the higher the level of amplification.

As has been stated, all electronic circuits generate "Noise" (much like seeing "Snow" on old-school analog televisions).  As the level of amplification increases the ratio of the noise level to the desired signal increases as well, hence the term "Signal to Noise Ratio".  A high SNR means that the level of the desired signal is much higher than the level of the noise so the overall signal will be very "Clean" or the noise is very low in comparison.


----------



## TimmyD11 (Aug 6, 2017)

So, as a photographer using daylight wouldn't I pretty much always have my camera set to 100?

And maybe 400 very early in the morning or late in the evening?

But for all intents and purposes wouldn't a photographer using natural daylight pretty much always keep his ISO set to 100?


----------



## Peeb (Aug 6, 2017)

SCraig said:


> Peeb said:
> 
> 
> > "Horrible" seems a bit harsh.  If sensors don't work the way the video describes - how do they work (sincere question- not rhetorical)
> ...


Thanks, Scott.  OK- so the sensor detects brightness and the bayer filter detects colors (as a very rudimentary explanation).  

Did not know that!  I'm afraid to learn much but I'm happy to know that much.


----------



## Overread (Aug 6, 2017)

Depends.

If you're shooting portraits of people chances are 100 might well be fine, but you'd fast want 800 or even 1600 or higher if indoors without using flash. 

However if you were taking photos of action you might well need a higher ISO as even with a wide aperture there might not be enough light (and middle of the day light brings its own problems). 

You've got three settings which you balance based upon your requirements and the situation. Indeed whilst keeping the ISO low is good, its generally bad advice for most new people to follow. Because, like yourself, you quickly  think that you must keep it low; when in reality you might find that ISO 400 is a more practical starting point outside and even 800 on very cloudy days. You've got to be willing to react and change based on the situation.


----------



## Designer (Aug 6, 2017)

TimmyD11 said:


> How does a "fixed" sensor in a digital camera become more or less sensitive to light by just selecting a different ISO? I'd like to know what it's doing so I know how to use it (and use it to my advantage when I can).


You are correct in thinking that the sensitivity of the sensor does not change.

ISO is applied gain.  The processor applies gain according to the ISO setting.

The signal-to-noise ratio (SNR) is also a function of the sensor design and firmware that is designed into your camera.  Some cameras have a wider dynamic range than others, and some can handle higher ISO (post-capture gain) better than others.

Here is a two-part tutorial on the topic of noise:

Digital Camera Image Noise: Concept and Types

Image Noise: Examples and Characteristics


----------



## SCraig (Aug 6, 2017)

TimmyD11 said:


> So, as a photographer using daylight wouldn't I pretty much always have my camera set to 100?
> 
> And maybe 400 very early in the morning or late in the evening?
> 
> But for all intents and purposes wouldn't a photographer using natural daylight pretty much always keep his ISO set to 100?


That would depend on the aperture and/or shutter that is needed for the particular shot.  If you're shooting birds in flight and need somewhere around 1/2000 second shutter speed and enough aperture to give some decent depth of field then you would likely need to increase the ISO.  How much would depend on the light level.  On the other hand if you're shooting a landscape using a tripod then, yes, ISO 100 would likely work fine.  You can't just say one particular setting will work for all circumstances.  That's the reason they are all adjustable.


----------



## Peeb (Aug 6, 2017)

Overread said:


> Depends.
> 
> If you're shooting portraits of people chances are 100 might well be fine, but you'd fast want 800 or even 1600 or higher if indoors without using flash.
> 
> ...


Agree- in fact many birders go out with their rig set to something like:  1/1000 [or faster] plus f/8 plus auto-ISO.

 The shutter speed is non-negotiable (long lenses and possibly moving subjects) and they need decent depth of field so the only variable to give is ISO.  It's amazing how well modern sensors can deliver usable images at previously unimaginable ISO levels.


----------



## Designer (Aug 6, 2017)

TimmyD11 said:


> So, as a photographer using daylight wouldn't I pretty much always have my camera set to 100?
> 
> And maybe 400 very early in the morning or late in the evening?
> 
> But for all intents and purposes wouldn't a photographer using natural daylight pretty much always keep his ISO set to 100?


Some photographers fiddle with the setting as if it were the third leg of an "exposure triangle", but in reality, changing the ISO has no effect whatsoever on the exposure. 

It does, of course, have an effect on how the processor generates the image.

If you don't want to keep adjusting the ISO, just put it on "auto".  Or if you wish to get ahead  of the curve when faced with a low-light situation, go ahead and turn up the ISO before shooting.  As you bump up the ISO, you will eventually run up against the processor's ability to produce a clean image, and you will get artifacts in the image called "noise".


----------



## TimmyD11 (Aug 6, 2017)

Do most cameras come with ISO set to auto as a default setting?


----------



## Designer (Aug 6, 2017)

TimmyD11 said:


> Do most cameras come with ISO set to auto as a default setting?


I don't know.  Because it's so easy to change that setting, what they come with out of the box is irrelevant.


----------



## TimmyD11 (Aug 6, 2017)

These ISO invariant sensors I have been hearing about...are they primarily only full frame cameras?


----------



## Ysarex (Aug 6, 2017)

TimmyD11 said:


> These ISO invariant sensors I have been hearing about...are they primarily only full frame cameras?



No. They come in any size. Fuji X system cameras are APS size sensors. The Fuji X-Trans II sensor is pragmatically ISO invariant.

Going back through this thread there's a lot of good information but there's also a lot of murky "grey" information that's developed over time as colloquial jargon but is in fact spurious.

First some of the best info:



Designer said:


> You are correct in thinking that the sensitivity of the sensor does not change.


 YES!!! Changing the ISO value does not alter the light sensitivity of the sensor.

And again:



Designer said:


> Some photographers fiddle with the setting as if it were the third leg of an "exposure triangle", *but in reality, changing the ISO has no effect whatsoever on the exposure.*[emphasis mine]


 This needs to be a little clearer but what's critical here is that ISO is not a determinant exposure factor. Photographic exposure is the amount of light per unit area that reaches the sensor and is a function of the scene illumination, attenuated through the lens (aperture) over time (shutter speed). Three factors and ISO is not one of them. ISO informs the choice of exposure but does not otherwise alter the exposure. This is a huge point of contemporary confusion.

The above becomes important in understanding the next topic: noise. *Raising the ISO value on a digital camera typically reduces noise. *I'm going to repeat that for effect, r*aising the ISO value on a digital camera typically reduces noise. *What's happened in the photo jargon is the adoption of a spurious correlation. As the light level drops and photographers need to reduce exposure in order to keep the camera shutter speed high, they raise the ISO. (I took photos in the garden today under a gloomy sky and set the ISO to 1600). Back with film we would switch to a high ISO film in the same circumstance. Now as we already know raising the ISO does not increase the light sensitivity of the sensor. It does however bias our camera meters to calculate a reduced exposure. AND noise is the direct result of underexposing the sensor. That's where the noise is coming from. *You get noise as a direct result of reduced sensor exposure; *ISO doesn't cause the noise* the underexposure causes the noise *and as we know ISO is not an exposure determinant.

Now once you've underexposed the sensor (and have lots of noise) you also have a very dark image. You're going to have to brighten that image and you can do it two ways. You can brighten the image digitally and/or electronically. Some cameras do a combination of both. The electronic method which you'll often hear referred to as gain or amplification is applied to the analog sensor signal before it's converted into numbers. If you do the number conversion first then the brightening of the image is done digitally by multiplying the numbers. This is digital camera ISO preforming it's primary function. Most of our cameras (vast majority) are using the electronic method of ISO brightening and for good reason -- it helps suppress the noise that occurred from the sensor underexposure (see statement above repeated for effect).

Your ISOless or ISO invariant cameras (just a few built around Sony sensors) show no noise suppression benefit from the electronic brightening method. As a result we can wait on the digital brightening method and do that later at the computer when we can bring substantially greater processing power to bear. In the field using the same camera we can ignore the ISO setting however that can make chimping your image a bit tricky as the camera JPEG could be very dark.

Joe


----------



## Bill The Lurker (Aug 7, 2017)

the trouble with pedantry is that it turns into an infinite regress of "well no, actually"

for example, "well no,  Ysarex, decreased exposure doesn't cause more noise, it causes a lower s/n ratio by reducing the numerator"

so what we actually do in practical terms is work with a simplified mental model that's good enough. rather than pedantically insisting that the sensor is the array of sensels, the bayer array, and the support circuits _but not the amplifiers_ then we get to do the dramatic bit about how changing the ISO doesn't change the sensor's sensitivity, and then the noobs look on in confused wonder and then we can wander off into the weeds mixing up noise and signal-to-noise ratio, but nobody will notice because so so so very many words

if instead you include the amplifiers that implement ISO into what you mean by the word sensor, which is a perfectly reasonable thing to do (I am an ex-systems guy) then by god changing the ISO setting does indeed make your sensor (or "sensor system" if you prefer) into amore sensitive, albeit noisier, sensor (-system)

which is a perfectly good mental model to use, it works fine for millions of people successfully making billions of photographs every year.

tl;dr - yes,  yes indeed, make ISO bigger makes them pichers brighter but noisier


----------



## Ysarex (Aug 7, 2017)

Bill The Lurker said:


> the trouble with pedantry is that it turns into an infinite regress of "well no, actually"
> 
> for example, "well no,  Ysarex, decreased exposure doesn't cause more noise, it causes a lower s/n ratio by reducing the numerator"
> 
> ...



No. If sensitivity were increased then additional data would be recorded. That doesn't happen. Amplifying existing data doesn't record more data. Raising ISO on a digital camera does not increase light sensitivity -- it doesn't allow the "system" to record data deeper into the shadows. The trouble with oversimplification is that it can encourage incorrect assumptions about cause and effect and false assumptions about what's possible. The mental model that the sun revolves around the earth worked for millions of people for a long time. I think you can still join in fact: The Flat Earth Society

Joe



Bill The Lurker said:


> which is a perfectly good mental model to use, it works fine for millions of people successfully making billions of photographs every year.
> 
> tl;dr - yes,  yes indeed, make ISO bigger makes them pichers brighter but noisier


----------



## Bill The Lurker (Aug 7, 2017)

well no it can record more data. suppose the sensor records volts, rounding down to the nearest volt, with an optional "amplify by 2x" circuit. this is simplified, of course.

consider a single sensel with a readout of 1.6 volts

without the amplifier, you read out a 1, which is off by 0.6 volts. with the amp you read a 3, which you interpret as 1.5 volts because you know about the amplifier for an error of 0.1 volts but most of this is irrelevant without considering the noise floor of the system which is determined in part by the support circuitry etc etc etc

see, you can play the "I know more fiddly details" game endlessly, but in the end it doesn't actually change the images you take one bit, it's just posturing. the fact that with some, but not all, modern sensors you can just shoot at ISO 100 and fix it in post is a mere curiosity.

"knowing" that changing the ISO "doesn't change the sensitivity" of the sensor has exactly zero practical impact on actually taking images

but i admit it sure makes you sound authoritative


----------



## fmw (Aug 7, 2017)

Sometimes we lose sight of the fact that this is beginners forum, not a place for scientific dissertations.


----------



## Overread (Aug 7, 2017)

Subsections are guidelines not rules in general - the OP asked a question in a way that leads into a higher level of discussion; there's no harm in that at all.


----------



## Ysarex (Aug 7, 2017)

Bill The Lurker said:


> well no it can record more data. suppose the sensor records volts, rounding down to the nearest volt, with an optional "amplify by 2x" circuit. this is simplified, of course.
> 
> consider a single sensel with a readout of 1.6 volts
> 
> without the amplifier, you read out a 1, which is off by 0.6 volts. with the amp you read a 3, which you interpret as 1.5 volts because you know about the amplifier for an error of 0.1 volts but most of this is irrelevant without considering the noise floor of the system which is determined in part by the support circuitry etc etc etc



That's just you being guilty of the same pedantry you're complaining about. Raising the ISO on a digital camera does not increase sensitivity -- because you do not record more data. If you raise the ISO 3 stops on a digital camera you do not record 3 stops of additional shadow detail. You get nothing more.



Bill The Lurker said:


> see, you can play the "I know more fiddly details" game endlessly, but in the end it doesn't actually change the images you take one bit, it's just posturing. the fact that with some, but not all, modern sensors you can just shoot at ISO 100 and fix it in post is a mere curiosity.
> 
> "knowing" that changing the ISO "doesn't change the sensitivity" of the sensor has exactly zero practical impact on actually taking images



This is where you're really wrong. Put all the "fiddly details" away and pragmatically let's go take some "pichers." My last camera was a Fuji X-E2 which is for all PRACTICAL purposes ISO invariant. I didn't buy it because of that and initially I didn't concern myself with that aspect of the camera. But eventually I tested it. I was impressed and so I tested it more rigorously and convinced myself that apart from a brighter image to chimp on the LCD the ISO function of the camera was without any real value.

Eventually I started to take advantage of that. I started to leave the ISO at base and didn't bother the trouble of raising it when I was forced to reduce exposure. There are some interesting PRACTICAL advantages to that. Taking photos is simpler when you have less buttons and dials to deal with. But also there's the fact that if you really must reduce exposure then withholding the ISO boost keeps your highlights from any threat of clipping. *When ISO brightens underexposed sensor data it does so equally for all data and can clip highlight detail. Any increase in ISO reduces sensor DR. If we don't increase ISO sensor DR remains at max.*

I used the X-E2 for 4 years before upgrading and eventually became accustomed to the PRACTICAL advantages I could access by understanding how it really worked. For example I was asked by friends to take some snaps at an indoor event -- be unobtrusive, no flash please. The venue was poorly lit and large windows added a unique complication. Here's a photo:




 

I left the ISO at base and the gentlemen at the table are basically 4 stops underexposed. Here's the camera processed JPEG:





And this is what I would have gotten if I had made that 4 stop ISO adjustment in camera:





Raising the ISO 4 stops on the camera would have reduced sensor DR by 4 stops. I would have gotten bupkis more data in the shadows but the highlights out the window and the highlight on the side of the man's face (blue shirt) would have been terminally clipped. Go back up to the first photo and look at the data out the window and that face highlight.

Under the circumstance in the above photo where I was forced to reduce exposure, my understanding that raising ISO does not increase light sensitivity but does reduce sensor DR had a huge PRACTICAL impact on the photo I was able to produce.

Joe



Bill The Lurker said:


> but i admit it sure makes you sound authoritative


----------



## Bill The Lurker (Aug 7, 2017)

if i am understanding ysarex right, he is asserting that increasing exposure (eg extending shutter speed) will get more shadow detail whereas increasing the ISO doesnt. this is probably true for newer cameras with ISO invariance.

testing my camera which is about 7 years old ISO works pretty much like film. taking identical images, one with a longer shutter speed at ISO 100 and the other with a shorter shutter speed at ISO  1600 reveals zero difference in shadow details. diffing the images shows differences on the edges and in the midtones. likely due to increased noise in the ISO  1600 shot and some noise reduction code.

in other words my camera was engineered so that the ISO  setting worked much like film ISO. the sensor is by modern standards pretty noisy, so the readout at ISO 100 leaves a lot low level signal behind (below the LSB) in favor of getting a clean signal. upping the ISO  does indeed recover more shadow detail at the expense of reading out additional noise. then the software does god knows what, and in the end, it's a wash and ISO works a lot like film's ISO.

point is you can engineer these things to work any way you like. modern systems favor ISO invariance which, as a consequence, means that upping the ISO will treat shadow detail differently from opening the aperture or increasing the shutter speed. which, if you want, I suppose you can define "sensitivity" to mean "not the thing ISO does" and then you get to grump at videos.

if you care about shadow detail, or color fidelity, or highlight detail, or haptics, or how easy it is to clean the sensor, or any of those things it will behoove you to learn about your camera a bit. if you don't care about one thing or another, just let it slide.

ISO make pichers brighter.

if that's not enough detail for you do figure out what ISO does on your camera until you know enough to satisfy you. every camera is gonna be a lil differet.


----------



## Bill The Lurker (Aug 7, 2017)

so you exposed for the highlights at ISO 100 and, because your sensor system is ISO invariant, were able to recover the men.

if instead you had set the ISO to 1600 and then exposed for the men rather than the highlights you'd have blown out the highlights, ok.that is unsurprising. again: if you'd set the ISO up and metered in a completely different way you'd have lost the highlights.

if you have set the ISO to 1600 and exposed for the highlights, that is, exposed in the same way you did originally rather than metering completely differently, you'd still have to recover the men the same way, and they'd be all noisy. but you could have used a faster shutter speed or a smaller aperture.

so.. let's see. setting the ISO to 1600 would have made the camera more sensitve, but noisier. right? did I get that right?


----------



## Ysarex (Aug 7, 2017)

Bill The Lurker said:


> so you exposed for the highlights at ISO 100 and, because your sensor system is ISO invariant, were able to recover the men.
> 
> if instead you had set the ISO to 1600 and then exposed for the men rather than the highlights you'd have blown out the highlights, ok.that is unsurprising. again: if you'd set the ISO up and metered in a completely different way you'd have lost the highlights.
> 
> if you have set the ISO to 1600 and exposed for the highlights, that is, exposed in the same way you did originally rather than metering completely differently, you'd still have to recover the men the same way, and they'd be all noisy.



yes.



Bill The Lurker said:


> but you could have used a faster shutter speed or a smaller aperture.



no. The reason I exposed the way I did originally was to get the faster shutter speed.



Bill The Lurker said:


> so.. let's see. setting the ISO to 1600 would have made the camera more sensitve, but noisier. right? did I get that right?



no.

Joe


----------



## Bill The Lurker (Aug 7, 2017)

far be it from me to judge how a persons thought process leads them to the asnwer and if yours takes a trip through details of sensor electrocics then so be it, more power to ya

me, i just expose for whatever highlights i want to keep, s ame as with slide film, same as any digital camera


----------



## TCampbell (Aug 7, 2017)

I too tried to watch the video and almost stopped just one minute in when most of what he said was wrong.  I forced myself to keep watching because sometimes to teach a concept, it's easier to fib a little to get a concept across ... and then clear up the fib later.  If you flood the person with too many facts up front, you'll lose them.   But at about 2/3rds of the way through the video it was still wrong and nothing was being clarified.  I realized that someone watching this video will probably come away knowing all the wrong stuff.



To clear a few things...

*Sensors don't have "pixels"*

 (photos have "pixels" but sensors do not).  Sensors have "photosites".  The photo-site is a single light sensitive cell that reads the luminosity of light at that position.  In other words, think of it as a black & white camera and not as a color camera.

To get color, the camera uses a "color filter array" (CFA).  The most common type is the Bayer Mask Bayer filter - Wikipedia 

Think of this as a chess-board type filter where each square is either red or green or blue transparent tile.   A photosite that happens to be located behind a "green" colored tile on the CFA is sensitive to "green" wavelength light, but not to "red" or "blue".  But most light is not a pure single color... for example teal light would be partially detected by "blue" photosites as well as "green" photosites.  

Still... the actual RAW data from the camera is a file which provides the light value for each photosite in single-color channel form.  It's a mosaic of color tiles... no blended color.  To get the blended color values, software has to "de-mosaic" the tiles.  

Suppose I'm a "green" photosite.  I know how "green" I'm supposed to be because that's the only color I can detect.  What I could not detect was how "red" or "blue" I was supposed to be.  But I have neighbors and I can borrow information from them.    If I'm green, then I'll have two red neighbors and two blue neighbors.  Suppose my red neighbors are above and below me and my blue neighbors are left and right.  If my red neighbor above has a value of 180 and my red neighbor below has a value of 178 then that tells me that if only I could have been able to detect "red" myself, I probably would have recorded a value of 179.  I can do the same with the blue.

It turns out the algorithms are a bit more involved than this simple example... there are several algorithms.  But that's the basic idea.  Most programs don't let you pick the algorithm (my PixInsight software that I use for astrophotography DOES let me pick from among several RAW decoding algorithms).  This means if you open a RAW file in one program and save the output to a non-lossy TIFF file... then do the same using a completely different program... then compare the two TIFF files, you'll find that they may have different results when you do a pixel-by-pixel comparison.

BTW, compare the file size of your RAW to the file size of the non-lossy TIFF... you'll notice the TIFF is MUCH larger and that's because it is made up of 3-color channel "pixels" whereas as the RAW is just single-color channel data where the 3-channel color will mathematically be derived when it is processed.



*As for the ISO...*

Sometimes it's easier to teach a concept by simplifying it.  The simplification can sometimes involve telling a fib.   Hopefully someone goes back and explains that "remember when we told you X was true... well we weren't entirely honest... we skipped the details for simplicity sake, but in reality... Y is true."

So yes, it's easier to say that changing the ISO changes the sensitivity of the sensor but that's not actually what it does.

When enough photons hit a photosite, it will bump up it's recorded light level by 1.  

All this happens before any ISO adjustment is applied.  In other words, whatever number of photons were collected is what it it was and that's the original data.

Changing ISO really just manipulates the data values that are reported (after the fact).

There are two main ways to do this... there's an "upstream" method and a "downstream" method (and a combination of both methods).  The camera manufacturer won't tell you what they do for any given sensor but there are ways to work it out via testing.

The sensor receives analog signal (not digital).   The camera sensor performs an Analog-to-Digital Conversion process (ADC).  That results in digital output so we call it a "digital" camera even though the light captured was analog data.

It turns out in the analog world, you can simply amplify the signal.  This is referred to as "upstream" amplification if it happens prior to the ADC step.

You can also take the digital output from the ADC step (so now you just have numbers) and multiply those numbers to increase the value.  If you do this, it's called "downstream" amplification.

Some cameras rely almost exclusively on "downstream" amplification and others rely on a combination of "upstream" and "downstream" amplification.  

Keep in mind that digital cameras have a maximum number of bits they use for each integer that represents the amount of light they counted.  An 8-bit JPEG file just has 256 values (0-255).  A 16-bit integer can store 65,536 values (0-65,535).  But most modern create 14-bit RAW files with 16,384 values (0-16,383).

So here's a problem:  Suppose that if I shoot at ISO 100 I end up with an image and if we just pick two of the photosites, one reported a value of 500, another reported a value of 9000.  This is no problem because both values are below the maximum value of 16,383.

Now I dial the camera up to ISO 200 and my camera only performs "downstream" gain.  This means it will perform simple multiplication and multiply all values by 2.  My photosite that reported a value of 500 will now be saved as if it reported 1000. So far so good.  But my photosite that reported a value of 9000 will now have to save a value of 18,000 and this is a problem because you can't store a value of 18,000 in a 14-bit integer.  That's an over-flow problem and it results in clipped data (loss of information).

This isn't just a loss of information, it's also a loss of the camera sensors dynamic range.

Some cameras have a feature to protect those highlights (e.g. "highlight tone priority").  This scales the the multiplication so that while the dark pixels are all multiplied by 2.0x, the lighter pixels are multiplied by some smaller value (say... 1.8x) so that they don't overflow and clip data... and then we linearly interpolate how much to boost each mid-range value (some are boosted by 1.81, 1.82, etc. all the way down until we reach the pixels that we multiply by the full 2.0x.)  Again, a simplification... but that's the idea.

This means we protect against full data loss... but we do lose part of the information.  Technically we compress the dynamic range into a smaller space to try to avoid losing data and the result looks pretty good so we're happy with it.

If you had previously believed that a change of ISO simply increased sensitivity and didn't realize the math behind what it is really doing (mathematically multiplying values) then it might have been missed that you are losing dynamic range and should take steps to protect against loss of data such as shooting bracketed images to perform an HDR merge... or enabling features such as highlight-tone-priority (which may have a different name depending on camera model).

But back to that "upstream" amplification... this results in a gain being applied BEFORE digital conversion (before ADC) and as a result it doesn't lose much in the way of dynamic range.  

If you were to test the dynamic range of a camera (using a test target) what you'd probably notice is that as you boost ISO, you don't seem to be losing much dynamic range... but there's a limit to this... and then suddenly you hit an ISO where you get a linear drop off in dynamic range for each boost in ISO beyond that point.  

Most cameras that do both "upstream" and "downstream" have some magic point where this trade-off occurs.  If you want to protect your DR then this is the highest ISO you should use.




*Noise*

When you boost ISO, you boost noise right along with it.  The reality is the noise was always there, you just didn't notice it.  Noise happens when a photosite reports a higher value than it should have (I'll skip discussions on why this happens but will mention that some of the reasons have to do with the quantum nature of the universe... no amount of electronic wizardry will make that go away because a sensor that reports this "noise" is actually reporting what really happened.)  Cameras that do try to make this problem go away are "cooking" the RAW data (quite a number of modern sensors on the market provide "cooked" RAW files.)    For some interesting reading... look up the Sony "star eating" issue (you'll find lots of hits on that search).  The summary for those who don't want to search is that in Sony's effort to produce sensors that have lower noise, they're averaging out data that they "think" is noise (because it seems to exhibit properties of noise) even when it's actually real data.  Astrophotographers starting noticing that stars are missing from their images that area really supposed to be there. The images look cleaner than they should because the computer assumed it was noise and was "cooking" the RAW data to get rid of it (and lots of Sony owners think they have better sensors because they don't see as much noise - unaware of what the camera sensor is really doing.)

Just remember that noise is "additive" in that it always results in a photosite reporting a HIGHER numeric value than it should have reported.  There is no "anti-noise" where the photosite reported a LOWER numeric value (at least if there is, I have yet to encounter it.)  

This is significant because it implies you're more likely to notice the noise in darker areas of your image and less likely to notice noise in whiter areas of your image.    If a pixel is nearly "white" already and there's  "noisy" pixel nearby, it won't be able to be that much brighter than the pixel that is already quite bright.  But when an anomalous high value pixel shows up in a "dark" area, it really stands out.

But there is an exception... suppose you shoot a photo in a very well lit "green" room.  So you're near saturating the green photosites but the red and blue photosites are porting very low values.  I can have noise that spikes the value of, say, a "red" photosite and this creates a bright red photosite where it should be dark.  My de-mosaicing algorithm then "blends" that spiked red value and it results in my green photosite being blended with red to create a yellow or orange pixel in the output.  In other words the "noise" didn't just make my pixel brighter... it shifted the color value of the pixel.

You are far more likely to notice noise in dark areas.
You are also far more likely to notice noise in regions of the image that have some relatively flat tone (non-contrasty areas).  

It isn't that there is no noise in the bight or contrasty areas... it's more than your eye is much less likely to notice it because it isn't so significantly different than the neighboring pixels.  (sort of a "where's waldo" problem... when the detail in an area of your image is very complicated, your eye is not likely to notice the flaw.  The flaw can hide in plain sight.)

Knowing this, you can smartly deal with noise by using software that more aggressively goes after darker regions more aggressively than lighter regions.

You can also create a mask that finds edges of contrast and protect those areas against de-noising ... and more aggressively smooth out the areas that lack contrast.


----------



## Designer (Aug 7, 2017)

I think this video is correct.  Tony does get off into the weeds toward the second half of the video, but as I understand it, he is correct in his concepts and terminology.


----------



## Ysarex (Aug 7, 2017)

Bill The Lurker said:


> if i am understanding ysarex right, he is asserting that increasing exposure (eg extending shutter speed) will get more shadow detail whereas increasing the ISO doesnt. this is probably true for newer cameras with ISO invariance.



It is. It is also true for older digital cameras that are not ISO invariant. Exposure determines the data you will record. ISO brightening is a post exposure process that can't create more data out of nothing. In a non ISO invariant camera ISO brightening of the sensor signal will advantageously suppress noise.

Joe



Bill The Lurker said:


> testing my camera which is about 7 years old ISO works pretty much like film. taking identical images, one with a longer shutter speed at ISO 100 and the other with a shorter shutter speed at ISO  1600 reveals zero difference in shadow details. diffing the images shows differences on the edges and in the midtones. likely due to increased noise in the ISO  1600 shot and some noise reduction code.
> 
> in other words my camera was engineered so that the ISO  setting worked much like film ISO. the sensor is by modern standards pretty noisy, so the readout at ISO 100 leaves a lot low level signal behind (below the LSB) in favor of getting a clean signal. upping the ISO  does indeed recover more shadow detail at the expense of reading out additional noise. then the software does god knows what, and in the end, it's a wash and ISO works a lot like film's ISO.
> 
> ...


----------



## Bill The Lurker (Aug 7, 2017)

Ysarex said:


> It is. It is also true for older digital cameras that are not ISO invariant. Exposure determines the data you will record. ISO brightening is a post exposure process that can't create more data out of nothing.



it may not "create more data" but it's certainly finding more data in the well and putting it into my file. it's just nicely blended with some noise. you can assert all you like that it's not true and yet here i am looking at these image files.

do you really think those blokes put the analog amplifiers in there because they're dumb? or because they just love amps?

what, exactly, is a non-ISO-invariant sensor, if the analog ISO  circuitry "can't create more data" in some user-visible way?


----------



## Bill The Lurker (Aug 7, 2017)

TCampbell said:


> Just remember that noise is "additive" in that it always results in a photosite reporting a HIGHER numeric value than it should have reported.  There is no "anti-noise" where the photosite reported a LOWER numeric value (at least if there is, I have yet to encounter it.)



huh? no

take a picture of a white piece of paper at high ISO, zoom in on it, and observe in astonishment.

discussing noise is extremely difficult and not one i would dare approach in a page or two. there are many sources of noise each with their own properties and each with their own appropriate models for analysis and so on. sadly it's also critical to understanding in any real
technical detail any of the issues here, which is why all the efforts to nupack what ISO settigs actually do are so hilarious and yet dull


----------



## Ysarex (Aug 7, 2017)

Bill The Lurker said:


> Ysarex said:
> 
> 
> > It is. It is also true for older digital cameras that are not ISO invariant. Exposure determines the data you will record. ISO brightening is a post exposure process that can't create more data out of nothing.
> ...



I have not and would not assert that is not true. You're getting extremely pedantic here but, yes, the gain to the sensor signal improves the system efficiency and saves recorded data that would otherwise be lost. This is happening post exposure however and does not constitute a sensitivity increase. Rather it's an efficiency improvement that prevents data loss in the system. You're just doing a better job extracting what's already there.

Joe



Bill The Lurker said:


> do you really think those blokes put the analog amplifiers in there because they're dumb? or because they just love amps?
> 
> what, exactly, is a non-ISO-invariant sensor, if the analog ISO  circuitry "can't create more data" in some user-visible way?


----------



## Ysarex (Aug 7, 2017)

Bill The Lurker said:


> far be it from me to judge how a persons thought process leads them to the asnwer and if yours takes a trip through details of sensor electrocics then so be it, more power to ya



I'm a photographer. My thought process in this regard has always been anchored in understanding my hardware so that I can take better photographs.

Joe



Bill The Lurker said:


> me, i just expose for whatever highlights i want to keep, s ame as with slide film, same as any digital camera


----------



## Bill The Lurker (Aug 7, 2017)

so let's see if i have this right.

my camera, a typical mainstream non-ISO-invarient camera of a few years ago. changing the ISO doesn't change the recorded data,
that is, to be precise, the data recorded in the wells of the sensor where it will reside for a few milliseconds before evaporating, the data which I  do not care about in the slightest. that data. the data i can't even access.

changing the ISO does change the data that is recorded in the file, though. the data that lasts for sometimes thousands or even millions of milliseconds. the data i can actually access. the data i care about. the picture i just took. that data, changing the ISO does give me more data there.

so you might almost say that the camera becomes more sensitive as a recording instrument, in terms of the bits it actually puts where i can access them, when i turn up the ISO on it.

now, you're certainly right, if you draw a red line on the chip that includes these transistors but not those transistors, and declare that set of analog circuitry to be "the sensor", that system doesn't record any more data. in fact, setting the iso doesn't even effect that circuitry at all. so, yeah, the iso doesn't do a single darn thing to the parts of the circuit its not connected to. you got me there.

but the camera considered as a whole system, w ell, the iso setting makes it, you imght almost say, "more sensitive"


----------



## Ysarex (Aug 7, 2017)

TCampbell said:


> But back to that "upstream" amplification... this results in a gain being applied BEFORE digital conversion (before ADC) and as a result it doesn't lose much in the way of dynamic range.
> 
> If you were to test the dynamic range of a camera (using a test target) what you'd probably notice is that as you boost ISO, you don't seem to be losing much dynamic range... but there's a limit to this... and then suddenly you hit an ISO where you get a linear drop off in dynamic range for each boost in ISO beyond that point.



What you're describing here is peculiar to Canon engineered cameras. This is a Canon only behavior that does not apply to the rest of the photo world. You need to preface this with that disclaimer. The basic rule is DR drops linearly with increasing ISO. Here's a visual on that looking at the Canon 5D III and Nikon D810: Photographic Dynamic Range versus ISO Setting

Joe


----------



## Bill The Lurker (Aug 7, 2017)

lest I lose the thread

every bit of technical twaddle I have trotted out here was merely to demonstrate that the technical fiddly bits go on forever

nobody in this thread actually understands how these things work every one of us is saying stuff that would make the sensor team at sony take to drink, or at least sigh loudly. i understand these issues better than the other people because I have a moderately relevant background, and am therefore in a position to illustrate how to nitpick endlessly

what you need to actually take pictuers is a mental model that is good enough for your work. it's going to be simplified and wrong, unless you were actually on the team that designed the sensor system you're using. but if you're lucky it's going to be good enough.

fussing about whether 'more sensitive but noisier' is strictly speaking true is silly. you can "correct" that but you'll still be wrong. ysarex has, in my opinion, failed to illustrate how that particular wrong mental model is going to  produce worse pictures than his mental model, w hich is also wrong in some details here and there.


----------



## Ysarex (Aug 7, 2017)

Bill The Lurker said:


> so let's see if i have this right.
> 
> my camera, a typical mainstream non-ISO-invarient camera of a few years ago. changing the ISO doesn't change the recorded data,
> that is, to be precise, the data recorded in the wells of the sensor where it will reside for a few milliseconds before evaporating, the data which I  do not care about in the slightest. that data. the data i can't even access.
> ...



You might say that but you'd be wrong. What you're describing instead is an improvement in system efficiency that's preventing slight data loss. Raising ISO on a digital camera does not record more data from the original scene -- sensor light sensitivity does not increase.

Joe



Bill The Lurker said:


> now, you're certainly right, if you draw a red line on the chip that includes these transistors but not those transistors, and declare that set of analog circuitry to be "the sensor", that system doesn't record any more data. in fact, setting the iso doesn't even effect that circuitry at all. so, yeah, the iso doesn't do a single darn thing to the parts of the circuit its not connected to. you got me there.
> 
> but the camera considered as a whole system, w ell, the iso setting makes it, you imght almost say, "more sensitive"


----------



## Ysarex (Aug 7, 2017)

Bill The Lurker said:


> lest I lose the thread
> 
> every bit of technical twaddle I have trotted out here was merely to demonstrate that the technical fiddly bits go on forever
> 
> ...



Well, you know I answered what and when I did because the OP asked:



TimmyD11 said:


> How does a "fixed" sensor in a digital camera become more or less sensitive to light by just selecting a different ISO? I'd like to know what it's doing so I know how to use it (and use it to my advantage when I can).



Good question! How does it do that? And then he asked specifically about ISO invariant cameras.

I gave him accurate information pertinent to his questions. You decided that "pedantic fiddly bits" weren't appropriate for him or helpful and he should just stick to taking pichers using a false mental model that works for millions. I'll trust he can sort those options out.

Joe


----------



## Designer (Aug 7, 2017)

I think the OP gave up.  He probably went to some other photo discussion site to ask his question.


----------



## waday (Aug 7, 2017)




----------



## Bill The Lurker (Aug 7, 2017)

Ysarex said:


> You decided that "pedantic fiddly bits" weren't appropriate for him or helpful



no, i actually decided that they're wrong and not helpful for anyone. saying that changing the iso doesn't alter the sensor's sensitivity has got to be one of the single most confusing bits of twaddle trotted out by "photography experts" today. it's only true at all if you squint and qualify the words very very carefully, and it presents absolutely forward path for useful action.

in fact, it strongly suggests to the unannointed that adjusting the iso serves no purpose, and yet, even the greenest tyro can see that changing the iso most certainly does do something. how can they not be confused at this pont? but the tyro certainly knows who the cleverest fellow in the rom is, a nd taht's the importantpoint, isn't it?


----------



## Braineack (Aug 7, 2017)

Peeb said:


> EDIT: Scott cleared it up for me- I was confused how colors were detected and identified if not by the sensor and I'm now told that they bayer filter does this. Awesome!



yeah, bayer array.   the video gives you a good general idea better than most try to explain.  the specifics, as shown by this thread, are boring.


----------



## TimmyD11 (Aug 7, 2017)

Well I have a lot to absorb and process here.

I have a few point and shoot cameras but I am researching cameras now to purchase a good one, a DSLR probably but perhaps a mirrorless camera. I am trying to figure out if I get an APSC DSLR or full frame DSLR, and if I should stick with Canon based on having some older early EF lenses from when I had an EOS A2 film camera. If I stick with Canon and go APSC I have to figure out how much camera is enough...t7? 77D? 80D? Or make the few hundred dollar leap to the 6D full frame (I think?)

OR cut my old lens losses and be free to go mirrorless APSC...

But I like what I have read about ISO invariance sensors and according to someone on another thread Canon sensors are STILL lagging behind other manufacturers, even with the latest improvements in their sensors...Is this true?

Wanna make a case for what direction I should go in?

I'm a nature / great outdoors fanatic that likes to photograph what I see on my journeys.

I am just getting back into taking photography a bit more seriously and I want to get a nice camera but I have to know how much camera may be the right amount for me and how much camera may be too much camera for me. This is the main reason I have not purchased anything yet. I have to figure out what the right camera for me is so I don't have buyer's remorse and wish I spent a few more bucks for a better camera. At the same time I don't want to go so crazy that I spend too much and get something I may not take full advantage of. 

Also, if I decide to get into astralphotography in the mountains can APSC handle that?

The thread subject will drift but that's fine by me if you guys don't mind.


----------



## Ysarex (Aug 7, 2017)

TimmyD11 said:


> Well I have a lot to absorb and process here.
> 
> I have a few point and shoot cameras but I am researching cameras now to purchase a good one, a DSLR probably but perhaps a mirrorless camera. I am trying to figure out if I get an APSC DSLR or full frame DSLR, and if I should stick with Canon based on having some older early EF lenses from when I had an EOS A2 film camera. If I stick with Canon and go APSC I have to figure out how much camera is enough...t7? 77D? 80D? Or make the few hundred dollar leap to the 6D full frame (I think?)



Yes the 6D is FF.



TimmyD11 said:


> OR cut my old lens losses and be free to go mirrorless APSC...



I would take the loss on the old lenses and start fresh even if you do go with Canon.



TimmyD11 said:


> But I like what I have read about ISO invariance sensors and according to someone on another thread Canon sensors are STILL lagging behind other manufacturers, even with the latest improvements in their sensors...Is this true?



In terms of ISO invariance yes. You want to look at Nikon, Sony and Fuji if you're interested in ISO invariance. I wouldn't rank that variable too high though -- Canon makes some nice and very usable hardware. But, yeah I'd go Nikon or Fuji. Nikon if you want FF and Nikon or Fuji for APS.

If you want mirrorless then Fuji with the caveat that Fuji's X-Trans CFA is a unique tech and special handling is indicated. Research that before you decide Fuji -- I can help. Sony could be an APS mirrorless fall back.



TimmyD11 said:


> Wanna make a case for what direction I should go in?
> 
> I'm a nature / great outdoors fanatic that likes to photograph what I see on my journeys.



In the great outdoors you face the problem of not being able to control the lighting. With difficult lighting like very high contrast, the ISO invariance ability of some newer cameras can prove a real positive -- you're better able to take full advantage of the sensor DR. But I'm starting to get technical there and we don't want to start another round of "don't worry stay stupid and be happy." ;-) Otherwise I fall back on my earlier comment of not ranking it too high.



TimmyD11 said:


> I am just getting back into taking photography a bit more seriously and I want to get a nice camera but I have to know how much camera may be the right amount for me and how much camera may be too much camera for me. This is the main reason I have not purchased anything yet. I have to figure out what the right camera for me is so I don't have buyer's remorse and wish I spent a few more bucks for a better camera. At the same time I don't want to go so crazy that I spend too much and get something I may not take full advantage of.
> 
> Also, if I decide to get into astralphotography in the mountains can APSC handle that?
> 
> The thread subject will drift but that's fine by me if you guys don't mind.



To get a better response from the group at large a new thread would be a good idea with a more appropriate title.

Joe


----------



## Braineack (Aug 7, 2017)

TimmyD11 said:


> But I like what I have read about ISO invariance sensors and according to someone on another thread Canon sensors are STILL lagging behind other manufacturers, even with the latest improvements in their sensors...Is this true?



yes.



> Wanna make a case for what direction I should go in?



the video posted in post #31: ISO discussion, digital SLR compared to film SLR

watch between 7:15 and 9:20.  That's the case alone for me.


----------



## Gary A. (Aug 7, 2017)

Thanks for hanging there TimmyD11.  (I'm not a techie ... so I hope I get this right ...)

For me, the bottom line in all this, was Joe saying that with the latest and greatest Sony sensors (used in Sony, Nikon and Fuji cameras), that changing the ISO in-camera doesn't matter because the same basic/technical process is used to lighten the image in the camera as on a computer.  There are two advantages to manipulating the image on a computer: 

1) a computer is more powerful and is capable of rendering a better manipulated image than in-camera manipulation (i.e. raising the ISO); and
2) Raising the ISO will often results in the camera blowing out highlights, while manipulation on a computer tends to reduce the chances of 'clipping the highlights'. 

Thank You Joe.


----------



## TCampbell (Aug 7, 2017)

Bill The Lurker said:


> TCampbell said:
> 
> 
> > Just remember that noise is "additive" in that it always results in a photosite reporting a HIGHER numeric value than it should have reported.  There is no "anti-noise" where the photosite reported a LOWER numeric value (at least if there is, I have yet to encounter it.)
> ...



I'm not sure where you're trying to go with this.  I mentioned that there are lots of reasons for noise (including quantum uncertainty) but I wasn't going to attempt to pick each one apart.

It looks like you are responding without having read what I wrote.

Noise is much easier to notice in "dark" areas than in "light" areas.  Shoot a wedding and look at the bride's "white" wedding dress next to the groom's "black" tuxedo.  Black tux has lots of noise.  White dress... not so much.  

When I do astrophotography (but really any photography) I more aggressively treat noise in dark tonal regions than in light tonal regions.  

It's easy to find examples of this.  

Now... can you severely under-expose an image and then boost ISO high enough that something that should be dark only appears bright under extreme ISO boost?  Sure.  Would you see noise if you did that?  Sure.   Why would you do that if you knew it was going to be a problem?


----------



## Gary A. (Aug 7, 2017)

As I shoot mirrorless, I imagine the ISO dial is useful to gauge if a potential image, in a low light situation, can be acceptably manipulated to a successful image. Sorta like a preview.


----------



## SCraig (Aug 7, 2017)

TCampbell said:


> ...It's easy to find examples of this.


Just Google "Expose To The Right".


----------



## TCampbell (Aug 7, 2017)

Ysarex said:


> TCampbell said:
> 
> 
> > But back to that "upstream" amplification... this results in a gain being applied BEFORE digital conversion (before ADC) and as a result it doesn't lose much in the way of dynamic range.
> ...



Hi Joe,

The graph varies by model and whether or not they employ the use of upstream gain.

Here's the profiles for several Nikon cameras:

Best ISO values for Nikon cameras

Notice the D3100 (first camera & graph on the list).  There's almost no significant loss of dynamic range as they boost ISO (comparing ISO's 100, 200, & 400).  At 800 there's a more noticeable drop... but it's not huge.  Then you get to 1600 and suddenly the graph takes on a linear slope.  (and the camera follows the graph just like the example you cited above.)

That indicates that the D3100 is doing "upstream" amplification and somewhere around ISO 800 it begins using a moderate blend of upstream + downstream amplification... but by the time you get to ISO 1600 it's all downstream amplification (digital side) and no additional upstream gain is applied.

Compare that to a D3200 ... it's all one linear slope.  This suggests that the D3200 is doing only downstream (digital) amplification and no upstream gain is applied at all.

If you go through the full list, you'll see some Nikon models do apply some upstream amplification... many apply none and it's all downstream.

Canon seems to always use upstream amplification for low amounts of ISO boost, then switches to downstream for high amounts of ISO boost.


----------



## Ysarex (Aug 7, 2017)

TCampbell said:


> Ysarex said:
> 
> 
> > TCampbell said:
> ...



Having a deja vu experience -- we've done this before. So I see the 3100 graph and when I check it against Bill's data he shows the same thing but not to the same degree. All these test sites show data pulled from raw files (post ADC) and camera manufacturers aren't talking so I'm not prepared to speculate exactly what they're doing. When I get a chance I'll pass this one by Illiah Borg and see what he has to say. He should have an answer. In the meantime the majority of cameras tend to show a pretty linear raise the ISO a stop/drop a stop of sensor DR behavior and I'd call that the basic rule and other behavior an exception. I'll ask Illiah.

Joe


----------



## Derrel (Aug 7, 2017)

Why are we addressing the Nikon D3100 here?

From WIkipedia: "The *Nikon D3100* is a 14.2-megapixel DX format DSLR Nikon F-mount camera announced by Nikon on August 19, 2010. It replaced the D3000 as Nikon's entry level DSLR. It introduced Nikon's new EXPEED 2 image processor and was the first Nikon DSLR featuring full high-definition video recording with full-time autofocus and H.264 compression, instead of Motion JPEG compression. It was also the first Nikon DSLR to provide high-definition video recording at more than one frame rate.[3]"

The D3100 is a VERY outdated camera...


----------



## fmw (Aug 8, 2017)

Derrel said:


> Why are we addressing the Nikon D3100 here?
> 
> From WIkipedia: "The *Nikon D3100* is a 14.2-megapixel DX format DSLR Nikon F-mount camera announced by Nikon on August 19, 2010. It replaced the D3000 as Nikon's entry level DSLR. It introduced Nikon's new EXPEED 2 image processor and was the first Nikon DSLR featuring full high-definition video recording with full-time autofocus and H.264 compression, instead of Motion JPEG compression. It was also the first Nikon DSLR to provide high-definition video recording at more than one frame rate.[3]"
> 
> The D3100 is a VERY outdated camera...



Now, now Derrel, if the idea is to make good images, a 14mp DX camera is hardly outdated.  It just doesn't have all the features of current models.  It is capable of stunning photographs in the hands of someone who makes stunning photographs.  I know you could use one to create any image you like and be happy with the result.  I know I could.  Sometimes we confuse need with want.


----------



## Derrel (Aug 8, 2017)

Holding up the D3100 as an example of what "Nikon does" as far as image signal processing is not appropriate in the context of discussion how a camera maker's image processing strategies are currently implemented...that's the issue. Just calling out a bit of shade-throwing that somebody else was trying to do.

I've occasionally shot the old D2x over the past couple of years; inage quality is very "2005". Like the now-old D2x, the five years newer D3100 has easily been bettered; the current D3400 is a vastly better iange maker from a dynamic range point of view. As far as what "Nikon deoes" today, let's look to their D7200, which represents an actual and CURRENT Nikon camera. In the D7200 the dynamic range is 14.6 EV. The now-old D3100 with the same-sized APS-C sensor has 11.3 EV of dynamic range; that is a HUGE improvement, of three and one-third MORE stops's worth of DR.

In terms of overall sensor score, the D3100 is at 67, the D7200 is at 87; right up there in what used to be "full-frame" territory as far as technical image quality.




A sensor score of 67 is now considered poor in Nikon circles; that score and that bad dynamic range is more like m4/3 mini-sensor territory. An improvement of 3.3 EV in DR is major; holding up the D3100 as an example of modern sensors or signal processing as done by Nikon "today" is disingenuous, and that's what I wanted to call attention to.

Ohhhhh--and the D7200 is for all practical purposes, ISO Invariant.


----------



## KmH (Aug 8, 2017)

TimmyD11 said:


> How does a "fixed" sensor in a digital camera become more or less sensitive to light by just selecting a different ISO?


 It doesn't. As said before in the thread, the voltage developed by the pixel gets amplified (gain added) after the exposure has been made but before the analog voltage is converted to a digital number in the Analog to Digital (A/D) converter circuitry.
But if you want to know more about how a CMOS pixel works: Active pixel sensor - Wikipedia


----------



## TCampbell (Aug 8, 2017)

Derrel said:


> Why are we addressing the Nikon D3100 here?
> 
> [snip]
> 
> The D3100 is a VERY outdated camera...



Derrel, follow the link and look at all the graphs.  The D3100 is simply the first camera they list and it does upstream amplification (and is is probably the most extreme example of it), then the D3200 doesn't do it all... then the D3300 does it again (but not to the extreme of the D3100) and you can keep going through the list of all the models.

I think the author says they pulled the data from DxO (and I'm always extremely skeptical of anything that comes from DxO) but I do personally know people who have tested their own individual cameras.    A friend of mine did a test to compare several of his bodies because he's an astrophotographer and wanted to find the optimal ISO prior to graphs like this being available.


----------



## fmw (Aug 10, 2017)

Derrel said:


> Holding up the D3100 as an example of what "Nikon does" as far as image signal processing is not appropriate in the context of discussion how a camera maker's image processing strategies are currently implemented...that's the issue. Just calling out a bit of shade-throwing that somebody else was trying to do.
> 
> I've occasionally shot the old D2x over the past couple of years; inage quality is very "2005". Like the now-old D2x, the five years newer D3100 has easily been bettered; the current D3400 is a vastly better iange maker from a dynamic range point of view. As far as what "Nikon deoes" today, let's look to their D7200, which represents an actual and CURRENT Nikon camera. In the D7200 the dynamic range is 14.6 EV. The now-old D3100 with the same-sized APS-C sensor has 11.3 EV of dynamic range; that is a HUGE improvement, of three and one-third MORE stops's worth of DR.
> 
> ...



So what?  Can you make great images with it or not?  Isn't that what matters?


----------



## Derrel (Aug 10, 2017)

Re-read the above discussion, and strive to place my comments into context. The discussion was -not- about "making pictures".


----------



## Designer (Aug 10, 2017)

The dynamic range of modern DSLRs is astounding!  I once tried to duplicate what I was seeing with my eye in my dining room.  The room was lit by very muted and shady daylight entering through the glass patio doors.  I wanted to capture what I saw because I liked the range of light (fairly bright fading to quite dark).

I tried several settings but was not able to get a photograph to look just like what I was seeing.


----------



## Overread (Aug 10, 2017)

fmw said:


> So what?  Can you make great images with it or not?  Isn't that what matters?



In technical discussions - no no its not what matters. 
Creative ability is limited by the creator and also by the tool they use. However when you are comparing tools against other tools you have to boil things down to technical differences. To actual boring dull maths and stats and to things that are really boring photos that look at things purely technically.

It's not art its not creative its a technical discussion. 

If you try to bring "Oh but I made this great photo with it" then its not really a valid argument. Because you can make great photos with mobile camera phones; with point and shoots; with film; with digital; with glass plate. So it doesn't really give people much to measure and compare with other than the fact that someone made  a great picture of something with it.


----------



## fmw (Aug 11, 2017)

Derrel said:


> Re-read the above discussion, and strive to place my comments into context. The discussion was -not- about "making pictures".



My apologies.


----------



## Gary A. (Aug 11, 2017)

Overread said:


> fmw said:
> 
> 
> > So what?  Can you make great images with it or not?  Isn't that what matters?
> ...


Significance is the odd element of all this tech stuff.  If I was purchasing a camera ... sure I'd want a camera with the greatest DR ... but for what I shoot ... I've never had a photo fail because of lack of DR ... no one ever told me "... that shot could have been successful if it had two more stops of dynamic range...". Often I will increase the contrast and darken the shadows to eliminate distracting details and to add more drama to the image.

I've never shot astro and maybe DR is vitally important in shooting the heavens ... I mostly shoot people ... with people, the impact of the moment is vital not the DR.  

Tech and specs are important and should be discussed, but we all need to realize that the bottom line is the final image. The photographer needs to know and understand a minimal amount of tech and specs in order to consistently capture the exceptional image. But also remember that tech and specs are merely the means towards the end.


----------



## petrochemist (Aug 15, 2017)

Ysarex said:


> Bill The Lurker said:
> 
> 
> > the trouble with pedantry is that it turns into an infinite regress of "well no, actually"
> ...



 But it DOES increase the information.
Considering a low light exposure, such as stars in the night sky, with the ISO low the result of the A/D conversion for the brightest pixel could easily be much lower than  1/10th of full scale.
 When the signal is amplified 8 fold (3 stops higher ISO) before the A/D the digital output of a hypothetical brightest pixel which was 10% maximum would now be ~80% of maximum.
Meanwhile the luminosity recorded by many dark pixels that may have a digital value of only 0-5 (out of 256 for 8-bit jpegs or more for RAW) would now be spread over the range 0-15 the extra data here could be very significant, or could only be down to shot noise from the random nature of individual photons.
Each pixel of the intermediate light levels will also be digitalized over a 3x larger number of values, shot noise will not be significant for these so meaningful extra data is generated.

If the brightness of the shot is adjusted in post processing (after A/D conversion) then no extra data is produced the existing values are just spread further apart, but this is not how cameras usually work!


----------



## Ysarex (Aug 15, 2017)

petrochemist said:


> Ysarex said:
> 
> 
> > Bill The Lurker said:
> ...



What you are describing is a post processing procedure that occurs after the exposure of the sensor. *You can't increase the data the sensor has recorded once the exposure has ended.* What you can do, which you describe, is improve the efficiency with which you retain the data recorded -- in other words do a better job avoiding data loss through the system.

The issue in question here is: does raising the ISO increase the sensitivity of the sensor. If it did that then you would be able to raise the ISO and expect the camera to reach deeper into the shadows recording more data. That doesn't happen. What does happen in most cameras is what you described, the ISO gain pre-ADC prevents loss that would otherwise occur in an inefficient system. That's very different from increasing the data recorded.

Work with a functionally ISO invariant system and you can easily see the above. Here's an example:




 

Exact same exposure in both with the only difference that the ISO was 200 in one exposure and 3200 in the other. Above are the JPEGs created by the camera.



 

The Fuji X-E2 is functionally ISO invariant. If raising the ISO made the sensor more light sensitive the shadow detail in the above images would bear that out. It bears out the opposite. Raising the ISO to 3200 provided no benefit in increased shadow detail. The light sensitivity of the sensor is unaffected. With an ISO invariant camera the ISO analog gain adds no value. That's not the case with most cameras including Fuji's new X-Trans III sensor cameras. With those cameras the process you describe is valuable but it's not increasing the light sensitivity of the sensor and allowing the sensor to record more data. It's an engineering difference that does a better job retaining the data recorded.

Joe


----------



## Overread (Aug 15, 2017)

You know I think we are at the point were it might be wise to call it quits. not only are we trying to cobble together how sensors and ISO works but we are trying to do it for different generations and types of sensor; with resulting arguments and differences of opinions.


----------



## petrochemist (Aug 16, 2017)

Ysarex said:


> With those cameras the process you describe is valuable but it's not increasing the light sensitivity of the sensor and allowing the sensor to record more data. It's an engineering difference that does a better job retaining the data recorded.
> 
> Joe


I don't know about ISO invariant cameras, I don't have one & don't expect to in the next few years.

For other cameras (still the majority unless I'm much mistaken), the data recorded is only the digital output from the A/D, so it's changing the data recorded to show a different range. It has no effect on retaining the data recorded at all that's down to systems after the A/D.

I'll try to take on @Overread's wisdom & leave it there.


----------



## Derrel (Aug 16, 2017)

THE BIGGEST difference between ISO in film and in modern d-slrs is the ability to change the ISO value now, from shot to shot, as-needed. No more "36 frames stuck at ISO 64" for super-quality color slides...no more being stuck at ISO 400 pushed two stops for detail-free shadows with pushed Tri-X B&W film...nope! With digital, ISO speeds can be changed, and exposures accordingly changed, as-needed or as-desired. THAT is the single, biuggest difference.

As far as equivalency: a photographer friend and I used his Canon EOS 10D (ten-Dee!) back in the day, with its 6 megapixel sensor: we felt that it had finer grain,and more detail than ISO 100 color negative film had.


----------



## Ysarex (Aug 16, 2017)

petrochemist said:


> Ysarex said:
> 
> 
> > With those cameras the process you describe is valuable but it's not increasing the light sensitivity of the sensor and allowing the sensor to record more data. It's an engineering difference that does a better job retaining the data recorded.
> ...



After exposure the data in the sensor wells is read -- it is fundamentally an electron count. That is the data recorded. That's what the sensor does -- it records data as an electron count. Reading that data is the beginning of processing that data. Amplified or not the read data is then converted into numerical form in the ADC. Both the analog gain if applied and the ADC conversion are applied to the recorded data and constitute post processing which cannot alter the original electron count. Post processing can do a more or less efficient job with the data. Changing ISO on a digital camera does not/can not change the data the sensor is able to record. It can change how the data is post processed.

Joe


----------

