# Noise: ISO push vs. PP push



## Stosh (Oct 19, 2010)

I've seen it mentioned here several times.  
A properly exposed high ISO image can have less noise than an improperly (dimly) exposed low ISO image.
Why would this be?  I'm not saying it's wrong, just trying to understand why.  I'd like to look at as fair of an example as I can think of.  Let's start by properly exposing a scene at... say ISO 3200.  Then let's keep aperture and shutter speed identical and reduce the ISO 2 stops to 800.  What should happen here?

The first thing that comes to mind is my experience with Canon DSLRs and Canon's PP software.  In their Digital Photo Professional, you can "push" your exposure by 2 stops and it "magically" brings out those 2 stops of previously invisible data.  So the data is there even though you couldn't see it before pushing.  Yes, it's definitely noisier than the same image with no push, but is it noisier than the properly exposed image at ISO 3200?

Another observation is that Digital Photo Professional's noise reduction settings.  If you take an exposure at ISO 800 by default it will use very little noise reduction.  At ISO 3200 it will use a significant amount of noise reduction.  So the comparison isn't fair until you bump up the noise reduction on the pushed image to match that of the ISO 3200 image.

The way I understand digital ISO is that the signal from the sensor is amplified higher (multiplied) at higher ISOs.  In post processing to achieve a brighter image you do the same thing in the digital realm (multiply the values).  Assuming I'm right about how digital ISO works, why would the analog gain from the sensor be less noisy than the digital gain in post processing?


----------



## Big Mike (Oct 19, 2010)

The more 'underexposed' your image is, the lower your signal to noise ratio will be...because there is just less information in the darker ranges of the digital photo.  

Expose Right


----------



## Stosh (Oct 19, 2010)

Big Mike said:


> The more 'underexposed' your image is, the lower your signal to noise ratio will be...because there is just less information in the darker ranges of the digital photo.
> 
> Expose Right



I understand that, but how would increasing the gain from the sensor's signal do any better with the signal to noise ratio?  When you amplify a signal, you amplify both signal AND noise therefore not effectively gaining anything.


----------



## JG_Coleman (Oct 19, 2010)

Because of the way a digital sensor records image data, there is exponentially less information in progressively darker portions of a photograph.

Let's say you've got an underexposed digital image on your computer and there's a huge, dark shadow that you want to brighten up. The problem is that, even though the RAW file does indeed have some data stored for details within this shadow, it is extremely minimal. When you brighten up the shadow in the RAW file, you're basically amplifying the brightness of a nearly pure black area of the digital photograph.

Let me try to clarify this... Consider the opposite end of the spectrum. If you have a digital image with blown-out highlights... those areas of the photo contain little or no information that can be recovered. They've been recorded by the camera as pure white (or nearly pure white) blotches that lack usable detail. The same goes for shadows, really. If any area is recorded as pure black (or nearly pure black) there's going to be very little saved data in that area of the photo to recover. Thus, when you brighten up a dark shadow, you will recover some detail... but it will be grainy, noisy, and lack acceptable color fidelity.

Now, what if we use a high ISO? From the start, this seems to put you at a severe disadvantage, since the higher ISO will produce a noisier image by default. While that's true, there's still an important difference between the noise produced by a high ISO shot and the noise produced by boosting exposure in post.

The difference is in the source of the data. You'll be able to pull better detail from shadow areas using a higher ISO, because that extra exposure achieved will be able to call upon the actual light coming from the subject. Even though high ISO shots are noisy, the shadow areas will still have a better signal-to-noise ratio than if you simply underexposed the shot and brightened it up in post. Adding exposure in post, if you try to go too far, can be equivalent to trying to "squeeze blood from a stone". The digital image can only hold so much data... there IS a point at which areas which are mostly black just don't have any more usable data to reveal. In contrast, the world itself possesses nearly infinite detail in the form of reflected light. When you're using a higher ISO, the subject you're trying to record is right in front of the camera, ready to yield all the details you desire if you only expose the shot thoroughly enough.


----------



## Jay Hsiang Studio (Oct 19, 2010)

helpful post. thanks! 

georgia from jay hsiang studio

Jay Hsiang Studio
Los Angeles Wedding Photography
Engagement Photography
Destination Wedding Photography


----------



## Stosh (Oct 19, 2010)

JG_Coleman said:


> Because of the way a digital sensor records image data, there is  exponentially less information in progressively darker portions of a  photograph.


Yup, totally agree.  Because of the linear nature of the sensor, each darker stop has half as much info as the previous brighter stop.



JG_Coleman said:


> Let's say you've got an underexposed digital image on your computer and  there's a huge, dark shadow that you want to brighten up. The problem is  that, even though the RAW file does indeed have some data stored for  details within this shadow, it is extremely minimal. When you brighten  up the shadow in the RAW file, you're basically amplifying the  brightness of a nearly pure black area of the digital photograph.
> 
> Let me try to clarify this... Consider the opposite end of the spectrum.  If you have a digital image with blown-out highlights... those areas of  the photo contain little or no information that can be recovered.  They've been recorded by the camera as pure white (or nearly pure white)  blotches that lack usable detail. The same goes for shadows, really. If  any area is recorded as pure black (or nearly pure black) there's going  to be very little saved data in that area of the photo to recover.  Thus, when you brighten up a dark shadow, you will recover some  detail... but it will be grainy, noisy, and lack acceptable color  fidelity.


Yup, again we agree.  What you call grain, I'll call posterization.  Yes, pushing dark areas definitely results in more noise than brighter areas.  Great, we agree here.



JG_Coleman said:


> Now, what if we use a high ISO? From the start, this seems to put you at  a severe disadvantage, since the higher ISO will produce a noisier  image by default. While that's true, there's still an important  difference between the noise produced by a high ISO shot and the noise  produced by boosting exposure in post.


I'm all ears.



JG_Coleman said:


> The difference is in the source of the data. You'll be able to pull  better detail from shadow areas using a higher ISO, because that extra  exposure achieved will be able to call upon the actual light coming from  the subject.


As opposed to........ light coming from somewhere else?  I don't understand what you're saying here.



JG_Coleman said:


> Even though high ISO shots are noisy, the shadow areas will still have a  better signal-to-noise ratio than if you simply underexposed the shot  and brightened it up in post.


You've just re-worded my question into a statement.  How does that answer my question?



JG_Coleman said:


> Adding exposure in post, if you try to go too far, can be equivalent to  trying to "squeeze blood from a stone". The digital image can only hold  so much data... there IS a point at which areas which are mostly black  just don't have any more usable data to reveal.


Yup, totally agree.  There is definitely a point in which you can not squeeze any more data out of the black areas.  But how does this answer my question?  My example uses 2 identical exposures.  One pushes 2 stops through ISO and one pushes 2 stops through PP.  I don't think you're telling me 2 stops in PP is like getting blood from a stone.  Certainly 6 stops is, but I realize that's way above and beyond the capability of good PP.   A 6-stop boost from ISO 100 is ISO 6400 which is noisy, yes, but at least you have SOMETHING.  If you push an image in PP by 6 stops you won't have very much.  I never suggested that PP pushing could match ISO pushing in extreme situations.  I tried to make the experiment fair because Canon seems to record at least 2 stops of shadow information that you can't even see until you use RAW processing to bring it out.  You just can't bring out any "hidden" shadow info from a JPG.  It doesn't exist.



JG_Coleman said:


> In contrast, the world itself possesses nearly infinite detail in the  form of reflected light. When you're using a higher ISO, the subject  you're trying to record is right in front of the camera, ready to yield  all the details you desire if you only expose the shot thoroughly  enough.


This was a nice summary, but again, I can't find an answer to my original question.  Please help me if I just didn't see it.


----------



## JG_Coleman (Oct 19, 2010)

Okay... the problem here is really just that, on the fly, it was a difficult concept to convey.  What I'll try to do is explain it a different way, with a specific example first.

To illustrate the concept, it's necessary to use a somewhat exaggerated example.  Bear in mind that this example is put forth just for the sake of illustrating the concept.

Let's say your in this scenario.

-You're taking a landscape photo on a beach at night illuminated dimly by the moon.
-You're working with an aperture of f/5.6 on a kit lens.
-You're working with a specific shutter speed... let's say 1/100 sec.

Now, imagine you take a photograph at ISO 100 with these settings.  You'll pretty much get a pitch black frame with almost no information at all.  If you try to push the exposure through software, you might get hazy shapes of the beach and the horizon... but it could barely be called a photograph.  The data to draw the image out just isn't there.

Now, let's say that without changing the aperture or the shutter speed, you take the exact same shot at ISO 12,800.  It will be a seriously grainy photograph... pure crap as far as a good photo goes... but you'll actually be able to see the scene rather well in comparison to the first shot.

Now, as I said earlier, this is a severely exaggerated scenario.  But it basically takes the issue of "ISO push vs. PP Exposure Push" to its most extreme in order to demonstrate the inherent weakness in pushing exposure in post.



> Quote: Originally Posted by *JG_Coleman*
> 
> 
> _The difference is  in the source of the data. You'll be able to pull  better detail from  shadow areas using a higher ISO, because that extra  exposure achieved  will be able to call upon the actual light coming from  the subject._
> ...


As opposed to having a photograph that is already exposed and cannot be further exposed to any light whatsoever.  That's the whole reason that the PP exposure push doesn't really stack up to ISO push.

Using our above example, why is it that the ISO 100 shot can't even come close to the ISO 12,800 shot?

With the ISO 100 shot, the shutter opened up for 1/100 sec and the sensor basically saw absolutely nothing... and that's what it recorded... nothing.  That's all it has... nothing can be drawn out, because nothing was recorded.

With the ISO 12,800 shot, the shutter opened up for 1/100 sec and the sensor saw, through plenty of color noise, a beach scene complete with sand and water and a few dunes.



> The way I understand digital ISO is that the signal from the sensor is  amplified higher (multiplied) at higher ISOs.  In post processing to  achieve a brighter image you do the same thing in the digital realm  (multiply the values).


This is where you get confused...

You're thinking of using high ISOs as "boosting" the brightness of the image.   In fact, that is not what's happening.  High ISOs don't "boost" anything except for the sensitivity of the sensor.  It's not as if the camera is just applying its own post-processing to make the image brighter.  The sensor itself is literally made more sensitive to light, and therefore exposes faster than it would otherwise.  It has the same _basic_ effect as if you left your ISO low and instead left the shutter open longer.  High ISO doesn't multiply data... it literally collects more data.  It does this at the expense of generating color noise... but it is nonetheless collecting more data.

Using post-processing to increase exposure, in contrast, _is just a boost_.  You're just brightening up... multiplying... the minimal data available.

If you can expose your photograph enough at the time of capture, either by lengthening shutter speed or increasing ISO, you are gathering MORE source data (in the form of reflected light).  If you increase exposure in post, you are not gathering any more source data... you're just brightening up what minimal data you already have.

Admittedly, this is a difficult thing to explain.  I feel like there's an ideal way to succinctly express this concept that I'm just not able to draw out of myself, which is why I'm left to make meandering explanations.

Maybe this explains it a bit better?


----------



## JG_Coleman (Oct 20, 2010)

So, given a situation where you can shoot one of two photos:

Photo A - shoot two stops underexposed with low ISO (then boost exposure in post)

Photo B - shoot properly exposed at a higher ISO

Quite frankly, at only two stops underexposure, there is probably not an _enormous _difference  between the two.  I would still say, based on my previous points, that  Photo B will be the better photograph overall.  That the RAW file  records "at least 2 stops of shadow information" is something of a  half-truth.  True, details can be pulled from shadows that are two stops  under-exposed... but the information in those under-exposed areas is of  markedly lower quality than anything else in the image that is properly  exposed.  And the closer an area is to being a full two stops  under-exposed, the worse it gets.

Brighter areas of a photograph hold exponentially more data than darker  areas, as you've agreed.  Using higher ISO to properly expose a scene  simply captures _much more_ data... it's really just that simple.  That high ISOs generate color noise is a point not to be ignored... but the signal-to-noise ratio is still higher than if you start out with minimal signal to begin with in an under-exposed shot.

I understand that I'm referring to extreme circumstances, and that  you're talking about a much less extreme PP exposure push.  The fact of  the matter, though, is that the principle found in extreme examples  carries right over to less extreme examples.  The difference between  Photo A and Photo B won't be as pronounced as the ISO 100 vs ISO 12800  shot I used as an example, but the same principle carries right up to  the difference between ISO 200 and ISO 400.

Both areas are going to show noise in their own way... but the properly exposed shot will have much more signal mixed in with that noise.


----------



## Garbz (Oct 20, 2010)

More signal is right. That's the problem here. The amplification change in the ISO is not just a software processed difference of the signal. It changes the way the signal is amplified and converted to yield better signal to noise ratio than simply bumping the brightness of the data already captured.

I see a lot of discussion here, but not a lot of testing. Let's fix that 
Here is our base image, shot at ISO100.






Here is in order the image with shutterspeed reduced 2 stops:
Firstly shot at ISO400.
Secondly shot at ISO100, with the exposure slider at +2 fill light cranked up, saturation reduced and contrast reduced.
Finally shot at ISO100, with JUST the exposure slider at +2 (note how the tone is totally wrong. Why Adobe do you do this? Why?)











Here is in order the image with shutterspeed reduced 4 stops:
Firstly shot at ISO1600.
Secondly shot at ISO100, with the exposure slider at +4 fill light cranked up, saturation reduced and contrast reduced.
Finally shot at ISO100, with JUST the exposure slider at +4 (note even more how screwed the basic exposure slider is compared to a real change in exposure)












Please ignore motion blur. Desk doesn't a good tripod make


----------



## Stosh (Oct 20, 2010)

JG_Coleman said:


> With the ISO 100 shot, the shutter opened up for 1/100 sec and the  sensor basically saw absolutely nothing... and that's what it  recorded... nothing.  That's all it has... nothing can be drawn out,  because nothing was recorded.
> 
> With the ISO 12,800 shot, the shutter opened up for 1/100 sec and the  sensor saw, through plenty of color noise, a beach scene complete with  sand and water and a few dunes.


OK, we're making progress because we found something we disagree on.  How did these 2 identical sensors with identical shutter speeds see 2 different things?  Back in film days I would agree with you 100%.  The film was vastly different in sensitivity to light depending on which film you loaded behind the shutter.  But today you don't change the sensor.  It sees what it sees and that's that.  The way I understand how ISO works, the gain is turned up on the sensor to make it more sensitive.  Gain is another way of saying amplifier which is another way of saying multiplying which is another way of saying post processing (although it's post processing that the camera does on its own and we have no control over it).



JG_Coleman said:


> This is where you get confused...
> 
> You're thinking of using high ISOs as "boosting" the brightness of the  image.   In fact, that is not what's happening.  High ISOs don't "boost"  anything except for the sensitivity of the sensor.  It's not as if the  camera is just applying its own post-processing to make the image  brighter.  The sensor itself is literally made more sensitive to light,  and therefore exposes faster than it would otherwise.  It has the same _basic_  effect as if you left your ISO low and instead left the shutter open  longer.  High ISO doesn't multiply data... it literally collects more  data.  It does this at the expense of generating color noise... but it  is nonetheless collecting more data.


It seems this is the meat of our discussion.  Again, I don't agree with you here and would love to find out which side is more accurate.  Is there anywhere to research this further?



JG_Coleman said:


> That the RAW file  records "at least 2 stops of shadow information" is something of a  half-truth.


Maybe we need to explore this a little further too.  The whole reason RAW is better than JPG is because it has a wider dynamic range (similar to how a negative held more info than a print).

If we express dynamic range in bit depth, it's easier to continue the discussion.  A normal JPG that you see on a monitor or send to a printer is 8 bits per color channel.  Off the top of my head, I don't know what the "standard" is for today's DSLRs, but to be safe, let's just say it's 10 bits of dynamic range.  Let's say for your final picture you have only 2 options in post processing (we're using budget software lol).  Option A is to use the lower 8 bits and option B is to the upper 8 bits.  In both cases your final picture is using real world actual data that the sensor recorded.  To use your words again, we're not trying to squeeze blood from a rock.  Admittedly, the info in the bottom 8 bits will have a slightly lower SNR, but I think both would be excellent quality photos.  Can I also say that photo B is "pushed" 2 stops beyond photo A because we're using a 2 bit shift?  Can I describe this as "lossless pushing?"

As I've already said, I agree that the upper levels hold much more information than the lower levels, but I think that concept is greatly exaggerated and I think my case above shows why.  Since a JPG only shows 8 bits per channel, you're not losing anything at all by using the lower 8 bits vs. the upper 8 bits.  Sure, you're truncating the top 2 bits, but if there was nothing in those fabulously data-rich upper 2 bits, you're not losing a thing.  You still end up with a full 8 bits of actual real-world data.

I contend that you lose exactly the same amount of data by dropping the upper 2 bits or the lower 2 bits.  Again, I agree the lower 2 bits will have slightly more noise.



JG_Coleman said:


> I understand that I'm referring to extreme circumstances, and that   you're talking about a much less extreme PP exposure push.  The fact of   the matter, though, is that the principle found in extreme examples   carries right over to less extreme examples.


I'm all for that line of thinking.  I use extremes to illustrate subtle differences all the time.  And in your extreme examples, I follow you 100% and agree with you 100%.  But it's likely I didn't state my original thought well enough.  I don't want to compare ISO push vs. PP push in situations where no info was recorded (which would be beyond however wide the dynamic range of the RAW output is).  I want to compare true, recorded data to true, recoded data using 2 different methods.  And just to be clear, here are the 2 different methods:

Method A:  properly expose a scene in a higher ISO to use the upper 8 bits.
Method B:  use a lower ISO to underexpose the same image only enough so that data is still recorded so that when you push the exposure in PP using the RAW file, you're not inventing data, you're using what's already there.

I think there are 2 keys to my viewpoint.  #1 is that I think sensor gain (ISO) is just multiplying the same internal data.  #2 using the lower 8 bits of a RAW image does not make a significant difference  in using the upper 8 bits.

After re-reading my post, I think I may see where I have 1 concept not quite right.  When I talk about upper 8 bits vs lower 8 bits, I'm thinking in the linear world which is definitely how the sensor records the data.  However, the camera applies curves to this linear data to make the final picture look more "real world" the way our eyes perceive the scene.  This conversion from linear to real-world curve may have something to do with my confusion.  Can you help with that?


----------



## Stosh (Oct 20, 2010)

I knew I could get you to post, Garbz!
Thanks for the examples.  I like your use of subjects lol.  You beat me to the hands-on portion, but I'm still interested in the why.

What about my noise reduction discussion?  Doesn't the camera automatically apply more noise reduction when you use a higher ISO?  And did you apply that same increased noise reduction to the PP pushed image to make it a little more fair?


----------



## Helen B (Oct 20, 2010)

> If we express dynamic range in bit depth, it's easier to continue the discussion.  A normal JPG that you see on a [COLOR=#990000 ! important][COLOR=#990000 ! important]monitor[/COLOR][/COLOR]  or send to a printer is 8 bits per color channel.  Off the top of my  head, I don't know what the "standard" is for today's DSLRs, but to be  safe, let's just say it's 10 bits of dynamic range.  Let's say for your  final picture you have only 2 options in post processing (we're using  budget software lol).  Option A is to use the lower 8 bits and option B  is to the upper 8 bits.  In both cases your final picture is using real  world actual data that the sensor recorded.  To use your words again,  we're not trying to squeeze blood from a rock.  Admittedly, the info in  the bottom 8 bits will have a slightly lower SNR, but I think both would  be excellent quality photos.  Can I also say that photo B is "pushed" 2  stops beyond photo A because we're using a 2 bit shift?  Can I describe  this as "lossless pushing?"





> After re-reading my post, I think I may see where I have 1 concept not  quite right.  When I talk about upper 8 bits vs lower 8 bits, I'm  thinking in the linear world which is definitely how the sensor records  the data.  However, the camera applies curves to this linear data to  make the final picture look more "real world" the way our eyes perceive  the scene.  This conversion from linear to real-world curve may have  something to do with my confusion.  Can you help with that?



I think that understanding the linear (or near-linear) nature of the output from the ADC is the key to this discussion. Sending the ADC a full-range analogue signal is clearly a good thing.

Yes, a JPEG has a tone curve applied, it is not a simple truncation of the linear data. There is, therefore, no fixed relationship between the scene brightness range that a JPEG represents and the 8-bit nature of a JPEG.

Best,
Helen


----------



## JG_Coleman (Oct 20, 2010)

Well... I'm running out of ways to explain it, really.  But upon further contemplation, I think I've hit on the best way to explain it.  Maybe this still won't work for you, but it's all I can really think of.

I've been somewhat simplifying the case of high ISO, but maybe being more technically specific information will help.  Indeed, when you change the ISO to a higher value... you aren't _technically_ increasing the sensor's sensitivity.  You are right in that respect... you can't actually make the sensor more sensitive.  Indeed, the signal is, in the strictest sense, just being amplified.

But, higher ISOs are amplifying the _analog _signal received by the sensor.  This amplification is occurring before the data is sent to the A/D converter to be recorded as digital information.

So, when you use higher ISO to fully expose those extra two stops, you're drawing the extra two stops of information directly from the _analog signal _received by the sensor.

In contrast, when you underexpose by two stops with lower ISO and increase the exposure in post, you're amplifying _digital information_.

This is really the crux of the matter.  The analog signal carries more image data than the digital information in a photograph that has already been captured.


----------



## Stosh (Oct 20, 2010)

Garbz said:


> More signal is right. That's the problem here. The amplification change in the ISO is not just a software processed difference of the signal. It changes the way the signal is amplified and converted to yield better signal to noise ratio than simply bumping the brightness of the data already captured.
> 
> I see a lot of discussion here, but not a lot of testing. Let's fix that
> Here is our base image, shot at ISO100.
> ...



Now that I had a few minutes to look at this, I noticed something interesting.  Of course the first thing I looked at was the dark area in the top center.  Concerning noise in dark areas, your pictures certainly show that ISO 400 did better than ISO 100 pushed 2 stops.  But now look down at the bottom book.  Look at the word "Mathematics".  Notice how much sharper it is in the ISO 100 shots?  Both methods of pushing ISO 100 were sharper than the ISO 400 shot.  My guess is back to what I said about noise reduction.  The camera (or some PP) applied more noise reduction to the ISO 400 shot _automatically_.  The noise reduction was not applied to the ISO 100 shots.  So although the noise reduction certainly did its job of reducing noise in the dark areas, it significantly wounded the brighter part of the image by reducing sharpness.

So back to what I suggested before - if you were to somehow find the amount of noise reduction that got applied to the ISO 400 shot, then also applied it to the pushed ISO 100 shot, would it look similar?


----------



## JG_Coleman (Oct 20, 2010)

Stosh said:


> #1 is that I think sensor gain (ISO) is just multiplying the same internal data.



I'm telling you, Stosh... you can look at sample shots all day long... but the conceptual under-pinnings are all you really need to know to understand the answer to your question.  Your above assumption is incorrect.

When you use a high ISO, you're amplifying the _original, analog signal_ produced by the camera's sensor.  When you compensate for under-exposure in post, you're amplifying a weak digital representation of the original signal.  

Thus, it's most definitely _not _"multiplying the same internal data".  The two data sources are much different.  One is the original signal produced by light striking your sensor, the other is an under-exposed, low-data, digital record of that signal.

Noise reduction may have it's various implications, as well.  But the fact still remains... you get more image data by boosting the original analog signal than by boosting the low-data digital representation produced by under-exposure.


----------



## Stosh (Oct 20, 2010)

Helen B said:


> Yes, a JPEG has a tone curve applied, it is not a simple truncation of the linear data. There is, therefore, no fixed relationship between the scene brightness range that a JPEG represents and the 8-bit nature of a JPEG.
> 
> Best,
> Helen


OK, you're right that any random 8-bit JPG image does not necessarily correspond to any real-world dynamic range, but that's because we as image processors make those 8 bits represent whatever we want.

Conversely, the camera outputs a RAW file that DOES correspond to a real-world dynamic range.  This we can't change that because it's written into the firmware of our cameras.  My whole point is that this RAW file has more dynamic range than we're used to seeing in our images.  When our 8-bit calibrated monitors try to display a RAW file, they look extremely dull from low contrast because we're squeezing 10, 12, or even 14 bits of dynamic range into 8.

Yes, I now realize and agree that curves are applied to the data, so it's not nearly as simple as truncating bits, but why can't we select good real-world data from the top, middle, or bottom of this wide dynamic capture?


----------



## Stosh (Oct 20, 2010)

JG_Coleman said:


> I'm telling you, Stosh... you can look at sample shots all day long... but the conceptual under-pinnings are all you really need to know to understand the answer to your question.


Well that's an angle I never heard before.  Most people say, "we can talk theory all day, but when it comes down to a real world example.........."



JG_Coleman said:


> Thus, it's most definitely _not _"multiplying the same internal data".  The two data sources are much  different.  One is the original signal produced by light striking your  sensor, the other is an under-exposed, low-data, digital record of that  signal.


Well now we're getting into semantics.  Both instances do come from the exact same signal if go back far enough in the process (the signal coming directly from the sensor before the amplifier).

I do get what you and others are saying - apparently the amplified analog signal can be more accurate because the digital signal is very low bit depth at the dark end of the spectrum.  I think the moral of this whole thing is it made little difference at 2 stops, but significant difference at 4 stops and above.  I'd like to do some examples myself, but I fear I have exhausted everyone's interest in this subject lol.


----------



## JG_Coleman (Oct 20, 2010)

Stosh said:


> Well that's an angle I never heard before.  Most people say, "we can  talk theory all day, but when it comes down to a real world  example.........."



Lol... no doubt that real-world investigation can be a good thing.  It's just that, in this case, the known way in which a camera handles signal and the known way in which exposure is boosted in PP will produce the same predictable results every time.


----------



## LokiZ (Oct 20, 2010)

Here are the three things I know...

- With my camera the higher the ISO the more noise.  

- With my camera the less light available the more noise.

With me so far?

- Thirdly with my camera the degree of difference noise wise with my ISO settings (100 through 1600) is minute compared that of the difference noise wise when comparing my max metered under exposure to my max metered over exposure.

My test went about it a little differently:

I took a base image correctly exposed at ISO 400.  I under exposed a 100 ISO image by -2 on the in camera meter and over exposed  the 1600 ISO by +2.  In PP I was able to acceptably lower the 1600 ISO image and match my 400 ISO image but adjusting the 100 ISO up was unacceptable and noise wise they did not match.

Flipping the test around I under exposed a 1600 ISO image by -2 on the in camera meter and over exposed  the 100 ISO by +2.  In PP I was able to more then acceptably lower the 100 ISO image and match my 400 ISO image but adjusting the 1600 ISO up was very unacceptable and noise wise they did not match.

So... In conclusion the effect of exposure has a much greater consequence on noise in an image then that of the the ISO setting on a camera.  That in turn makes it possible for images to be made where higher ISO value images have less noise then that of lower ISO images (after PP adjustments have been made.)

Basically if more light comes in on a higher ISO shot then does on a lower ISO shot it is then possible to have less noise in the high ISO shot through the use of PP to decrease the exposure.


Edit: line 1 two changed to three.


----------



## Garbz (Oct 21, 2010)

Stosh said:


> but I'm still interested in the why.
> 
> What about my noise reduction discussion?  Doesn't the camera automatically apply more noise reduction when you use a higher ISO?  And did you apply that same increased noise reduction to the PP pushed image to make it a little more fair?



Noise reduction can only come into it if I shot in JPEG. I didn't. The noise reduction was equal on all parts of the image due to camera corrections not being applied in RAW.

The why is still simple. ISO is increasing the sensitivity of your light capturing device before conversion to digital. Increasing brightness is a linear increase on the data after it has been converted to digital. The former has very complicated non-linear relationships between noise and increased photo conversion. The latter has a linear relationship. Double the brightness, double the noise.



Stosh said:


> Look at the word "Mathematics".  Notice how much sharper it is in the ISO 100 shots?  Both methods of pushing ISO 100 were sharper than the ISO 400 shot.  My guess is back to what I said about noise reduction.  The camera (or some PP) applied more noise reduction to the ISO 400 shot _automatically_.  The noise reduction was not applied to the ISO 100 shots.  So although the noise reduction certainly did its job of reducing noise in the dark areas, it significantly wounded the brighter part of the image by reducing sharpness.
> 
> So back to what I suggested before - if you were to somehow find the amount of noise reduction that got applied to the ISO 400 shot, then also applied it to the pushed ISO 100 shot, would it look similar?



No sorry, the real explanation is far more sinister. The camera was not on a tripod, and the focus was not locked, the camera attempted to refocus every time it takes a picture. In some cases it pushed it out. Notice how the sharpness effect is exactly the opposite in the ISO1600 shots as the ISO400 shots? Well... my bad


----------



## Stosh (Oct 21, 2010)

Garbz said:


> The why is still simple. ISO is increasing the sensitivity of your light capturing device before conversion to digital. Increasing brightness is a linear increase on the data after it has been converted to digital. The former has very complicated non-linear relationships between noise and increased photo conversion. The latter has a linear relationship. Double the brightness, double the noise.


Thanks Garbz.  This is what I've surmised over the previous few days discussion, but you've summarized it nicely in a couple of sentences.



Garbz said:


> No sorry, the real explanation is far more sinister. The camera was not on a tripod, and the focus was not locked, the camera attempted to refocus every time it takes a picture. In some cases it pushed it out. Notice how the sharpness effect is exactly the opposite in the ISO1600 shots as the ISO400 shots? Well... my bad


OK, that actually makes much more sense.


----------

