# Lens f-stop and ISO - is there a direct relationship?



## dennybeall (Jan 13, 2016)

I was wondering if there was a direct measureable relationship between the maximum f-stop of a lens and ISO. In other words if you have a lens that's F 3.5 and another that's F3.0 could you use the f3.5 with a Higher ISO and get the same results as the f3.0 with a lower ISO.
Disregarding the other qualities of the lens since lower f is usually a better quality lens.


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## Ysarex (Jan 13, 2016)

dennybeall said:


> I was wondering if there was a direct measureable relationship between the maximum f-stop of a lens and ISO. In other words if you have a lens that's F 3.5 and another that's F3.0 could you use the f3.5 with a Higher ISO and get the same results as the f3.0 with a lower ISO.
> Disregarding the other qualities of the lens since lower f is usually a better quality lens.



In your example the difference between f/3.5 and f/3.0 is negligible -- less than a 1/2 stop. So no difference. Instead compare lenses with a max apertures of f/2.8 and f/5.6 -- that's a two stop difference or the difference between ISO 200 and ISO 800. Now there's a difference.

Joe


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## dennybeall (Jan 13, 2016)

OK, the difference is 2 stops, makes sense, can we make an ISO change to make up for 2 stops of difference?


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## astroNikon (Jan 13, 2016)

as you use a smaller aperture
and keeping shutter constant
you will be getting less light in to the sensor
thus to compensate you increase the ISO to make the sensor more sensitive to light.

that's your exposure triangle.

The aperture, if you physically look at it reduces quite quickly from a large opening to a smaller opening.  This is a mathematical model associated with that.  This may help ==> Please Make A Note: The Mathematics of f/stop Aperture Numbers


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## Ysarex (Jan 13, 2016)

dennybeall said:


> OK, the difference is 2 stops, makes sense, can we make an ISO change to make up for 2 stops of difference?



Of course. ISO 100 to ISO 400 is two stops. Changing ISO however, as your question asks, does result in a measurable difference in the end result. Measurable and visible are two different things and how much they matter will depend a lot on what you're trying to achieve. A camera has a base ISO. Any increase in the camera ISO setting above base is used by the meter to calculate an exposure that underexposes the sensor. Underexposure of the sensor results in less data recorded. Most folks don't need all the data a sensor is capable of recording to render a satisfactory photo so no big deal. In high contrast lighting conditions where you might want all that data it becomes an issue.

Joe


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## Ysarex (Jan 13, 2016)

astroNikon said:


> as you use a smaller aperture
> and keeping shutter constant
> you will be getting less light in to the sensor
> thus to compensate you increase the ISO to make the sensor more sensitive to light.
> ...



You can't make the sensor more sensitive to light -- that's physically impossible. The light sensitivity of a digital camera sensor is fixed in manufacture and can't be altered otherwise.

Joe


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## Braineack (Jan 13, 2016)

exposurewise, yes.
qualitywise, debatable.

see: exposure triangle.


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## dennybeall (Jan 13, 2016)

"Of course. ISO 100 to ISO 400 is two stops. Changing ISO however, as your question asks, does result in a measurable difference in the end result. Measurable and visible are two different things and how much they matter will depend a lot on what you're trying to achieve. A camera has a base ISO. Any increase in the camera ISO setting above base is used by the meter to calculate an exposure that underexposes the sensor. Underexposure of the sensor results in less data recorded. Most folks don't need all the data a sensor is capable of recording to render a satisfactory photo so no big deal. In high contrast lighting conditions where you might want all that data it becomes an issue.

Joe[/QUOTE] "

That's what I was asking I think. 100 to 400 equals 2 stops. I ask because I've seen where an ad for an action photographer will specify an f2.8 or 3.0 lens and an f3.4 lens is not acceptable. It seems intellectually that with modern digital cameras you could make up for the lens with ISO setting. Perhaps it's just a way to want people that are are presumed to be more professional since they have the better lens????


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## KmH (Jan 13, 2016)

Ysarex said:


> astroNikon said:
> 
> 
> > as you use a smaller aperture
> ...


Yep.
What is actually done is changing how much the signal (voltage) recorded by each pixel is amplified.

Part of the image noise all digital photos have is amplifier circuit induced electronic noise.


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## astroNikon (Jan 13, 2016)

dennybeall said:


> That's what I was asking I think. 100 to 400 equals 2 stops. I ask because I've seen where an ad for an action photographer will specify an f2.8 or 3.0 lens and an f3.4 lens is not acceptable. It seems intellectually that with modern digital cameras you could make up for the lens with ISO setting. Perhaps it's just a way to want people that are are presumed to be more professional since they have the better lens????


Do you have a copy/link to the ad?

The lower f stops let's one isolate the subject, which I use for sports all the time.  But if the ad is poo-poo'ing f/2.8 .. I'm guessing they aren't supporting ANY zoom lens.


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## Ysarex (Jan 13, 2016)

dennybeall said:


> "Of course. ISO 100 to ISO 400 is two stops. Changing ISO however, as your question asks, does result in a measurable difference in the end result. Measurable and visible are two different things and how much they matter will depend a lot on what you're trying to achieve. A camera has a base ISO. Any increase in the camera ISO setting above base is used by the meter to calculate an exposure that underexposes the sensor. Underexposure of the sensor results in less data recorded. Most folks don't need all the data a sensor is capable of recording to render a satisfactory photo so no big deal. In high contrast lighting conditions where you might want all that data it becomes an issue.
> 
> Joe


 "

That's what I was asking I think. 100 to 400 equals 2 stops. I ask because I've seen where an ad for an action photographer will specify an f2.8 or 3.0 lens and an f3.4 lens is not acceptable. It seems intellectually that with modern digital cameras you could make up for the lens with ISO setting. Perhaps it's just a way to want people that are are presumed to be more professional since they have the better lens????[/QUOTE]

Less than a 1/2 stop isn't generallyworth fussing over. However action photography is demanding. You can't rely on a tripod and the action itself requires faster shutter speeds. Take that action indoors and you're not talking about a stop or two of underexposure between ISO 200 and 800 you're talking about a stop or two between 1600 and 6400. Bottom line is that increasing ISO is underexposing the sensor. We can get away with that such that there's no visible difference if we're working near the sensor base ISO to begin with. The camera tech is going to play a role as well. Many cameras today produce pretty fair results even with a sensor underexposure of 6 stops. But an indoor basketball gym? Do we push for 8 stops or 9 stops? Eventually you cross the line.

Joe


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## Dave442 (Jan 13, 2016)

As Jo noted, at some point you run into where you can't make up for the smaller aperture by raising the ISO. In this case the sports photographer knows what ISO he will go up to and he knows what minimum shutter speed he needs and if he has hit both those limits then the only thing left to change is the aperture. 

A change on one Stop in any one of those three is designed to adjust the exposure by the same amount.  I now do to my grandson what my grandfather would do to me, we both take a shot and then we compare settings and then he tells me how much difference we had in exposure.


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## Derrel (Jan 13, 2016)

A few years back,I recall an item written by a member of the dPreview staff (Barnaby Britton maybe?) that suggested that lens testing folks had stumbled onto a type of what I would classify as a kind of *digital reciprocity failure*, in which digital camera sensors seemed to break the reciprocal arrangement in which one stop wider and aperture resulted in one full EV more exposure--once lens apertures climbed above (_wider hole than_) f/1.4 I believe it was. This was as I recall, based on test results using some of the fairly new ultra-speed lenses which have recently proliferated in the mirrorless realm--lenses like the new-ish f/0.95 Cosina-made lenses, for example.

It's been a while, but as I recall, the piece seemed to suggest that digital sensors were just not delivering fully reciprocal exposure levels as the lens apertures grew wider and wider...that there was in effect, what one could describe as a loss of effective ISO speed. I sure as heck don't have any uber-speed lenses to test this out, and I almost never shoot any of my lenses at wider than f/2.8, even though I have a number of f/2 and f/1.8 and f/1.4 lenses...

******Okay....I found one thread about this....

Light loss on current CMOS sensors at big apertures: Open Talk Forum: Digital Photography Review

Light Loss on current CMOS Sensors at Big Apertures. The simplified concept would be that as pixel wells have grown smaller and smaller as sensor density has elevated, that light coming in at oblique angles has a tougher time at "filling" the so-called pixel wells.

dPreview's Andy Westlake chimes in in the above thread with test results from a Leica 50mm f/0.95 Leica Noctilux lens that shows that the depth of field blurring does NOT follow the proper linear relationship at apertures wider than f/1.4. Again...this discussion was a while back, and it might be that smaller-sized sensors and super-speed lenses don not deliver all of their theoretical performance.

Speaking of *aperture and ISO and the relationship*: I think a much bigger,much more real-world issue might be effective light transmission, aka *T-stop*, and how that relates to the number of lens elements, and how many air-to-glass surfaces there are in a lens, and how well-designed the anti-reflection coating is for the lens design. SOME lenses, like say a certain Nikon 70-200 f/2.8 zoom with 21 elements in 15 groups, seem to me to have an actual T-stop that's well below what say, my 135 D.C. lens achieves...the 135 DC is my "brightest" lens, with  a mere 7 lens elements in 6 groups.

T-stop is a big deal to cinematographers. Their lenses are marked in T-stops.


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