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ISO Invariance: an experiment

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FingerPainter said:
Nearly every exposition of the exposure triangle I have seen says that noise increases with ISO, or that increasing ISO increases the noise.
Click to expand...
That is an almost meaningless statement without further context.

I've noticed from this forum that a lot of photographers have a very weak understanding of the importance of distinguishing between constants and variables (and distinguishing between dependent and independent variables).

"Increasing ISO increases the noise" is true if the final image brightness is kept constant. By final image, I mean the jpeg produced by the camera.

"Increasing ISO increases the noise" is generally false if the exposure (shutter speed and f-number) is kept constant.

Of course, both the above assume that the subject brightness is kept constant, as well as several other basic things.
 
D Cox said:
confused circle said:
FingerPainter said:
The "exposure triangle" model, as usually taught, would have us expect the ISO 6400 shot to be about 5 stops noisier than the ISO 200 shot.
Click to expand...
Why. The exposure settings are the same, the iso 200 shot is not receiving more light.
Click to expand...
However, this requires you to manually set some combination of aperture and time, and then change the ISO number without changing either of these. Which is perfectly possible, but it's not the way we normally work.

If you are shooting in A or S mode, for instance, then changing the ISO number will cause the camera to change one of the exposure settings. If you are in manual mode, there will be some kind of marker that tells you the shot will be "wrongly" exposed, and normally you would change speed or aperture to re-centre the marker.

Also, the view on the LCD or EVF will look too dark or too light if you change only the ISO number and keep the exposure unchanged.

So I think these tests are interesting and worth doing, but don't match normal camera usage. They may confuse inexperienced users even more than the "exposure triangle" does.
Click to expand...
That is a good point. I should have said that I was using full manual settings: shutter speed, f-number and ISO.
 
D Cox said:
confused circle said:
FingerPainter said:
The "exposure triangle" model, as usually taught, would have us expect the ISO 6400 shot to be about 5 stops noisier than the ISO 200 shot.
Click to expand...
Why. The exposure settings are the same, the iso 200 shot is not receiving more light.
Click to expand...
However, this requires you to manually set some combination of aperture and time, and then change the ISO number without changing either of these. Which is perfectly possible, but it's not the way we normally work.

If you are shooting in A or S mode, for instance, then changing the ISO number will cause the camera to change one of the exposure settings. If you are in manual mode, there will be some kind of marker that tells you the shot will be "wrongly" exposed, and normally you would change speed or aperture to re-centre the marker.

Also, the view on the LCD or EVF will look too dark or too light if you change only the ISO number and keep the exposure unchanged.

So I think these tests are interesting and worth doing, but don't match normal camera usage. They may confuse inexperienced users even more than the "exposure triangle" does.
Click to expand...
A problem is that normal camera usage is sub-optimal, at least for a raw shooter who is willing to put at least a minimal amount of effort into processing.

Should we try to educate these inexperienced users, at the risk of initially confusing them, or should we leave them to their misunderstandings?
 
Last edited:
Tom Axford said:
Two recent threads (this and this) have generated a lot of discussion of "ISO Invariance" and related matters. These discussions have been almost entirely theoretical and largely centred on dynamic range and noise.

However, the experimental approach ("suck it and see") raises a couple of extra points that seem to be often overlooked.

For example, here are two images with the same shutter speed and aperture, but differing ISO settings:

Taken at ISO 6400 and at the exposure indicated on the camera's meter. Default processing in Lightroom.

Taken at ISO 6400 and at the exposure indicated on the camera's meter. Default processing in Lightroom.

Taken at ISO 200 and the same exposure as the previous shot. Processed in Lightroom with Exposure +5.0

Taken at ISO 200 and the same exposure as the previous shot. Processed in Lightroom with Exposure +5.0

The first point I would like to make is that to do this experiment, you need to choose not only the camera, but also the software to do the raw processing.
Click to expand...
It may also depend on the Process Version you select in Lr/ACR.

Could you please make raw files available?

--

About LibRaw | LibRaw

www.libraw.org www.libraw.org
 
stevo23 said:
FingerPainter said:
Tom Axford said:
FingerPainter said:
The "exposure triangle" model, as usually taught, would have us expect the ISO 6400 shot to be about 5 stops noisier than the ISO 200 shot.
Click to expand...
I wasn't aware that the "exposure triangle" told us anything about noise.
Click to expand...
Nearly every exposition of the exposure triangle I have seen says that noise increases with ISO, or that increasing ISO increases the noise.
Click to expand...
I wasn't aware that exposure triangle tells us about noise. That's an semi-unrelated consequence I think.
Click to expand...
Well, perhaps your personal knowledge of exposure theory is so complete that you are unfamiliar with how the "exposure triangle" conceptual model is usually presented.

Here's an example from the popular (but frequently incorrect) site Cambridge in Colour:

9d7d400a4cb84f0f9584a11410a47f36.jpg.png


In the diagram, the "ISO Speed" is shown as directly affecting (only) image noise. In the text, the claim is that "ISO speed affects image noise", and that "ISO Speed: controls the sensitivity of your camera's sensor to a given amount of light". You seem to be aware that the second claim is incorrect. Except in the case of dual conversion gain sensors, the ISO setting has no effect on the sensor's sensitivity. The sensor reports the same voltage for the same amount of incident light per unit area regardless of ISO setting.

The diagram and text together model three settings: Aperture , Shutter Speed and ISO Speed, and three effects: depth of field, motion blur and noise. That doesn't make ISO's claimed effect on noise a "semi-unrelated consequence".
 
richarddd said:
D Cox said:
confused circle said:
FingerPainter said:
The "exposure triangle" model, as usually taught, would have us expect the ISO 6400 shot to be about 5 stops noisier than the ISO 200 shot.
Click to expand...
Why. The exposure settings are the same, the iso 200 shot is not receiving more light.
Click to expand...
However, this requires you to manually set some combination of aperture and time, and then change the ISO number without changing either of these. Which is perfectly possible, but it's not the way we normally work.

If you are shooting in A or S mode, for instance, then changing the ISO number will cause the camera to change one of the exposure settings. If you are in manual mode, there will be some kind of marker that tells you the shot will be "wrongly" exposed, and normally you would change speed or aperture to re-centre the marker.

Also, the view on the LCD or EVF will look too dark or too light if you change only the ISO number and keep the exposure unchanged.

So I think these tests are interesting and worth doing, but don't match normal camera usage. They may confuse inexperienced users even more than the "exposure triangle" does.
Click to expand...
A problem is that normal camera usage is sub-optimal, at least for a raw shooter who is willing to put at least a minimal amount of effort into processing.

Should we try to educate these inexperienced users, at the risk of initially confusing them, or should we leave them to their misunderstandings?
Click to expand...
The larger problem is that while shutter time and aperture are fairly straightforward and work in the same way on all cameras, the ISO number and the +/- EV control are complex and work differently on various camera models.

As I see it, increasing the ISO number usually has two effects: it changes the speed or aperture (or both) to reduce exposure; and it sets variables in the processing, such as noise reduction, to reduce the bad effects of underexposure.

You can forcibly block the first effect by shooting in manual mode and ignoring warnings, in which case you will get the second effect on its own.
 
stevo23 said:
FingerPainter said:
stevo23 said:
FingerPainter said:
The "exposure triangle" model, as usually taught, would have us expect the ISO 6400 shot to be about 5 stops noisier than the ISO 200 shot. Instead, what I see is the ISO 6400 shot being approximately one stop less noisy than the ISO 200 shot. The result seems consistent with Bill Claff's measurements (no surprise there).
Click to expand...
Not sure what you're implying here. That at ISO200, the camera isn't ISO invariant?
Click to expand...
I'm implying that the OP's experiment proves (once again) the typical exposition of the "exposure triangle" to be incorrect.
Click to expand...
Maybe the typical exposition of the exposure triangle starts with an incorrect premise? I've not really followed exposure triangle discussions of late.
Click to expand...
Yes, I think that is evident.
stevo23 said:
Noise is noise and does tend to go up with ISO,
Click to expand...
Yes, but there is no causal relationship between those two factors alone. Noisiness tends to go up with ISO because ISO tends to go up as exposure goes down. The actual causal relationship is an inverse one between exposure and noisiness.
stevo23 said:
but exposure should only be about getting correct exposure - 18% grey etc. etc.
Click to expand...
I'd suggest exposure should be about maximizing information, by collecting as much light as possible without blowing desired highlight detail. Brightness is easier to correct in development than is SNR, DoF and various other types of blur.
stevo23 said:
Noise is another discussion altogether. For instance: Shots at ISO800 on my Fuji X-Pro2 actually seem to display less noise than shots at ISO400-ISO600 due to the dual gain structure. That would tend to invalidate that typical exposition.
Click to expand...
 
Tom Axford said:
FingerPainter said:
Nearly every exposition of the exposure triangle I have seen says that noise increases with ISO, or that increasing ISO increases the noise.
Click to expand...
That is an almost meaningless statement without further context.
Click to expand...
Well I have provided further context in another post ITT where I provide an example of a typical exposition of the exposure triangle, as found at CiC.
Tom Axford said:
I've noticed from this forum that a lot of photographers have a very weak understanding of the importance of distinguishing between constants and variables (and distinguishing between dependent and independent variables).

"Increasing ISO increases the noise" is true
Click to expand...
Not really. It misses the part about the increase in nosiness being caused by the accompanying decrease in exposure, not directly by the increase in ISO.
Tom Axford said:
if the final image brightness is kept constant. By final image, I mean the jpeg produced by the camera.

"Increasing ISO increases the noise" is generally false if the exposure (shutter speed and f-number) is kept constant.
Click to expand...
Quite correct. However, most expositions of the exposture triangle do not make such a distinction
Tom Axford said:
Of course, both the above assume that the subject brightness is kept constant, as well as several other basic things.
Click to expand...
Such assumptions are usually not explicitly explained in presentations of the "exposure triangle" model.
 
FingerPainter said:
stevo23 said:
FingerPainter said:
Tom Axford said:
FingerPainter said:
The "exposure triangle" model, as usually taught, would have us expect the ISO 6400 shot to be about 5 stops noisier than the ISO 200 shot.
Click to expand...
I wasn't aware that the "exposure triangle" told us anything about noise.
Click to expand...
Nearly every exposition of the exposure triangle I have seen says that noise increases with ISO, or that increasing ISO increases the noise.
Click to expand...
I wasn't aware that exposure triangle tells us about noise. That's an semi-unrelated consequence I think.
Click to expand...
Well, perhaps your personal knowledge of exposure theory is so complete that you are unfamiliar with how the "exposure triangle" conceptual model is usually presented.
Click to expand...
I don't know about that. I just know what I need to get what I want and I know how noise happens in my own camera. Should I get another one at some point, I'll head straight to Bill Claff's site and see how it behaves.
 
FingerPainter said:
Tom Axford said:
FingerPainter said:
Nearly every exposition of the exposure triangle I have seen says that noise increases with ISO, or that increasing ISO increases the noise.
Click to expand...
That is an almost meaningless statement without further context.
Click to expand...
Well I have provided further context in another post ITT where I provide an example of a typical exposition of the exposure triangle, as found at CiC.
FingerPainter said:
I've noticed from this forum that a lot of photographers have a very weak understanding of the importance of distinguishing between constants and variables (and distinguishing between dependent and independent variables).

"Increasing ISO increases the noise" is true
Click to expand...
Not really. It misses the part about the increase in nosiness being caused by the accompanying decrease in exposure, not directly by the increase in ISO.
Click to expand...
I'm sure you could prove black is white with arguments like that!

I can see no point in continuing this discussion.
 
Tom Axford said:
FingerPainter said:
Tom Axford said:
FingerPainter said:
Nearly every exposition of the exposure triangle I have seen says that noise increases with ISO, or that increasing ISO increases the noise.
Click to expand...
That is an almost meaningless statement without further context.
Click to expand...
Well I have provided further context in another post ITT where I provide an example of a typical exposition of the exposure triangle, as found at CiC.
Tom Axford said:
I've noticed from this forum that a lot of photographers have a very weak understanding of the importance of distinguishing between constants and variables (and distinguishing between dependent and independent variables).

"Increasing ISO increases the noise" is true
Click to expand...
Not really. It misses the part about the increase in nosiness being caused by the accompanying decrease in exposure, not directly by the increase in ISO.
Click to expand...
I'm sure you could prove black is white with arguments like that!
Click to expand...
Pardon me, but no; while "Increasing ISO increases the noise" is the kind of argument that attempts to prove black is white, because increasing ISO tends to decrease read noise, while decreasing exposure decreases the signal-to-noise ratio.
 
Iliah Borg said:
Tom Axford said:
FingerPainter said:
Tom Axford said:
FingerPainter said:
Nearly every exposition of the exposure triangle I have seen says that noise increases with ISO, or that increasing ISO increases the noise.
Click to expand...
That is an almost meaningless statement without further context.
Click to expand...
Well I have provided further context in another post ITT where I provide an example of a typical exposition of the exposure triangle, as found at CiC.
Iliah Borg said:
I've noticed from this forum that a lot of photographers have a very weak understanding of the importance of distinguishing between constants and variables (and distinguishing between dependent and independent variables).

"Increasing ISO increases the noise" is true
Click to expand...
Not really. It misses the part about the increase in nosiness being caused by the accompanying decrease in exposure, not directly by the increase in ISO.
Click to expand...
I'm sure you could prove black is white with arguments like that!
Click to expand...
Pardon me, but no; while "Increasing ISO increases the noise" is the kind of argument that attempts to prove black is white, because increasing ISO tends to decrease read noise, while decreasing exposure decreases the signal-to-noise ratio.
Click to expand...
The full statement I wrote (most of which was cut out by FingerPainter) was:

"Increasing ISO increases the noise" is true if the final image brightness is kept constant. By final image, I mean the jpeg produced by the camera.

This is a statement about mathematical dependence, not physical cause and effect.

The second statement I made was:

"Increasing ISO increases the noise" is generally false if the exposure (shutter speed and f-number) is kept constant.

This statement is also about mathematical dependence, not physical cause and effect.
 
FingerPainter said:
Well, perhaps your personal knowledge of exposure theory is so complete that you are unfamiliar with how the "exposure triangle" conceptual model is usually presented.

Here's an example from the popular (but frequently incorrect) site Cambridge in Colour:

9d7d400a4cb84f0f9584a11410a47f36.jpg.png


In the diagram, the "ISO Speed" is shown as directly affecting (only) image noise. In the text, the claim is that "ISO speed affects image noise", and that "ISO Speed: controls the sensitivity of your camera's sensor to a given amount of light".
Click to expand...
This site is correct to say that ISO speed affects image noise because this is in the context of the exposure triangle.

This means that the important text is: one can therefore use many combinations of the above three settings. This means that changing ISO changes exposure.

The exposure triangle is often criticised for wrong reasons.
 
Tom Axford said:
Iliah Borg said:
Could you please make raw files available?
Click to expand...
Follow this link to access the raw files. The exposure of the first was determined by the camera's metering (in matrix mode), I did not attempt to ETTR it.
Click to expand...
Thank you. The problem is caused by the rounding of and close to the black level. It manifests as typical magenta contamination. The magenta tint is present even with simple matrix profile, profile twists are not the main reason of the problem here.

--
http://www.libraw.org/
 
Last edited:
Iliah Borg said:
Tom Axford said:
Iliah Borg said:
Could you please make raw files available?
Click to expand...
Follow this link to access the raw files. The exposure of the first was determined by the camera's metering (in matrix mode), I did not attempt to ETTR it.
Click to expand...
Thank you. The problem is caused by the rounding of and close to the black level. It manifests as typical magenta contamination. The magenta tint is present even with simple matrix profile.
Click to expand...
Thank you.
 
Tom Axford said:
The full statement I wrote was:

"Increasing ISO increases the noise" is true if the final image brightness is kept constant. By final image, I mean the jpeg produced by the camera.
Click to expand...
Decreasing exposure is the reason for the increase in the perceived noise. Compensating lightness by increasing ISO doesn't compensate the decrease of SNR that is caused by the decreased exposure.
 
Iliah Borg said:
Tom Axford said:
The full statement I wrote was:

"Increasing ISO increases the noise" is true if the final image brightness is kept constant. By final image, I mean the jpeg produced by the camera.
Click to expand...
Decreasing exposure is the reason for the increase in the perceived noise. Compensating lightness by increasing ISO doesn't compensate the decrease of SNR that is caused by the decreased exposure.
Click to expand...
 
confused circle said:
Iliah Borg said:
Tom Axford said:
The full statement I wrote was:

"Increasing ISO increases the noise" is true if the final image brightness is kept constant. By final image, I mean the jpeg produced by the camera.
Click to expand...
Decreasing exposure is the reason for the increase in the perceived noise. Compensating lightness by increasing ISO doesn't compensate the decrease of SNR that is caused by the decreased exposure.
Click to expand...
Yeah, but the effects of shot and read noise in the image changes as iso is increased. There can be an overall improvement in the image by raising iso if the electronics architecture of the camera will provide it.
Click to expand...
I know of only one camera where raising ISO by a stop decreases the read noise by so much that the decrease in exposure is very close to being fully compensated noise-wise, Canon 5D original, ISO 100 to ISO 200. Highlight clipping however remains to be one stop earlier at ISO 200 compared to ISO 100.
 
Iliah Borg said:
Tom Axford said:
The full statement I wrote was:

"Increasing ISO increases the noise" is true if the final image brightness is kept constant. By final image, I mean the jpeg produced by the camera.
Click to expand...
Decreasing exposure is the reason for the increase in the perceived noise. Compensating lightness by increasing ISO doesn't compensate the decrease of SNR that is caused by the decreased exposure.
Click to expand...
Agreed, but if the ISO is increased while the final image brightness is kept constant, then it follows that the exposure decreases and the noise increases. Mathematically, the noise is an increasing function of the ISO. This statement does not imply any physical cause and effect.

Mathematically, noise can be considered to be a function of a whole load of variables including shutter speed, f-number, ISO, subject brightness, camera characteristics, etc.

The final image brightness is another function of these various variables. It is perfectly possible to impose the constraint that the image brightness remains constant (which is what happens if you use one of the auto modes on your camera). That constraint is just as valid as the simpler constraint of saying that the exposure does not change.
 
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