# Real iso's?



## Valvebounce (May 28, 2013)

Hi guys, 
I have read before about intermediate iso's being interpolated from real iso's, but always thought that the last "real" iso was the one before the H setting. 

I have no knowledge of the 5D2 in the quote below, is 3200 the last iso before the H or are there more before the H?

Where does one find out about the real iso's for a particular camera? Is it just a case of my 7D has 6400, so that is the last real iso, or is that also lifted from 3200?

I hope this isn't a hanging offence, but this is a quote of dilbert from the post that raised the question.

Topic was "Why is my 5D III so noisy?"

[/quote]

It doesn't matter whether it is ISO 3200 or ISO 6400. Once the ISO is past the point where IQ drops more than 1 stop per ISO stop, increasing the ISO and then overexposing does not result in a better picture because you lose more than you gain by moving the ISO higher.

On the 5D2 the last real ISO is 3200 and everything over that is software underexposing and pulling up.

Whether it is the same on others...
[/quote]


----------



## neuroanatomist (May 28, 2013)

There are basically three 'types' of ISOs:


Base ISO - this is the 'real' ISO for the sensor before any amplificaition. For most sensors, this is actually in the ISO 60 to ISO 80 range, not ISO 100 as many people assume.
Native ISOs - analog amplification applied to the base signal, prior to analog to digital conversion (ADC). These values have numbers for selection (e.g. ISO 3200)
Expanded ISOs - digital amplification applied to the signal after the maximum analog amplification, occurring after the ADC. These valuse have letters for selection (L, H1, etc.).

You're talking about 'tweener' ISOs - those are seen in some cameras but not others. Some analog amplifiers (most of them, for Canon sensors, actually) are only capable of full stop incremental amplification, so digital amplification (pushing/pulling occurring after the ADC) is applied to the signal after the 'native' full-stop analog amplification. If you look at Bill Claff's data for Canon sensors, you can see the zig-zag curves that result from these 'tweener' ISO settings, with all their cameras except the 1D X.

So, dilbert is wrong about the 5DII's ISO settings - ISO 6400 is the highest 'real' (aka native) ISO. He may be confused because ISO 3200 is the highest available ISO when ISO is set to Auto. I have no idea what he means by stating, "_ Once the ISO is past the point where IQ drops more than 1 stop per ISO stop_" - what the heck is '1 stop of IQ'?!? Last time I checked, IQ was not measured in stops anywhere outside of dilbert-land (a fanciful place where lenses are sometimes cameras). But the reality is, noise and DR scale linearly in the 5DII from ISO 1600 all the way through H2, so the 'IQ difference' between ISO 1600 and ISO 3200 is the same as the difference between ISO 3200 and ISO 6400. 

Having said that, the highest ISO one will use is a personal judgement call. With the 5DII, ISO 3200 was my highest setting for non-emergency use (but that doesn't make ISO 6400 'not real' - it's still a native ISO setting).


----------



## Rienzphotoz (May 28, 2013)

interesting


----------



## privatebydesign (May 28, 2013)

I turned off 1/3rd stops for iso on my 1Ds MkIII's after reading this a few years ago. http://www.northlight-images.co.uk/article_pages/cameras/canon_1ds3_noise.html

It is easy, if a little involved, to test for yourself using Keith's methodology. Some would argue this is all getting a bit too anal, in general I would agree and many people will never notice the difference, but there are occasions where you are pushing and pulling your file to the absolute limits and that is when little things like this can make a difference. This kind of thing only ever makes a difference if you are shooting RAW and you are comfortable with post processing.


----------



## RGF (May 28, 2013)

Neuro

Great stuff. I learn something from your posts. I had heard about to avoid 1/3 stop ISO , but know know why. Do you have a reference for Nikon? Just wondering if they have similar behavior. Also what about point and shots?

Thanks


----------



## Pi (May 28, 2013)

privatebydesign said:


> I turned off 1/3rd stops for iso on my 1Ds MkIII's after reading this a few years ago. http://www.northlight-images.co.uk/article_pages/cameras/canon_1ds3_noise.html



I find this site to be misleading, at least. He tests for read noise (visible mainly in the deep shadows). It is true that "software ISO" gives you higher read noise than hardware one. The shot noise however depends on the total light, and it is not much affected by the ISO. In other words, if you are concerned with the deep shadows, ISO 200 is better than ISO 160. Shooting at ISO 200, in, say Av mode, would lower the total exposure by choosing a faster speed. As a result, the noise away from the deep shadows will increase. If you are going to keep the black level at moderate levels, you will not notice the increased shadow noise anyway. 

The best thing in that scenario is to shoot at ISO 200 and overexpose a little (keep the speed the same) assuming you do not blow the highlights.


----------



## privatebydesign (May 28, 2013)

Pi said:


> privatebydesign said:
> 
> 
> > I turned off 1/3rd stops for iso on my 1Ds MkIII's after reading this a few years ago. http://www.northlight-images.co.uk/article_pages/cameras/canon_1ds3_noise.html
> ...



Anybody pushing the limits of Canon RAW files is interested in deep shadows, everything else is easy! So your recommendation is full iso stops and expose to the right, isn't that basic knowledge and in agreement with the article?


----------



## sanj (May 28, 2013)

VERY interesting indeed. 
So Mr. Scientist sir (Neuro!), how do I find out the 'base' ISO for 1dx and 5d3. It would be great to know. You are the only one I know who can help.
Regards!
Sanjay


----------



## privatebydesign (May 28, 2013)

sanj said:


> VERY interesting indeed.
> So Mr. Scientist sir (Neuro!), how do I find out the 'base' ISO for 1dx and 5d3. It would be great to know. You are the only one I know who can help.
> Regards!
> Sanjay


1DX iso 251 
5D MkIII iso 159
both as per the Bill Claff link.


----------



## Pi (May 28, 2013)

privatebydesign said:


> Anybody pushing the limits of Canon RAW files is interested in deep shadows, everything else is easy! So your recommendation is full iso stops and expose to the right, isn't that basic knowledge and in agreement with the article?



Not so simple. If you go to ISO 400, instead of ISO 260 (which is ISO 200 pushed), you can blow the highlights with the same exposure to light. So you have to decrease the total light, and this will increase the noise everywhere instead in the shadow area.


----------



## privatebydesign (May 28, 2013)

Pi said:


> privatebydesign said:
> 
> 
> > Anybody pushing the limits of Canon RAW files is interested in deep shadows, everything else is easy! So your recommendation is full iso stops and expose to the right, isn't that basic knowledge and in agreement with the article?
> ...



Using full iso stops does not negate the need to not blow highlights! Show me two images files where your scenario makes an appreciable difference to the overall image quality. You might be making a valid technical point, but I believe I am making a valid user point, shadows are far and away the biggest problem for corrections with Canon RAW files, if there is a setting that elevates a problem from the shadows to anywhere else in the image, as an actual photographer processing RAW files everyday, I'll take it.

It is a shame that the Bill Claff site doesn't have data for my cameras.


----------



## Pi (May 28, 2013)

privatebydesign said:


> Pi said:
> 
> 
> > privatebydesign said:
> ...



I just pointed out the misleading nature of that page (the guy has much worse articles, BTW, like the one on diffraction). What is important to you is something I cannot comment on. 



> It is a shame that the Bill Claff site doesn't have data for my cameras.



Why do you trust that site so much? I prefer sensorgen. I have communicated with some of the authors, and I am fairly sure that they know what they are doing.


----------



## Pi (May 28, 2013)

Pi said:


> privatebydesign said:
> 
> 
> > Pi said:
> ...


----------



## privatebydesign (May 28, 2013)

Pi said:


> privatebydesign said:
> 
> 
> > It is a shame that the Bill Claff site doesn't have data for my cameras.
> ...



I don't trust any site to any substantive degree, but I like user interface direct comparisons, I can choose what information is displayed but must assume that his measurements, even if they are not "accurate", are consistent.

As for Kieth's site, I like it (and he is a member here) because he is an actual working pro with an above average interest in the technical side which he not only demonstrates, but he always lays out his methodology so you can repeat his results of you want to. He also addresses technical ideas and solutions from the perspective of a working pro, he is happy to say things like 'this might be a better way but you'll never see the difference', and that, to me, makes a lot more sense than the Lloyd Chambers style of analyze to death regardless of actual real world differences.

YMMV.

P.S. With regards sensorgen, anybody that measures an output 14 bit file as having 14.0 stops of DR is showing signs of method failure, add in the fact that they take most of their actual measurements off the DxO site and they have lost me.


----------



## bdunbar79 (May 28, 2013)

Can I get some credit here?? In the post that the OP is speaking of, I DID tell dilbert he was wrong!


----------



## privatebydesign (May 28, 2013)

bdunbar79 said:


> Can I get some credit here?? In the post that the OP is speaking of, I DID tell dilbert he was wrong!



We give you credit 

Now go and fix that 400 mount


----------



## MxM (May 28, 2013)

I did an ISO test with my 7D.

Put the camera in a dark room (basement) leave the lenscap on, close/block the viewfinder, use RAW or disable ISO long exposure/noise reduction. Choose the settings that you want (Manual mode). Take every picture for at least 10 seconds and you will come up with al list like me.







The 7D is an 18MP camera which is equal to approx. 18MB file size. (the smaller the file size, the cleaner the image is)

As you can see is ISO 160, 320, 640 the camera's sweet-spot... The golden rule is that the limit is MP/MB+1(file size) so 19MB is in the safe/clean zone and 20MB is pushing your camera. It's no math, just hard numbers.

MxM


----------



## Rocky (May 28, 2013)

Based on Bill Claff's data, The canon 30D has quite low readind noise. Any camera after that the read noise has been at least doubled up, includind the 1D-X. Why??? Is Canon is going backward on the read noise???


----------



## privatebydesign (May 28, 2013)

Rocky said:


> Based on Bill Claff's data, The canon 30D has quite low readind noise. Any camera after that the read noise has been at least doubled up, includind the 1D-X. Why??? Is Canon is going backward on the read noise???



It is because you are comparing 12 bit to 14 bit files.


----------



## Sporgon (May 28, 2013)

privatebydesign said:


> Rocky said:
> 
> 
> > Based on Bill Claff's data, The canon 30D has quite low readind noise. Any camera after that the read noise has been at least doubled up, includind the 1D-X. Why??? Is Canon is going backward on the read noise???
> ...




At Building Panoramics we shoot a lot of skys for our library and of all the Canon cameras we've used the 30D was by far the worst for noise, in fact worse than the 20D for some unknown reason. 

With regard to Neuro's link, this confirms what I have found in practice. I could never see this '160 ISO is less noisy than 100' statement that's bandied about, but I do find 50 gives noiseless data due to it's 'overexposure pulled back' value. 

Actually it was Privatebydesign's link. I can't find the practical link with Claff's data. So the 5D mk2 has the same read noise at ISO 640 as it does at 100 ? Yet shoot a sky at ISO 640a and it's full of noise whereas 100 is ( reasonably) clean. Likewise ISO 125 is similar to 100.

If you're exposing in total darkness with the lens cap on surely you are under exposing never mind how long you leave the shutter open. What about heat generated at these long 'exposures-that-aren't' ?

?


----------



## bdunbar79 (May 28, 2013)

privatebydesign said:


> bdunbar79 said:
> 
> 
> > Can I get some credit here?? In the post that the OP is speaking of, I DID tell dilbert he was wrong!
> ...



:'(


----------



## dppaskewitz (May 28, 2013)

Has anyone done any testing of the 6D? According to Claff's charts, the 5DII, 5DIII (and 30D, for that matter) seem to follow one pattern (less noise at 160, 320, etc.) while the 5D classic (my other body) follows a 100, 200, 400 pattern).


----------



## Click (May 28, 2013)

Very interesting thread. I'm following this closely. Thanks for the great info. 8)


----------



## RGF (May 28, 2013)

MxM said:


> IThe golden rule is that the limit is MP/MB+1(file size) so 19MB is in the safe/clean zone and 20MB is pushing your camera. It's no math, just hard numbers.
> 
> MxM



Can you explain the golden rule? Why is it true? What does it measure?


----------



## RGF (May 28, 2013)

Click said:


> Very interesting thread. I'm following this closely. Thanks for the great info. 8)



+100


----------



## Pi (May 28, 2013)

MxM said:


> As you can see is ISO 160, 320, 640 the camera's sweet-spot...



When you shoot with the cap on...


----------



## Sporgon (May 28, 2013)

MxM said:


> I did an ISO test with my 7D.
> 
> Put the camera in a dark room (basement) leave the lenscap on, close/block the viewfinder, use RAW or disable ISO long exposure/noise reduction. Choose the settings that you want (Manual mode). Take every picture for at least 10 seconds and you will come up with al list like me.
> 
> ...




But what practical data is established by exposing the sensor in the dark for ten seconds ? Surely you are chronically under exposing never mind how long you leave the shutter open. The sensor will start to generate heat.

I see your data matches the Bill Claff charts for the 7D. Looking at the 5D mk2 data it stated that the read noise is the same at ISO 640 as it is at 100. This will give people the impression that your data will show the same amount of noise at these exposures, but this just isn't the case. Shoot a sky at ISO 640 and it is very noisy. Shoot at ISO 100 and it's pretty clean: there is no comparison in data quality - so what's all this read noise meant to mean in practice ?

I think it is very misleading - unless of course you like to shoot inside of lens caps in the dark.


----------



## privatebydesign (May 28, 2013)

Pi and Sporgon, you are missing the point. 

The point is that the read noise displayed is added to a regular shot in lesser amounts at shorter exposures. You can argue the relevance, but when you are trying to pull that last bit of detail from a dark shadow that tiny extra amount of noise might make the difference.

It is all about setting yourself up to get the last 1 or 2 % of your sensors capabilities, this small difference is best illustrated with long dark exposures, but is just as valid for shorter brighter ones.


----------



## Pi (May 28, 2013)

privatebydesign said:


> Pi and Sporgon, you are missing the point.



No, you are just misreading my posts.


----------



## Sporgon (May 28, 2013)

privatebydesign said:


> Pi and Sporgon, you are missing the point.
> 
> The point is that the read noise displayed is added to a regular shot in lesser amounts at shorter exposures. You can argue the relevance, but when you are trying to pull that last bit of detail from a dark shadow that tiny extra amount of noise might make the difference.
> 
> It is all about setting yourself up to get the last 1 or 2 % of your sensors capabilities, this small difference is best illustrated with long dark exposures, but is just as valid for shorter brighter ones.



Well I'm going to have a look at this tomorrow. By the nature of what I shoot I'm nearly always on ISO 100, so don't have many files I can use as a comparison, but at the moment I believe that ISO 100 is _much_ better than 640 in every way, quality and post process -wise.


----------



## Marsu42 (May 28, 2013)

privatebydesign said:


> So your recommendation is full iso stops and expose to the right, isn't that basic knowledge and in agreement with the article?



This isn't so, the Magic Lantern devs have figured it out, see this good read about what iso is best (see q&a section): http://magiclantern.wikia.com/wiki/ISO



> Then, what is the best ISO?
> 
> To the best of my knowledge, the best ISOs are the ones available in recent Magic Lantern versions (April 2012 or later), obtained from ISO 100 multiples adjusted with a small amount of negative digital gain:
> 
> ...


----------



## Zen (May 29, 2013)

neuroanatomist said:


> There are basically three 'types' of ISOs:
> 
> 
> Base ISO - this is the 'real' ISO for the sensor before any amplificaition. For most sensors, this is actually in the ISO 60 to ISO 80 range, not ISO 100 as many people assume.
> ...



How in the world do you guys remember all this stuff, or even better, how do you take advantage of it when shooting your images? I'm completely lost when it comes to this highly technical data. Does it actually make a difference in your images?

Guess I'm a real boob! Just can't seem to get my arms around it all . . .

Thanks for trying to pound it into my thick skull just the same.

Zen ???


----------



## Valvebounce (May 29, 2013)

WOW
Thanks guys for all the answers, I am going to confess that I am still unclear as to where my native (real to use my incorrect description) iso's run out and when they become expanded, is it in theory the one before H, or is it in multiples of the base iso, 60 to 80 which I also don't know for the 7D! Or is it still best to work in multiples of the lowest selectable iso of 100 (no L setting on 7D)?
Thanks to Neuro for his in depth answer, perhaps I should have asked for the simplified version, might make more sense when less tired and stressed!
Really enjoy reading this forum, not sure how much is going to stick.
So much info so little ram left I wonder what just got over written! 

Cheers, Graham.


----------



## privatebydesign (May 29, 2013)

Valvebounce said:


> WOW
> Thanks guys for all the answers, I am going to confess that I am still unclear as to where my native (real to use my incorrect description) iso's run out and when they become expanded, is it in theory the one before H, or is it in multiples of the base iso, 60 to 80 which I also don't know for the 7D! Or is it still best to work in multiples of the lowest selectable iso of 100 (no L setting on 7D)?
> Thanks to Neuro for his in depth answer, perhaps I should have asked for the simplified version, might make more sense when less tired and stressed!
> Really enjoy reading this forum, not sure how much is going to stick.
> ...



The 7D's best iso level, the one with least noise, is 160iso; 100 and 200 are a touch noisier, then 318 and 126, then 251 and then they all get progressively more noisy.

So for shooting photos the "best" iso would be 160, as you needed more go up 1/3rd to 200, skip 251 (2/3rds up), after that just keep it as low as is practical. Although these are odd numbers they are the actual iso values registered when the camera is adjusted in 1/3rd stops of iso.

All this will only make the absolute tiniest of differences to dark areas of shadow detail and even then only if you shoot RAW.


----------



## LetTheRightLensIn (May 29, 2013)

MxM said:


> I did an ISO test with my 7D.
> 
> Put the camera in a dark room (basement) leave the lenscap on, close/block the viewfinder, use RAW or disable ISO long exposure/noise reduction. Choose the settings that you want (Manual mode). Take every picture for at least 10 seconds and you will come up with al list like me.
> 
> ...



Don't forget that the clipping point also shifts though. ISO160 is nothing more than over-exposing ISO200 by 1/2rd stop (in RAW). So for RAW it really doesn't matter. For JPG and movies it can matter though because of how they apply the tone curves.


----------



## Pi (May 29, 2013)

privatebydesign said:


> The 7D's best iso level, the one with least noise, is 160iso; 100 and 200 are a touch noisier, then 318 and 126, then 251 and then they all get progressively more noisy.
> 
> So for shooting photos the "best" iso would be 160, as you needed more go up 1/3rd to 200[...]



This is wrong. ISO 160 is pushed ISO 200 by 1/3 stop (ETTR). You can push ISO 100 to ISO 80, if you want. ISO 100 and ISO 200 have the same shadow noise but away from the shadows, ISO gets better and better, up to 2.6 dB better at the top. Nowhere in the range is ISO 200 better. 

The illusion of the "better" ISO 160 comes from the hidden overexposure by 1/3 stop. That does not change the DR; in fact, the DR at ISO 200 (or 160) is slightly worse than at ISO 100.


----------



## privatebydesign (May 29, 2013)

Pi said:


> privatebydesign said:
> 
> 
> > The 7D's best iso level, the one with least noise, is 160iso; 100 and 200 are a touch noisier, then 318 and 126, then 251 and then they all get progressively more noisy.
> ...



Well if you take empirical measurements, or read the page that was linked to http://home.comcast.net/~NikonD70/Charts/RN_ADU.htm you would see that the 7D has the lowest read noise at iso 159 (160).

If you look further on the site you will find this page http://home.comcast.net/~NikonD70/Charts/PDR.htm this measures DR to be the same at 100 and 200 iso, well within 0.01 of a stop.


----------



## Pi (May 29, 2013)

privatebydesign said:


> Pi said:
> 
> 
> > privatebydesign said:
> ...


But higher shot noise above the shadows, per DXOmark. This makes the statement that ISO 160 is better incorrect. It has to be better everywhere, to be declared better.

And of course, you can just dial in EV=+1/3 at ISO 100 and beat ISO 160 everywhere (DXOmark). With FF bodies, you can do even better: ISO 50. 



> If you look further on the site you will find this page http://home.comcast.net/~NikonD70/Charts/PDR.htm this measures DR to be the same at 100 and 200 iso, well within 0.01 of a stop.



I said "slightly worse" (DXOmark). But getting the same DR does not make it better. The shot noise is still worse.


----------



## wickidwombat (May 29, 2013)

Anyone got any idea how many shots you miss while agonising over this? ???

seriously I dont think it makes much of a difference


----------



## privatebydesign (May 29, 2013)

> This makes the statement that ISO 160 is better incorrect. It has to be better everywhere, to be declared better.



No it doesn't and, no it doesn't.

Look we are talking about tiny differences in the ability to process a RAW file, an actual picture. Specifically with a 7D shadow detail is the problem, it is so easy to adjust highlights and midtones 1/3rd stop either way with no loss of IQ, but where we struggle is getting that 1/3 stop extra in the shadows. If using 160 instead of 200 gets you less shot noise and no reduction in DR then surely that is a no brainer?

Better can mean many things, in my experience anything you can do with Canon RAW files to lessen problems in the shadows makes for better images.


----------



## privatebydesign (May 29, 2013)

wickidwombat said:


> Anyone got any idea how many shots you miss while agonising over this? ???
> 
> seriously I dont think it makes much of a difference



It doesn't make much difference. I have never agonised over it and I have never missed a shot because of it, I set my camera to only do full stops of iso because that is where empirical tests showed it had less noise. Set once and done, never touched it again.


----------



## neuroanatomist (May 29, 2013)

privatebydesign said:


> I set my camera to only do full stops of iso because that is where empirical tests showed it had less noise. Set once and done, never touched it again.



I also set my camera for only full ISO stops...while I certainly appreciate lower noise, honestly the reason is that having selection restricted to full stops means it's a lot faster to ramp the ISO up and down as needed, taking 67% fewer clicks of main dial rotation.


----------



## privatebydesign (May 29, 2013)

neuroanatomist said:


> privatebydesign said:
> 
> 
> > I set my camera to only do full stops of iso because that is where empirical tests showed it had less noise. Set once and done, never touched it again.
> ...



Another good point 

Although your 1DX has a much wider iso range than my cameras so it would take a lot longer........ heck I could do individual iso points and still have less clicks than you with 1/3 stops, I top out at 1600 but never go above 800!


----------



## Pi (May 29, 2013)

privatebydesign said:


> > This makes the statement that ISO 160 is better incorrect. It has to be better everywhere, to be declared better.
> 
> 
> 
> ...



Wait, you were talking about ISO 100 vs. ISO 160, and now it is ISO 200 vs. ISO 160?

What the camera is doing with ISO 160 is ETTR with ISO 200. Why do you need a fake ISO for that? Isn't it simpler just to dial in EV=+1/3?

The best ISO is ISO 50 or ISO 80, whatever does not blow the highlights. It is called ETTR. It does not matter if you camera "has it". It is a simple thing to do.


----------



## Sporgon (May 29, 2013)

Well I have had a look at this Claff data in practice, and as I suspected it is taking the sensors response to a particular test, and then applying the results across the board. ( To be fair to Claff he seems to be just stating the fact of read noise vs ISO - it's others that are drawing 'practical" conclusions form the data).

In terms of the 5Dmk2, ISO 160 does not give the highest image quality. When under exposed in a 16 bit tiff file it is true that blacks and greys show smoother data when pushed from ISO 160, with a fraction less luminance noise. However the chroma noise in the colour data is worse than ISO 100, as is everything else. Don't even mention ISO 640.

Classic case of producing data from an empirical experiment and then other observers drawing erroneous conclusions from it.


----------



## mememe (May 29, 2013)

To the iso darkroom tester:

Dont do this test with a lens that is faster than f2.8 (or not stopped down) cause Canon does strange stuff to get the brightness level properly under 2.8 (cause film collects the light which comes in at certain angles better than sensor)


----------



## Sella174 (May 29, 2013)

neuroanatomist said:


> If you look at Bill Claff's data for Canon sensors ...



Hey, my 30D has at ISO160 the lowest noise of all the Canon cameras, apart from some 1D-series at lower ISO. Unbelievable!


----------



## Sporgon (May 29, 2013)

Sella174 said:


> neuroanatomist said:
> 
> 
> > If you look at Bill Claff's data for Canon sensors ...
> ...



And yet when we used one at Building Panoramics to shoot skys for our library the 30D produced more noisy images than any other Canon camera we have used, including the 20D. 

This read noise / ISO data is being taken out of practical context.


----------



## Pi (May 29, 2013)

Sella174 said:


> neuroanatomist said:
> 
> 
> > If you look at Bill Claff's data for Canon sensors ...
> ...



_These raw values are not appropriate for comparing camera models because they are not adjusted for gain or area._


----------



## dougkerr (May 29, 2013)

Just a little background here.

"ISO" is an intialism that stands for an international standards body.

*There is no property of photographic film or of a digital imaging chain whose actual designation is "ISO".
*
The ISO publishes various standards that give ways of stating what we might call the "sensitivity" of a photographic film or of a digital imaging chain.

With regard to film, the basic sensitivity metric, determined as prescribed by the appropriate ISO standard, is called the I_SO speed_. The name of course comes from the fact that with film of a higher sensitivity, an appropriate exposure (for given scene luminance and given aperture) is given by a shorter exposure - a "faster" exposure process.

To provide continuity with practice in the film realm, the basic metric for the sensitivity of a digital imaging chain is also called the _ISO speed_. (It is of course determined in a wholly different way.)

Putting aside some complications, the basic "equation" we use (for example, in an exposure meter or automatic exposure control system) to arrive at a "recommended" photographic exposure (that is, combination of exposure time and aperture) for a given scene luminance, taking into account the sensitivity of the film or digital imaging chain (as its _ISO speed_) is the same for either medium.

But, especially taking into account the greater sophistication of modern exposure metering and control systems, it was observed that, in the case of digital cameras, this equation typically led to a lesser exposure than was "optimal" - typically about 1/2 stop "short". This gives a less-good noise result than we might actually enjoy.

Camera manufactures could have "tweaked" the operation of their exposure control systems to "take advantage" of this in two ways:

a. Change the "equation" used for automatic exposure control.

b. Keep the equation the same but "rate" the sensor system at a lesser _ISO speed_ than would be determined by the ISO test method.

Had they done (a), the result would have been that the internal automatic exposure system would have produced a different exposure for a given shot than would have been "recommended" by a properly-calibrated free-standing exposure meter. This would have led to complaints about "inaccuracy" of the exposure control system of the camera.

So they did (b) instead.

But of course this led knowledgeable enthusiasts to complain that the "_ISO speed _ratings" of the digital camera were incorrect.

So the ISO defined a new metric for the sensitivity of a digital camera chain, the _ISO standard output sensitivity_ (_ISO SOS_). This is a different measure than the ISO speed. In fact, again putting aside some complications, for a given sensor chain, the _ISO SOS_ is about "1/2 stop" less than the _ISO speed_. (That is, the _ISO SOS_ is about 0.7 times the _ISO speed_.)

And that "solved" the problem!

Today, the sensitivity of many digital cameras is stated in terms of the _ISO SOS_, not the _ISO speed_ (and that often is stated in the "specifications", although it is sometimes hard to find).

Now for a second chapter of this story, i must refer to the matter of _exposure index_. For our purposes we can say that exposure index is "*what we tell the exposure meter *is the ISO speed of the film of digital camera chain".

Going back to traditional analog exposure meters, we recall that if we want to bump the exposure the meter recommends for some reason (compensate for backlighting, perhaps), we can set the "ISO speed" dial to a lower value than the actual _ISO speed _of the film or digital sensor. And of course, what we set on that little dial is the exposure index (and, as I just illustrated, it is not always the ISO speed of the film or digital sensor chain).

Now, returning to recent developments in the description of digital camera sensitivity, when the ISO introduced the ISO SOS, it also introduced another new "metric", the _ISO Recommended Exposure Index_ (ISO REI).

This is a metric that works the same way as the ISO speed or ISO SOS (as a parameter of the exposure equation). It is defined as (and I paraphrase) "the exposure index that the camera manufacturer feels will, as an input to the exposure equation, give a 'desirable' exposure result in many cases."

In many case, digital camera manufacturers now indicate that the sensitivity "ratings" of their camera (at its various "ISO" settings) are in terms of ISO REI. That is, there is not any way that these values can be "correct" of "incorrect".

As a practical matter, in most of these cases, the "ISO" ratings are actually intended to be the ISO SOS values (and often turn out to closely conform to that definition), but the manufacturers do not care to commit to that.

A more extensive discussion of this matter is given here:

http://dougkerr.net/pumpkin/articles/SOS_REI.pdf

Best regards,

Doug


----------

