What does ISO setting actually do? Michael?

[samirkharusi]

Perhaps a useful future article on LL. I have yet to find a satisfactory explanation of what the ISO setting does in a DSLR. I am therefore requesting Michael (or anyone else) to talk to his Dr Know or somebody who actually KNOWS what is going on so that we can all get an authoritative briefing on what that ISO setting actually does. Let me explain why I find the topic rather muddy:

Astro CCDs normally have only one speed or ISO. Many output 16 bits. If you have each photosite have a fill of 65,000 electrons (or less) then your analog to digital digitiser has ample quantification "resolution" with 16 bits, 1 to 65536. It is rumoured that DSLR sensors also have only one fixed sensitivity. I do not know whether this is true or false. Assuming it is true then it is further speculated that if your output is only 12 bits (1 to 4096) then obviously you want to spread the most important part of your image signal to within 12 bits. Presto! Low signal/high ISO, use the 12 bits to examine the region closest to nil electrons. High signal/low ISO, use the 12 bits to examine the upper end of the electron well fill. I.e. the ISO button changes the information flow window only, not "sensitivity". I am not totally convinced that this is true, because no matter how you post process, it seems impossible to take an image at, say, 1/1000sec ISO 100 (16x under-exposed) and make it even approach an image with the same exposure (this time the "correct" exposure) at ISO 1600. I would have expected that if it were only a matter of data flow then one can achieve a reasonably close result in post processing, with some extra noise naturally. Corollary to all this? When DSLRs get 16-bit A/D converters (like some MF backs already?) then there will be no need for any ISO setting at all (like for astro CCDs). And Michael will no longer have to quibble about the placing of the ISO dial/display on future DSLRs. All the discussions I have come across to date are conjecture and guessing with some reverse engineering. Somebody somewhere must know what is going on. Anybody?

[mahleu]

I always assumed that at higher ISOs the camera put more current through the CCD making it more sensitive but at the trade-off of noise.

[Graeme Nattress]

I think ISO is made up of two components - AtoD gain, and software gain.

Graeme

[madmanchan]

Adjusting the ISO simply adjusts a voltage gain applied to the captured analog signal before it is digitized (and therefore quantized) by the A-to-D converter. In other words, the gain is applied in the analog domain at a very early step of image processing. This is an important technical difference from applying an exposure boost later on in the RAW conversion software.

Of course, any unwanted parts of the signal (e.g., noise) will also get boosted by the voltage gain.

Eric

[BJL]

Adjusting the ISO simply adjusts a voltage gain applied to the captured analog signal before it is digitized (and therefore quantized) by the A-to-D converter. In other words, the gain is applied in the analog domain at a very early step of image processing. This is an important technical difference from applying an exposure boost later on in the RAW conversion software.
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Mostly right, but I believe that some of the higher ISO settings on DSLR's go beyond the limits of this analog gain, and are done in the digital stage, reproducing what can be done later in raw conversion or post-processing. This is perhaps the case with ISO speed settings distinguished as "boost" or "HI", the ones above about 800 or 1600 in many DSLR's.

Indeed, I have read that in raw files from the Olympus E-1, there is no difference between ISO 800, 1600 and 3200 when one uses equal shutter speed and aperture, indicating that the latter are "digital pushes" of ISO 800 sensor and A/D convertor output. (1600 and 3200 are the optional "boost" settings on the E-1, disabled by default.)

[madmanchan]

Mostly right, but I believe that some of the higher ISO settings on DSLR's go beyond the limits of this analog gain, and are done in the digital stage, reproducing what can be done later in raw conversion or post-processing. This is perhaps the case with ISO speed settings distinguished as "boost" or "HI", the ones above about 800 or 1600 in many DSLR's.

Yes, I forgot to mention that in most of the current DSLRs, there is different behavior for the extreme ISO modes that are usually turned off (or made inaccessible) by default. Examples include the "H" ISO mode for Canon DSLRs which is ~ISO 3200, and the "L" ISO mode which is ~ISO 50) behave differently.

Essentially, the "H" mode applies the same voltage gain as with ISO 1600, but after the signal has been digitized, the digital values are then doubled, thereby increasing by 1 stop. This means that the effective dynamic range is smaller by 1 stop in the shadows. Since this "doubling step" can be performed during RAW conversion, I see no advantage to using "H" mode for people who shoot RAW except perhaps a shorter workflow.

Conversely, the "L" mode applies the same gain as with ISO 100, but after the signal has been digitized, the digital values are then halved, thereby decreasing by 1 stop. This means that the effective dynamic range is smaller by 1 stop in the highlights.

Eric

[John Sheehy]

Corollary to all this? When DSLRs get 16-bit A/D converters (like some MF backs already?) then there will be no need for any ISO setting at all (like for astro CCDs). And Michael will no longer have to quibble about the placing of the ISO dial/display on future DSLRs.

Not having ISO settings means a big change in method.  How do you get a review image of the correct brightness?  Or JPEGs saved on the card?

Sure, it would be nice if there was exactly the same digitization for all "ISOs", but you still need to hold onto the concept for default rendering purposes, and also if you want to use something like Av-priority mode and instruct the camera to use a fast shutter speed (as in setting a higher ISO).

All the discussions I have come across to date are conjecture and guessing with some reverse engineering. Somebody somewhere must know what is going on. Anybody?
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You're not going to get the kind of response you want.  Reverse engineering is the only way you can figure this stuff out.  Companies do not disclose their methods in any kind of detail.

When ISO 400 has only 18% more readout noise than ISO 100, then you know that they received unique amplifications.  When ISO 3200 is all even numbers in the RAW data, and the noise is exactly the same as an ISO 1600 image doubled, you know that some simple math is used for 3200, and it has 1600-level amplification.  When every 5th value is missing in a RAW histogram of an ISO 125 image, and the readout noise is exactly 1.25x as high, you know that the camera metered for ISO 125, amplified for 100, and multiplied the RAW data by 5/4, clipping away 1/5 of the linear dynamic range.

Canon's spokesperson has already said that they don't disclose such details about their cameras (a good way to avoid critical discussion).  I would doubt that any of the other companies are willing to share their "secrets", either.  Even if they made a statement, it may be incorrect.  Reverse engineering is the only way to know these things.

[BJL]

Astro CCDs normally have only one speed or ISO. Many output 16 bits.
... Corollary to all this? When DSLRs get 16-bit A/D converters (like some MF backs already?) then there will be no need for any ISO setting at all (like for astro CCDs).
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The pre-amplifcation gain helps to reduce the effect of any noise introduced after that stage up to and including in A/D conversion. With some CMOS sensors, I believe that the pre-amplification is done very early, at the photosite itself, and so there might be distinct advantages in terms of reducing the effects of noise arising between the photosite and A/D conversion.

Doe anyone know the relative magnitude of the various noise sources that contribute to "read-out" noise including analog noise in the A/D convertor?

At what frame rates are astro CCD's read out? Higher read-out rates, as needed for standard digital cameras, increase read-out noise. Asrto CCD's might have negligible read noise. In fact I have read of other scientific CCD setups where read noise is less than one electron RMS. In that scenario, variable pre-amp gain (variable ISO speed) is probably irrelevant.

[John Sheehy]

In fact I have read of other scientific CCD setups where read noise is less than one electron RMS. In that scenario, variable pre-amp gain (variable ISO speed) is probably irrelevant.
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One electron's worth of read noise from 65K wells with 16-bit digitization?  I'll take it on my next DSLR.  Can I get 5fps?

[dlashier]

One electron's worth of read noise from 65K wells with 16-bit digitization?  I'll take it on my next DSLR.
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Just pack your camera in dry ice

- DL

[bjanes]

Does anyone know the relative magnitude of the various noise sources that contribute to "read-out" noise including analog noise in the A/D convertor?
At what frame rates are astro CCD's read out? Higher read-out rates, as needed for standard digital cameras, increase read-out noise. Asrto CCD's might have negligible read noise. In fact I have read of other scientific CCD setups where read noise is less than one electron RMS. In that scenario, variable pre-amp gain (variable ISO speed) is probably irrelevant.
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Yes, higher read-out rate increase noise. I do know that astronomical CCDs are read out at low rates and this is one reason why they must be cooled. Otherwise dark noise at the low readout rates would degrade the noise unacceptably during the long time interval of the read process.

To get faster frame rates I believe the Nikon D200 has 4 readout channels, each reading its own portion of the sensor. If these are not balanced properly this leads to banding and you have to send in the camera for adjustment.

This article explains some of the physics:

[a href=\"http://www.mssl.ucl.ac.uk/www_detector/optheory/darkcurrent.html#Readout%20noise]http://www.mssl.ucl.ac.uk/www_detector/opt...Readout%20noise[/url]

[samirkharusi]

Not a reply to anybody in particular, but simply some extra info. A Canon 20D has been measured to have about half the Read Noise of many astroCCDs, despite slower reading in the astroCCDs. A 20D also seems to have its lowest Read Noise at ISO 1600, slightly lower compared to other ISO settings. Nobody is more obsessive regarding noise than astrophotographers, and most have come to a consensus that it does not matter much what ISO you use. An hour integration time, made of, say, 12x5minute exposures, will end up with the same Signal to Noise Ratio in the final processed image regardless as to whether it was shot at ISO 1600, 800, 400 or 200 (keeping away from the extreme ISO settings). Previewing an image when we have no ISO settings at all is not a major issue. Just an auto-stretch (contrast/brightness) setting in the camera's firmware will give a reasonable view of what was captured. All astroCCD software that I have used offers this facility.

We need Dr Know...

[John Sheehy]

Not a reply to anybody in particular, but simply some extra info. A Canon 20D has been measured to have about half the Read Noise of many astroCCDs, despite slower reading in the astroCCDs. A 20D also seems to have its lowest Read Noise at ISO 1600, slightly lower compared to other ISO settings.

ISO 1600 has the lowest readout noise in units of electrons, or relative to absolute exposure.  The difference is not small, however, with other ISOs.  The absolute read noise is 7x as high at ISO 100, 3.7x at ISO 200, 2.1x at ISO 400, 1.4x at ISO 800.

Nobody is more obsessive regarding noise than astrophotographers, and most have come to a consensus that it does not matter much what ISO you use. An hour integration time, made of, say, 12x5minute exposures, will end up with the same Signal to Noise Ratio in the final processed image regardless as to whether it was shot at ISO 1600, 800, 400 or 200 (keeping away from the extreme ISO settings).

That contradicts what was already discussed, at least as far as the Canon is concerned.  Your statement is kind of vague, though.  Parts of the recipe are missing.

We need Dr Know...
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I still don't understand what you think is such a mystery.

[samirkharusi]

ISO 1600 has the lowest readout noise in units of electrons, or relative to absolute exposure.  The difference is not small, however, with other ISOs.  The absolute read noise is 7x as high at ISO 100, 3.7x at ISO 200, 2.1x at ISO 400, 1.4x at ISO 800.
That contradicts what was already discussed, at least as far as the Canon is concerned.  Your statement is kind of vague, though.  Parts of the recipe are missing.
I still don't understand what you think is such a mystery.
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In astrophototography one has to separate the noises one can do something about from those beyond the user's control. Read-Noise in a 20D is smallest at ISO 1600, but the noise in a final, fully processed image, usually made up of a large stack of frames, depends on many other factors, the major one being photon-statistical (or quantum statistical) noise in the skyfog (light pollution) itself. There is also thermal noise, etc. Current understanding is such that one can actually put up equations that desribe much of this stuff very satisfactorily, except that ISO setting (!), because most of our understanding as to what it does is guesswork. Hence we need Dr Know to say what the basic principles in setting ISO are, e.g. on-chip, off-chip, analog domain or after A/D conversion, prior to reduction to 12-bits or after, etc. I expect that all camera manufacturers use the same basic principles and then tweak here and there. The tweaks would presumably be the proprietary areas, the principles probably common knowledge in the industry. There is no ISO term we can confidently put into these equations to optimise the SNR in the final image. There is also no agreed test procedure/protocol (e.g. like for measuring Read-Noise) that leads to an answer like "use ISO 400 for this much skyfog and ISO 1600 for that much skyfog. Here is a simplistic note I have written up on the minimal length of sub-exposures in a stack:
[a href=\"http://www.samirkharusi.net/sub-exposures.html]http://www.samirkharusi.net/sub-exposures.html[/url]
and here are some of the equations I was alluding to (for astroCCDs and hence no mention of ISO):
http://www.starrywonders.com/snr.html
For the daytime shooter I think it would be of great interest to learn if ISO setting would be going the way of the dodo in a couple of years (when 16-bit A/D converters come on stream) or not. Personally I find it all very intriguing, much more than pixel peeping (which I also do occasionally).

[jani]

For the daytime shooter I think it would be of great interest to learn if ISO setting would be going the way of the dodo in a couple of years (when 16-bit A/D converters come on stream) or not. Personally I find it all very intriguing, much more than pixel peeping (which I also do occasionally).

Considering that certain digital back manufacturers claim 16-bit A/D, and still offer several ISO settings, it doesn't appear to be going the way of the dodo because of that.

And the ISO settings are useful for other things related to exposure, such as achieving a longer or shorter shutter time without changing DoF.

But perhaps that setting will go away with the dodo, if ISO 50 through 6400 are equivalent in terms of final noise; then I could just set the exposure in terms of shutter speed and f-stop, and adjust the exposure compensation afterwards to get the "right" exposure.

It's a bit dizzying to think of what photography would be like without choosing a "film speed", so maybe I'm way off the mark here.

[John Sheehy]

It's a bit dizzying to think of what photography would be like without choosing a "film speed", so maybe I'm way off the mark here.
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My vision of the Camera of the Future is one where you choose Av and Tv values in manual mode, and the feedback the camera gives is a logarithmic histogram based on high-res metering, with some kind of flickering or speckling of the lower ranges so that you recognize when S/N is being compromised, so you can let more light in if you don't need the high f-stop or fast shutter speed.  I couldn't say "good-bye" fast enough to discreet one-stop ISOs with high read-out noise.

For auto-exposure, user-programmable EV to AV/Tv mappings based on the lens mounted would be used.

[John Sheehy]

It's a bit dizzying to think of what photography would be like without choosing a "film speed", so maybe I'm way off the mark here.
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I've imagined it so many times, I'm ready *now*.  All I need is a high-res metering that gives a logarithmic histogram in the edge of the viewfinder to give me an idea of S/N, and I'm set to go.

[BJL]

A 20D also seems to have its lowest Read Noise at ISO 1600, slightly lower compared to other ISO settings.
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Does this mean that by applying that extra amplification (to "ISO 1600") early, right at the photo-sites, the relative impact of subsequent noise in the read-out and A/D conversion is minimized?

Naively, this makes sense, and suggests something like Samir's strategy, or at least my understanding of it, which is as follows.
1) amplify a lot and early, say to "ISO 1600", to minimize the effect of noise introduced later in the process (read-out, A/D).
2) use an A/D converter than can handle the resulting high maximum signal strengths, for example in situations where one is using "exposure index ISO 100", and thus amplifying to a level of four stops of "over-exposure". That is, add four bits or so of highlight headroom to the A/D convertor. So yes, go to 16 bits A/D or even more, in that direction.

Then selected ISO speed affects shutter speed/aperture choice in AE modes, but goes into the raw file as a mere suggestion for default raw conversion, like WB, sharpening, and such. With manual shutter speed and aperture setting, the light meter reading could be used to record a "suggested" ISO speed in the raw file.


My question is how large a signal-to-noise ratio an A/D convertor for a portable camera can have these days, or in the foreseeable future. That limits its useful bit range, regardless of what the spec. sheets say. No point the A/D being able to handle very high input levels if the noise added by the A/D convertor is much more that 1/65536=1/2^16 of that maximum signal, because then the least significant of those 16 bits are all A/D noise, no signal, and "16-bit" becomes purely a paper spec.

Aside: with current SLR sensors, the only possible benefit of 16 is covering up to four stops of exposure level error, which is related to what I (and Samir?) propose as a deliberate strategy. With correct exposure, abut 12 bits is enough to record the S/N ratio of any current SLR sensor itself, and trends at the high end (Kodak and Dalsa) seem to be holding the line or even dropping S/N ratio for the sake of increased pixel count. (Leaving the options of binning or down-sampling when the user prefers extremes of high DR at less high resolution.)

[bjanes]

Hence we need Dr Know to say what the basic principles in setting ISO are, e.g. on-chip, off-chip, analog domain or after A/D conversion, prior to reduction to 12-bits or after, etc. I expect that all camera manufacturers use the same basic principles and then tweak here and there

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I too wish that Dr. Know would enter into the discussion and enlighten us, but in the meantime, here are a couple of posts by Dr. Clark (an astrophysicist and avid photographer) that deal with read noise, dynamic range, and ISO.

He states that when unity gain is reached (the ISO where one photo-electron results in one increment in the ADU [analog to digital unit or raw pixel value], there is no reason to increase the ISO on the camera any further since it will only decrease the dynamic range without  recording additional shadow detail. Unity ISO is shown for various cameras and ranges up to about 1600.


[a href=\"http://www.clarkvision.com/imagedetail/evaluation-1d2/index.html]http://www.clarkvision.com/imagedetail/eva...-1d2/index.html[/url]

http://www.clarkvision.com/imagedetail/doe...tter/index.html

[John Sheehy]

I too wish that Dr. Know would enter into the discussion and enlighten us, but in the meantime, here are a couple of posts by Dr. Clark (an astrophysicist and avid photographer) that deal with read noise, dynamic range, and ISO.

He states that when unity gain is reached (the ISO where one photo-electron results in one increment in the ADU [analog to digital unit or raw pixel value], there is no reason to increase the ISO on the camera any further since it will only decrease the dynamic range without  recording additional shadow detail. Unity ISO is shown for various cameras and ranges up to about 1600.
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I've never agreed with that conclusion of his.  He ignores the issue of 1-dimensional noise - line banding.  This is *the* demon of Canon high ISO shadows; it is much more visibly potent than random noise alone, in many images.  Banding is still greatly improving from ISO 800 to 1600 (relative to absolute signal) with recent Canons, even when random noise is starting to taper off, so just to have less banding, it seems worthwhile to go to ISO 3200.

He draws conclusions about ISO 3200 based on what the camera actually does; not on what real ISO 3200 amplification would be.  His camera just doubles the data that is digitized for ISO 1600 but metered for 3200 - and then draws conclusions based on the noise of ISO 3200, which doesn't really exist on his camera, the way ISOs 100 through 1600 do.

The idea that digitization ends at unity (1 ADU = 1 electron) is not logical to me at all.  I believe 3 or 4 ADUs per electron will have slightly less posterized data.  Don't forget, the ADC is not counting electrons, it is digitizing a buffered, amplified fascimile of the charges in the wells.  Oversampling the electrons allows more accurate counting, as the noise can't flip the result as far in either direction.

Here's the read noises of the 20D blackframe at various ISOs; the yellow line is the total noise (divided by 10 to fit on the graph with the other two lines).  ISO 100 is straightforward, and the other ISOs are divided by 2, 4, 8, 16, and 32 to show their strength relative to absolute exposure (values proportional to electrons, IOW):



As you can see, the values for the artificial ISO 3200 are abruptly flattened, more than the trend suggested by the other ISOs.  This is hardly true of the total read noise (which is the only thing he and most others measure), but very true of the blue line which traces horizontal banding noise, *the* demon of Canon high ISO shadows (despite being only 10% as strong as the total noise).  I believe that a gain-based ISO 3200, and maybe even 6400, would improve the pattern noise situation, despite the likelyhood that no significant gains in total noise may occur.