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Can anyone tell me what the base ISO is of a Canon 5D MK III?

I was reading this great piece on ETTR (1) and (2) and it repeatedly mentions how important shooting at Base ISO is when using ETTR. I understand his reasoning, but I can't seem to find a dependable resource that tells me what my camera's native ISO is.

There is a lot of conjecture and answers ranging from 100 - 640.

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I have a Canon 5D3 and find that the blinkies and LV histogram, being based on a quickie JPEG conversion, are not at all accurate in detecting RAW saturation. Even with a histogram showing significant (say 1/2 stop) overexposure, there's still plenty of headroom to extract a well-balanced image from the RAW file. While ETTR is clearly the best way to maximize image quality, the techniques describe in the linked articles are a little simplistic. –  Jim Garrison May 3 at 2:33
    
I've definitely found the same regarding headroom. I regularly overexpose by two stops without clipping. I avoid the blinkies and as he mentions, the histogram displayed when viewing a captured image is more accurate. I'm interested that you call his explanation 'simplistic'. Especially the second page. Do you have an alternate source that you find more accurate? (This is meant in a genuine, not a snarky way). –  Pedr May 6 at 12:47
    
Not taken as snarky :-) I don't have a source, just observations and experience. It seems that the histogram and blinkies are derived from the 1-megapixel preview jpeg that is displayed on the rear screen, which makes for some interesting questions. At that size each display pixel contains 22 image pixels, so it would be useful to know how those 22 pixels are combined (average? maximum? something else?) to derive data points for the histogram. –  Jim Garrison May 6 at 18:58
    
@JimGarrison Thanks. So out of interest, what is your approach for achieving accurate ETTR? –  Pedr May 7 at 10:33
    
I don't really do anything special. Unless you're seriously underexposed the actual difference isn't going to be visible in a print and I just don't worry about it. It's all empirical and gut feel. –  Jim Garrison May 7 at 20:18

1 Answer 1

up vote 6 down vote accepted

The base ISO of all Canon cameras is ISO 100. This is the ISO with the lowest gain, without any in-camera magickry to achieve the setting (like ISO 50, which mucks with the actual exposure settings behind the scenes).

There is a lot of conjecture and misunderstanding about Canon's ISO settings because they use a "real/push/pull" model for ISO settings, rather than using native analog gain for all ISO settings. Canon only uses a standard analog gain (per-pixel amplification) for full stop ISO settings up to a certain high ISO level. So, ISO 100, 200, 400, ..., 3200 are usually achieved by using standard analog signal amplification. After a certain point, really high ISO settings in Canon cameras are achieved via a base analog amp., plus a secondary downstream (post-read/pre-ADC) amp, and possibly a digital boost.

For Canon cameras pre-5D III generation (i.e. 5D II, 7D, 50D, 550D, etc.) the highest directly amplified ISO settting is usually 1600, after which a combination of downstream amplification, push/pull, and possibly digital boost is used. For post-5D III generation (i.e. 5D III, 1D X, 6D, 70D, etc.) cameras, the highest directly amplified ISO setting is either 3200 or 6400, with the possibility that rebels still top out at ISO 1600.

Finally, third-stop ISO settings are achieved by using a 1/3rd stop "push" or "pull" using a downstream amplifier after read. ISO 125 is a third-stop "push" of ISO 100, where as ISO 160 is a third-stop "pull" of ISO 200. ISO 250 is a third-stop "push" of ISO 200, where as ISO 320 is a third-stop "pull" of ISO 400. This is why the 2/3rds stop ISO settings in Canon cameras are a little cleaner...the "pull" shoves the noise floor down a little bit.

The use of push/pull does not make ISO 160, 320, or 640 the "base" ISO...that is an internet myth, breed by conjecture, misunderstanding, and a simple lack of real knowledge of how Canon achieves the necessary signal boost for each ISO setting. All Canon cameras (with the exception of some of the earliest models) use ISO 100 as base ISO.

It should be noted that Base ISO is different than Unity ISO. Unity ISO is the ISO setting where gain is one, or in other words for every electron of charge, the ADC converts one DU (digital unit). It is pretty rare that Unity ISO is actually selectable, however there is usually a close third-stop setting to unity. What Unity ISO is depends on the camera, the sensor, the full-well capacity, and other factors (Roger Clark has lots of information on his site for popular Canon camera models, and within them he usually derives unity gain, if knowing it is important (some applications may benefit from unity gain).)


Regarding ETTR, and the use of Base ISO. That is a simple fallacy, really. ETTR simply means Expose to the Right. That's it. There should not be any additional caveats to that. ETTR is a very simple, strait forward rule, and it does not require that one always shoot at ISO 100 or base ISO. I am a Canon user, and rarely shoot at ISO 100 for most of my photography, which is birds and wildlife. Most of my photography is at least ISO 400, more often between 800 and 1600. I generally follow ETTR, regardless of the ISO setting.

ETTR offers a simple benefit: It puts more of the signal above the noise floor, which improves your editing latitude. ISO 100 is not, and should not be, a requirement for ETTR. Using ISO 100, assuming you have enough light, simply maximizes your dynamic range, within which ETTR is still shifting more of the signal above the noise floor. In Canon cameras, ISO 100, 200, and to some degree 400 have more read noise than all higher ISO settings. The higher read noise floor means you technically get more benefit from ETTR. If you are a landscape photographer, then ISO 100 is probably a given, but it is still not a requirement for ETTR.

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The way Canon uses "push" when figuring exposure and then "pull" when converting from RAW for ISO settings of 160, 320, 640, 1250, etc. is a form of ETTR in and of itself. You are, for example, using a sensor set at ISO 400 sensitivity but figuring exposure based on ISO 320 which means you just exposed 1/3 stop to the right and then pulled it 1/3 stop the other way when converting from RAW. Just as ETTR always does, you give up a little headroom in the highlight areas in exchange for more signal (less noise) in the shadows. –  Michael Clark May 3 at 1:03

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