How is ISO determined?

[bigblue1ca]

Thanks Neuro that makes sense.

[Jon Ruyle]

If Canon wanted, they could release a camera today with ISO ten billion just by taking the image at base sensitivity (assume ISO 100) and ratcheting the exposure up +27 EV (give or take).

This would be somewhat problematic since Canon only has a 14 bit raw

[ChadS]

This would be somewhat problematic since Canon only has a 14 bit raw

The quantization of the voltage is independent of the amount of gain added.

[Daniel Browning]

[...] there is a actual ISO that dictates the standards.

There is enough leeway in the "standard" that manufacturers can pretty much make up whatever they want for the "ISO" -- and they do. The ISO standard did include a simple and obvious method that everyone should use, but no one does. Too bad, because it would have been nice to be able to go from one camera to the next without having to spend the time figuring out what the iso setting *really* means. Many manufacturers, like Canon, aren't even consistent across model lines in the same generation, let alone different generations and different manufacturers.

[Jon Ruyle]

The quantization of the voltage is independent of the amount of gain added.

Ideally, sure. But Canon currently achieves ISO above 1600 by pushing the raw.

[neuroanatomist]

Ideally, sure. But Canon currently achieves ISO above 1600 by pushing the raw.

Is that true for all cameras? I thought Canon defined 'native' ISOs as analog gain and 'expanded' ISO as digital gain ("pushing the RAW").

[Daniel Browning]

Is that true for all cameras?

It was true for all cameras until a few years ago.

I thought Canon defined 'native' ISOs as analog gain and 'expanded' ISO as digital gain ("pushing the RAW").

Not really. There is a strong correlation between native ISO and analog gain, but there are several exceptions. For example, on the 5D2, 6400 is a 'native' ISO, but it is based on digital gain (3200 is the highest analog-only gain).

Personally, it doesn't really matter what the highest analog gain is -- I'm more interested in the highest *useful* analog gain. For example, the D7000 has analog gain up to ISO 1600, but going above ISO 200 in raw has only downsides -- no benefits. (Same with going above 1600 on the 5D2 -- although there is a slight reduction in pattern noise, for me it's not usually worth the loss in highlight headroom.)

[Jon Ruyle]

Is that true for all cameras? I thought Canon defined 'native' ISOs as analog gain and 'expanded' ISO as digital gain ("pushing the RAW").

I don't know for sure that 1600 is the highest real iso on all of them, but a friend who has examined raw files for more than one canon camera (including the 5DII) tells me that for those he has looked at, everything above 1600 is achieved by pushing the raw. Thus it is definitely not the case that pushing the raw only happens with "expanded" iso settings.

[Jon Ruyle]

on the 5D2, 6400 is a 'native' ISO, but it is based on digital gain (3200 is the highest analog-only gain).

Really? I was told 1600 was the highest.

If 3200 is the highest analog-only gain, then what do you mean when you say 6400 is 'native'? Do you simply mean it is not an expanded iso?

[Daniel Browning]

If 3200 is the highest analog-only gain, then what do you mean when you say 6400 is 'native'? Do you simply mean it is not an expanded iso?

Yes, that's what I meant.