Readers who are bored by/can't stand technical issues, stop reading now!

From time to time I have come across internet discussions about so-called 'native' ISO settings for cameras.

It seems that some cameras (eg Canon?) may have sensors which have a base ISO of 100, and use analog amplification of the output of the sensor to produce the standard 'native' sequence of ISO values (200, 400, 800 etc). However, typically, modern DSLRs such as my Canon 80D also have 1/3 stop values between these figures eg ISO 160. These intermediate values are produced by subsequent changes to the signal output using digital processing, and are therefore not considered 'native' by some definitions.

However I also recall seeing articles that suggest that some cameras (eg those using Sony sensors?) have 'native' ISOs which are within the '160' sequence - ie 160, 320, 640 etc. For example, I recall reading articles some years ago now about early Sony DSLRs eg the original A900 which implied that ISO 320 was optimal in terms of signal to noise output (but I may be recalling this incorrectly).

The argument appears to be that it is better to use 'native' ISOs to achieve the best output from the sensor, and that settings between the native values either exhibit more noise or lower dynamic range. For example, some cameras allow one to use ISO 64 but this does not provide the same dynamic range as ISO 100 and is essentially a 'modified ISO 100'. From memory, my old Sony DSLRs did this and the manual explained that there may be some reduction of dynamic range.

For speed of operation, as much as anything else, I have set my camera to only use the ISO 100 scale, to minimise the number of clicks when manually adjusting ISO. However I have noticed that a number of BLP advanced users commonly use ISOs from the '160 scale' such as ISO 640 or 1280. I am interested to know whether there is any reason for this, or this is just what was needed to get the job done in the light conditions with the lowest possible ISO. Is there any advantage in using a particular ISO sequence, and avoiding others? I note in passing that this is probably further confused by the fact that 'ISOs ain't ISOs' and that different manufacturers measure ISO slightly differently.

I realise this is a bit obscure and very unlikely to impact on my day to day photography (and other variables, such as the person behind the camera, are far more important) but I am just interested to know on whether anyone knows if this is all just internet myth or whether there is any substance to it.

Simon

Hi Simon,

This is the sort of question that I would be asking Ian Wilson but I haven't seen him post for a while, he might be away again. . . so so lucky this bloke, always away on birding trips
I'm sure that when he logs in, this question will be right up his alley.
Just as an aside, I too have read that different sensors in different cameras have a preferred ISO setting. Whether this is true or media hype, it's out of my league I'm afraid. . .

Bruce

Hi Simon,
Good question. This effect is not folk lore but real for some sensors. I say some sensors because it depends on the particular amplifier circuitry used in the camera. Cameras with an additional amplifier to provide the gain required for intermediate (non-native ISO steps) have slightly more noise at these ISO values than at the so called 'native' ISO values. The effect of the extra amplifier stage is to slightly increase the noise and the corollary of this is that the dynamic range takes a slight hit. As an illustration, I have attached measurements, snipped from Bill Claff's website, for the Canon 80D, which if I remember right is your camera. In both the read noise and photographic dynamic range graphs you will notice some little wiggles; this is the result of the additional amplifier kicking in and out. Hope this helps,
Regards, Ian

Just to round this topic off, a couple of interesting articles I have found:

The Riddle of Intermediate ISO settings:
https://photographylife.com/riddle-intermediate-iso-settings

ISO Invariance explained:
https://photographylife.com/iso-invariance-explained

Simon