I heard (another poster on this forum) say the "native iso" of a 7d is 160. does anyone have any (ie, concrete) proof of this? (can't google, at work).
remember a test on the 30D that showed that 160/320/640 etc were cleaner than the interspersed numbers. I though Canon had gone back to 100/200 etc, but I'm going on what I've read, not what I'm able to prove myself.
Only Canon knows what is 'native'. But I believe that the 'full stop' ISOs of 100, 200, 400, etc., are native. The forum posts claiming that ISO 160 is native, and that 160/320/640 are cleaner, are not incompatible with that. ISO 160/320/640 etc. are 'pulled' (although only by 1/3 stop, where ISO 50 is pulled a full stop). When you intentionally overexpose then reduce the exposure in post (even if that 'post' is in-camera before the RAW file is written), you reduce noise, so the '-1/3' tweeners have less noise than full stop ISOs, but also less highlight headroom.
You can see the effects of ISO on read noise in the data in the lower plot on this linked page for the 40D
(which is probably the same source for the 30D test that motorhead refers to, and is linked from that page). ISO 160/320/640/etc., have a higher S:N ratio than the full stops (but there are no corresponding data for dynamic range, which if there were, would show that it's reduced). If you roll your mouse down the list of ISO values in the top graph, you can see the 'combing' in the histogram at the 'fake' 1/3-stop ISOs, whereas the full stop curves are smooth.
Interestingly, the page linked above suggests that the 'tweener' ISOs are not 'fake' (i.e. pushed or pulled) on the 5D
- those cameras apparently use an additional analog amplifier. As a result, all the ISO curves are smooth, and in that case, the S:N is highest at the full stop ISOs (which includes ISO 50). I have no idea if an additional analog amplifier is also used in the 5DII (or the 7D, for that matter). If so, it's consistent with Sanjayfg's statement that ISO 50 is better than ISO 100 (lowest noise without sacrificing the highlight headroom).
Note that all of the data in the above links are read noise only (e.g. shots with the lens cap on and at the fastest shutter speed). Shot noise (and other, less significant sources like thermal noise) are ignored.