I have to admit that when it comes to ISO, I've always assumed that the base ISO was going to have the cleanest output. I had read articles on how some models of cameras were noisier at the 'in-between' ISOs (ISO 125 and 160) than the base numbers (100, 200, 400). This was true on a camera I had a while back, so I didn't really give it another thought.
I read an article in a magazine (link here
) where someone took a Canon 7D with a lens cap on and compared the noise with the levels boosted up. It isn't as dramatic as the article made out, but the results surprised me a little. Turns out my 7D is just as good (or better) at ISO 160 than it is at ISO 100. That is a 2/3 of a stop bonus with no penalty! Try getting that much more aperture on all your lenses for free :)
I'm not going to argue that this will change your life or anything, but after doing some testing I will be sticking to ISO 160, 320 and 640 over the 'whole' numbers. The point in the article is there is more going on than just the base ISO - that all the electronics and processing come into the picture. It is worth a few minutes taking a few shots and convince yourself that the ISO values you think are cleanest, really are. I was surprised to find banding in the shadows at ISO 400, but not ISO 640. I should hasten to add that I'm talking about boosting overexposure by +4 stops to make it obvious - but the point is that if in a carefully controlled experiment I can't tell the difference between ISO 100 and 160 or 400 and 640, why would I hesitate to use the higher value?
Let me know what you find?