Hm, I've not thought about such a function much... I don't have it in my 20D.
Here's how I think: I'd like the best possible image quality the lighting allows. So as the light fails, I push up the ISO to get shutter speed to sufficient levels so as not to produce motion-blurred images. In general, I use as low an ISO setting as possible to produce minimum noise.
If the automatic function worked like this - and one could understand and anticipate - or even better temper - its decisions, it could be quite useful.
However, in a decade of using digital cameras I've learned to check and set the ISO without thinking about it too much. It's second nature for me now.
-Noel
White Balance so easy, even our 5 year old can do it.- Melissa Strickland
I used Auto ISO a lot when I shot Nikons, but since I now shoot Canon, I don't have it. It worked great with the Nikons, and I wish the Canons had it. The 5D only has it in the Idiot mode, and it doesn't allow RAW in that mode, so I don't use it.
__________________ Dennis
White Balance so easy, even our 5 year old can do it.- Melissa Strickland