The Auto ISO mode sets ISO values between 100 and 3200 ISO. Compared to what I am used to (no auto-iso at all on a Canon 10D) this is pretty convenient. Some thoughts so far:
- I noticed that Auto ISO has a strong tendency to go for 100 ISO. That’s nice if there is enough light to get a short shutter speed and a safe amount of depth-of-field. But the camera also used 100 ISO on a 28mm, 1/30 s, f/4 shot in the evening. Arguably this still adheres to the old 1/f-rule for shutter speed. But the quality (noise, dynamic range) difference between 100 ISO and 200 ISO is minimal (although visible in DxOmark ‘s measurements). I thus wonder how often that stop could be put to better use like having a 2x faster shutter speed or getting more depth of field.
Update (16-Mar-09): a work-around to increase the minimum ISO from 100 to 200, is to enable “Highlight Tone Priority”. This, as a side-effect, sets the minimum ISO value to 200. - Maybe this is overoptimistic, but are the “only” conditions when one would overrule Auto ISO might be for either landscapes using tripods (where 100 ISO is a good idea) or sports photography (where increased ISO is a good idea).
See manual page 58.
I have also found auto-iso a very handy feature. I have been a bit irritated of it´s tendency to stick with 100 ISO. I tried your trick with highlight tone priority. I was shooting on a somewhat foggy day with snow on the ground. I was not correcting the exposure, but thought, that the highlight priority will shoot to the right, i.e. use the maximum available range of it´s 14-bit resolution. My logic might not have been right on how the highlight priority works, since all the pictures were roughly 1 stop underexposed. However, the lowest ISO was 200, which was the reason for trying this out. I have to do some experiments still, but it would be good to understand the highlight priority mode logic better. BR, Sakke
There is quite some info on the feature. See http://luminous-landscape.com/forum/index.php?showtopic=23250.
Summary and claim (Feb 2008, based on HTP on the Canon 1Ds Mark III): “there is a visible advantage only if you shoot JPEGs. In RAW, the Highlight Tone Protection setting has no effect other than underexposing by one stop.”
I don’t know if this is true for the Canon 5D Mark II as well.