A note on Canon Auto-ISO

I discovered a useful implementation of Auto-ISO on canon cameras (at least on the 40D & 50D), which doesn't appear to be in the manual. Some of you might find it useful/interesting.

In Av mode, the Auto-ISO feature prioritises ISO over shutter speed. That is, the camera aims to set the shutter speed before changing ISO until it hits a lower limit shutter speed. Simple stuff. However, it has a smart implementation of minimum shutter speed. The camera reads the focal length from the lens and then uses the 1/f rule for setting the minimum.

Here's an example: Mount a 70-200, zoom at 200mm. Set Auto-ISO. Set aperture. Shutter is adjusted until it hits 1/250s, then ISO is adjusted up until the maximum, then shutter is further reduced to match metering.

Mount a 500mm lens, minimum shutter target is 1/500s. Add the 1.4x tele-converter (700mm) and it goes to 1/750s.

Image Stabilisation doesn't affect the minimum shutter speed, which is nice.

The draw-back with the 40D is the Auto-ISO range is limited to 400-800. (I'm sure they could fix that with firmware.) On the 50D that's been fixed to be 100-1600. If I was a 50D owner, I'd use auto-ISO all the time.