There are several reasons why autofocus may not work:
- The autofocus switch on the lens is set to M(anual).
- Your subject moves quickly towards or away from the camera.
- You're trying to focus on a subject that has no contrast, e.g. a plain sheet of paper, a featureless wall, a clear blue sky...
- You are using LiveView (a mode of operation similar to compact cameras where you use the LCD screen as a viewfinder).
- Not enough light.
- A problem with the lens.
The solutions are:
- Set the autofocus switch on the lens to Auto. (d'oh!)
- Catch the subject when it doesn't move too much, or reposition yourself a bit farther away (and zoom in). Try setting the AF mode to AI-Focus or AI-Servo, this works better when tracking moving subjects, but requires a snappy lens.
- Find a feature to focus on, change the autofocus point if required.
- Autofocus does not work in LiveView mode on the 40D, you must use manual focus - but you can use the magnifier function to focus more precisely.
- Autofocus needs a minimum amount of light to work well. If it's too dark, you can use the camera's AF-assist feature. This uses the built-in flash to illuminate the subject while you try to focus, but it's a bit annoying to say the least. Most external flashes have an autofocus beam, a small red light that's just enough to focus up to 15-20 ft, this works much better and is much less annoying. Also remember that the central autofocus point is more sensitive than the peripheral points. If you have trouble focusing with the peripheral points in low light, selecting the central point might improve things a bit.
- Some lenses have better (quicker and more precise) autofocus mechanisms than others. A lens with a fast autofocus will have less trouble with moving subjects than a more sluggish lens. Some lenses (especially the EF-S 17-85) are known to have trouble with defective autofocus. If you suspect this is the problem, try a different lens and see if autofocus works.