Strange autofocus problem with 300D, can anyone help?

mojones

I've just come across a strange problem with my canon 300D (Digital Rebel).

The centre autofocus point has stopped working. Normally I have the centre AF point selected and the camera gets a focus lock quickly (in good light, etc.) But now the centre point has stopped working; if I have it selected, the lens just hunts around without getting focus. If I select one of the other AF points, or select 7-point AF, it seems fine.

Has anyone heard of this happening? Any ideas?

mojones

mojones

OK, panic over.

To answer my own question, I used a sensor brush to clean the mirror and mirror box, and that seems to have fixed it. Not entirely sure why though.... any ideas still appreciated

wxwax

Very odd.

GadgetGav

mojones wrote:

OK, panic over.

To answer my own question, I used a sensor brush to clean the mirror and mirror box, and that seems to have fixed it. Not entirely sure why though.... any ideas still appreciated

I would guess that it's something to do with the way the image is projected into the AF system. The main mirror is only a partial mirror. There's a second mirror behind it that reflects an image down into the AF and AE system. There must have been debris on the mirror or the sensor that confused the AF system - the AF need to see contrast-y edges to focus on.
There's a neat see-through diagram of the 300D here;
http://canon.nyherji.is/eos300d_seetrough.jpg