I have a Sigma 28-200 and when I look through the viewfinder of my Canon 300D everything is extremely dark, almost impossible to see anything and the autofocus does not work. Yes I have taken the lens cap off. Any ideas?

[agoodey]

Sounds like the aperture is closed right down inside the lens - I am not familiar with the 300D (I use Nikon kit) but is there a 'depth of field preview' button on the camera that is jammed in?

A camera's autofocus system requires quite a bit of light, and normally only works with the aperture fully open - which should be the default for a lens when attached to a camera.

[John7]

Remove the lens, wipe the contacts on the lens and camera body with a lens cloth or similar and try again. Try another lens and see if you still have problem - worth a try?

I tried 4 other lens that I have and no joy. I tried the 28-200 on a Canon film SLR that I have and the same problem.

Sorry guys - I tried 4 other lens including my Sigma 12-24 and they all work perfectly with my Canon 300D and 3000V film camera. The 28-200 is the only one that does not work. The depth of field button is ok and I cleaned the connections as you suggested - still no joy.

[Jazz Monkey Jr]

Is it new? I would take it back and exchange it.