I have a Canon EOS 400D with a EFS 17-85 mm lens. The camera doesn't capture a picture with this lens attached. It focusses back and forth exactly once per click. I tried with different modes and its the same. When set to auto, the camera correctly auto adjusts the settings like exposure time, but its not changing the F stops. Looking through the viewfinder also shows a dark preview.
I think it is a problem with the lens. Can anyone explain what seems to be the problem, and how I can fix it, if the problem is repairable.