I've been using this lens for over a year now with great results. The other day I shot about 100 pics with it. When I opened them with Capture One they were very dark. I had to up the exposure 2 stops to even see the photos. I tried other lens with the same camera settings and everything was fine. I put the 70-200 back on and once again, very dark images. Does anybody know what could be causing this?

The only thing I can think of is some sort of iris problem. Underexposure is cause by one of two things--Too small an f-stop or too fast a shutter setting. Something is not allowing an adequate amount of light to hit the sensor, thus producing an underexposed photo.

The lens reacts to whatever the camera tells it to do.. If you use the Av mode, you set the f-stop on the camera and the signal is sent to the lens.

Because you say you are having problems with just the 70-200, it might be an iris problem, causing it to close down too much or it might be dirty contacts on the lens. Try some different exposure modes after wiping the contacts..That might help.

Thanks WPMAN for your suggestions. I cleaned the contacts as you suggested and that seemed to do the trick. Expose is once again spot on.