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Any chance http://support.apple.com/kb/TS2034 is what's happening? Or if you're able to take a picture but the resulting image is blue or green, consider http://support.apple.com/kb/TS2095. Do you notice the same thing if you try using the iSight camera in iMovie?

When using iSight in iMovie I get a blank screen... I'll check those other two out. All my software is up to date, and it is not the problem with the image being blue or green. When I turn the camera on I get a green screen in photobooth and black screen in other things..
 
iSight camera not working properly, but it is a software issue

My iSight camera has stopped working properly. It loads in Photo Booth only to show a "green screen" and in iChat to show a "black screen." I have read a number of lists for help, followed the instructions and the problem remains. I have rebooted, reset the PRAM, (wondered which QuickTime preferences to delete), reinstalled my Mac OS X 10.4.11, and disconnected all power inputs for 6 hours, but no help. After the 6 hours downtime, I rebooted in SafeMode where in Photo Booth I could not even activate the camera, but in iChat the video preview activated the camera and my image appeared. I closed in SafeMode and rebooted in normal only to get the green and black screens as before. It is thus not a hardware problem with the camera or connection, but some kind of software issue. What is the fix involving QuickTime? What other software fixes are available? Thanks. Bob G

Update: Now the iChat video preview alternates between "No camera connected" and "black screen" and Photo Booth is still "green screen". My iMovie keeps looking for my DV Camera and then notes that "Video capture cannot begin, because a camera could not be found". However, the little green light next to the camera comes on with each attempt.
 
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