I'm working on a job that requires some footage of a computer monitor to be shot (two presenters in front of the monitor and cutaways navigating through a website).
I've been doing some tests with my 7D but I'm getting some undesirable results. I've been shooting the monitor of my 27 inch iMac and the footage keeps showing flickering and / or ugly patterns across the monitor (see this video: http://www.vimeo.com/13091498, note that the H264 compression on this video has 'removed' the problems of some of the test shots, but the ugly patterns still remain in the full-res 1080p video on all shots). I've trying changing the shutter speed on the camera, altering the iris & ISO, lowering the screen resolution of the iMac monitor, changing the position of the 7D (shooting from lower and higher angles, moving the tripod closer and further away from the monitor) but none of these measures seem to fix the problem.
Does anyone know why this is happening and are there any work-arounds?
"Does anyone know why this is happening and are there any work-arounds?"
Yes, this is happening for the same reason that computer monitors have haunted video shooters for years... the refresh rate of the computer screen is out of sync with any of the shutter speeds you have tried.
The best "workaround" is probably not one that you want to hear... which is to shoot not with the 7D but with a proper video camera that has a "clearscan" mode. Most mid-range and higher video camera will allow clearscan shooting, which allows you to dial in very exact shutter speeds to compenstate for strobing screens (or, flickering lighting instruments, etc.). I'm don't remember exactly and would have to test, but I think in my past experience 1/60.1 was the shutter speed I've most frequently used to get rid of screen flicker.
Now, I maybe be speaking out of turn... not being a 7D user I can't difinitively say that it does not have a clearscan mode... but I don't think so. Being primarily not a video camera, I don't imagine that it does.
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Why not use a screen capture app for the cut-a-ways?
OSX 10.5.7; MAC Book PRO (EARLY 2008); Camcorders: Sony Z7U, Canon HV30, Sony vx2000/PD170, Canon xl2; Pana, Sony, and Canon consumer cams; FCP certified; write professionally for a variety of media; teach video production in L.A.