I'm going to guess the camera shutter 'speed' is at fault. With the cameras set to automatic exposure, the shutter speed may be set by available light and you can get this sort of problem. Other cameras will be 'aperture prefered'', causing depth of focus shifts with changing light. If so the only way to control this is use total manual control of aperture and speed in your shoots. .
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System - Athlon 1.4GHz, Win98, Hauppauge PVR250 receiver and compressor.
Software -Magix Movie Edit Pro 10, Nero 6 + NeroVision Express, Moho 4.61, PSP 8.1, Bryce, Quicktime 6.52 pro, Goldwave 5, DVD-Lab.
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