Hi Baluk

I've heard of people using a light meter (can't remember which type - it might be a spot then maybe someone else can remember) to check nine points on the cloth you are using. You are looking for consistent readings at the top left, top middle, top right and then centre left, centre centre and centre right and the same along the bottom. I know it might sound a bit excessive but if it gets rid of your problems so you don't have to spend hours in post production trying to get rid of artifacts or shadows then it's worth it. You could also use the clone and eraser tools in After Effects if you have access to it.

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Cinematographers do it with a telephoto lens.