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Flat Frames - White or Grey?

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HI, I'm still getting to grips with the whole calibration frames thing, and I seem to recall reading somewhere that the flat frames shutter speed needs to be adjusted so that the resulting image is a kind-of medium grey colour, and not white. is this correct?

Appreciate any pointers.

Thanks :o) 

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What it means by grey and not white, is that the flats shouldn't be saturated.

If you have an exposure that's too long, then every single pixel will be at maximum brightness and there'll be no variation across the image. Whereas you should be able to see any vignetting, finger prints, dust bunnies, etc. These will appear as slightly darker areas, but only if the image isn't saturated.

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  • 5 years later...
38 minutes ago, quigley said:

I see images of flat frames that are a slight magenta in color. When I shoot mine using a white T shirt they come out white...am I overexposing?

I've always looked at the histogram of the image; as long as that isn't clipped then I think it's OK.  Mine usually peak aroune 1/3 of the way along.

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The goal is to have the range of exposures (if there weren't a range, you wouldn't need flats!) in the linear-response range of the sensor so the math can work out right. As Olly and Stefan say, an unclipped histogram is perfect, and a peak at 1/3 works out very well.

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