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Paul, I still take flats with a 50mm lens. There may be some vignetting, especially if you use a lens shade. In this case there may be a brightness dropoff around the edges of the frame which may or may not be noticeable depending on how bright the object is and how much low-level signal you try to tease out of it by adjusting the levels/curves.
Also, tiny dust motes on the inside of the lens, or on the inner-most lens, or anywhere near the chip, will cause shadows on the chip that look like donuts in the resulting image. The closer the dust is to the chip, the smaller and darker (more prominent) the donut will be.
Again these may or may not be noticeable for the same reasons as vignetting. Flat fielding will deal with these issues and remove the uncertainty, so I always do them.
Last edited by EddieT; 15-05-2005 at 12:28 PM.
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