In message ,
"Woollyzone" wrote:

Although I haven't yet tried testing it to see if it was light leakage from
through the viewfinder, I'm pretty confident that its AG, as it matches your
description of it. Its easily cropped out, but to ensure it all goes, it
means losing about 20% of the width of the picture.

If you take an image immediately afterward with the same ISO and
exposure time, with the lens cap on and the viewfinder cover, you have
the necessary data to subtract it. I don't know how well software can
take advantage of it; for best results, the subtraction should be done
to the RAW data, and I don't know if there are any programs that will do
that and still do a quality RAW conversion. The 20D has this as a
feature; it subtracts the black frame from the image before writing the
RAW file (if you enable the feature, and it only works with 1 second or
longer).
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John P Sheehy