> I have noticed that when shooting stuff on a white backdrop (back lit
> exposed about +2 stops to white it out) that I am getting a fall off of
> light on my 1ds files at the bottom of the frames, the top of the frame is
> deffinetly brighter / cleaner

Assuming it is not shutter speed, the reason may be the uneven response to
light across the chip.
I have noticed that when photographing an evenly lit surface (2 lights, 2
metres away at 45 degrees, no variation in brightness across an A4 area)
that an exposure made to give a density of, say RGB 180ish will have a
variation of about 10 points RGB between one spot and another.
I can't see it in any image with a subject, just when testing. I have used
different cameras, and they are each different.

I recall that once I tried to test an enlarger the same way, & was surprised
by the unevenness of brightness across a 5x4 neg area. Again it mostly
didn't matter when making prints, but if it had had an eyedropper & and info
palette I would have seen a numerical representation of the varying density
that I see on digital cameras.

Tim Wheeler

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