> Date: Thu, 16 Jan 2003 12:39:50 -0800
> From: Winsor Crosby <wincros@xxxxxxxxxxxxx>
> Uniform illumination of the field
> is necessary to make this kind of comparison which is hard to do with
> a gray card. It would have to be very large to fill the field of a
> 21 mm lens and still be in focus. I would suggest that you use a
> uniformly lit wall.
Or don't focus. Unless you need to compensate for magnification at close
focus, I find it better to leave focus at infinity when using a grey card.
Ideally you'd meter it at the focus you're going to use for the shot, but in
most situations it's not necessary.
The results would imply the camera is seeing the centre as being darker than
the rest. Some grey cards are a bit shiny at oblique angles - such as you'd
get at the edge with the 21 - look through the viewfinder and see whether
your eyes agree with the meter. If not, I'd say there is likely some method
flaw or a camera problem.
The 21/3.5 can vignette easily with filters, but that would be the opposite
effect.
Andrew
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