----- Original Message -----
From: "Joe Gwinn" <joegwinn@xxxxxxxxxxxx>
To: <olympus@xxxxxxxxxxxxxxx>
Sent: Saturday, December 29, 2001 6:17 PM
Subject: [OM] XA cosine^4 falloff and vignetting
> >
> >Shot a grey card or northern sky; scan that. Bring into PhotoShop and
> >turn it into a negative and do the add layers operation. I'm not all
> >that familiar with PhotoShop; but I've read this technique in the
> >AstroPhoto mailing lists. It is common with astrophotos.
>
> This sounds about right, but does one _add_ the inverted gray-card photo,
or does one do a pixel-by-pixel _multiply_? By physics, I would expect that
one would need to multiply, or the contrast would fall off at the edges,
even if the addition corrected the average brightness.
>
I don't really know enough about the way photoshop does its operations to be
certain, but I think adding is best. You're right about needing to
multiply, but the eye's response is logarithmic, so the RGB values in an
image represent a logarithmic brightness scale, not a linear one. Adding on
a logarithmic scale is the same as multiplying on a linear scale (so that
log(10) + log(5) = log(50)), so adding the grey card photo would seem to be
the way to go.
Sorry if that was an unintelligible explanation. I'll not be at all
surprised if theory and practice differ.
Roger
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