Correcting for uneven illumination

[Rawcoll]

If I want to reproduce some artwork, what is the best way of correcting for slight evenness in illumination? Can I replace the artwork with white card, re-photograph under identical conditions, and use this second image in a composite with the first to automatically correct? Or is there another way?

Thanks in advance.

[Dieter Mayr]

If you can make that white card shot it should work well for correcting for light eveness.
I would in any case convert that white card image to black and white to avoid any colorcast it may introduce during the composite process.
I've done something similar but with a hand created gradient, it is quiet tedious to get that right, both from orientation and strength of the gradient.

Hope it works well :)

[Rawcoll]

Thanks Dietr. OK, so I have my image and the b/w "mask" (for want of a better term) in which the brightest bits are white and the darkest bits grey. What blending mode should I use? Lighten? (Sorry, I find some of the terms which describe the different blending modes rather obscure!). Pesumably I'd have to adjust the percentage slider as the mask won't exactly cancel the variatons in level when at 100%.

[Dieter Mayr]

I did the following:

Inverting the mask (Grey - Negative)
Adjusting the brightness of the mask so that the brightest spot is just white (Grey - Brightness , Preseve set to "Neither")
So the darkest spot in the original is the brightest in the mask and wont be affected by the following step:
Composite - Input is the original, overlay is the mask, operation Filter.
So you should get the most even brightness all over.

A good way to test the whole procedure is to use it on the original white card image, you should get a pure white (or at least light grey) image.

[Rawcoll]

Thank you Dietr. I tried fiddling with some images I took a while ago and it seems to work, though I can't make up my mind whether it is just adding a varying amount of a grey tone to the original in order to even out the original, or whether it is actually darkening the lighter parts of the original, i.e effectively changing the exposure (and I can't decide whether the net result is the same or different; I somehow don't think it is the same thing). Forgive me, I'm a bit muddled about this. I suppose it comes down to what the "filter" operation actually does.

[jsachs]

Filter, used in the way Dieter described, darkens the lightest parts - the effect being analogous to shooting with a graduated neutral density filter.

[Dieter Mayr]

It darkens the brighter areas of the level of the darkest area.
So it would be wise to expose the original to the right, but of course not to clip it.

[Rawcoll]

Thank you Dieter and Jonathan.