[Garth Herrick]

I should add that zoom lenses have varying distortion like you describe at different focal lengths. At the widest angle the distortion will differ from the longest telephoto setting. Sometimes there is an ideal focal length that has the minimum of pincushion or barrel distortion present. You may have to try photographing your painting from different distances until you find the least distortion.

Then if you later have to photograph a different size painting, you may have to test this out all over again. One work around for this is to note down the relationship of the size of the painting to the distance you are standing from it, and keep this ratio as the one you can depend upon.

I hope this helps.

Garth