A couple of thoughts with respect to copying flat originals. There is
a reference to distortion. This common problem is caused by lenses
not being symmetrical (front elements the same as the rear elements).
There are lenses designed for producing color separations. They are
known as Apocromats. They are corrected for three colors instead of
two. If there is any way to use them, they can probably be found
second hand at bargain prices. They were once very expensive. One
name to look for would Artar produced by Goertz American Optics. The
images these lenses produce from color originals are very much better
than those produced with regular lenses.