Hi Raid, For a full explanation of the problems between enlarging lenses, longitudinal chromatic aberrations (LCA) and variable contrast papers I recommend the book "Post exposure" by Ctein. It is an excellent book for advanced Darkroom workers and the only modern source I know that has extensive testing and commentary on darkroom enlarging lenses. Ctein has a Web site.(posted 8737 days ago)By the way, Ctein was not the 1st to realize the difficulty LCA causes with variable contrast papers. Concerns were raised by Arthur Cox in his book Optics: The Technique of Definition as far back as 1950-60. Look into it, you will find it very interesting.