Lee: The concern I expressed had to do with distortion that might result from putting filters between the lens and the negative, not below the lens. I've never put filters between lens and negative and know of no enlarger that facilitates it. I just think that any waviness in or crud on the filter could only be bad for the image, plus there's no need to put the filter there in the first place. I had an old enlarger years ago that required placing filters below my enlarger lenses, and I was not aware of distortion in the images. Then I got a C23, which puts the filters above the condenser, and this seems like the simplest way to color the light without involving filter material in the image formation.