Quote Originally Posted by Poisson Du Jour View Post
I have read Alan's original post over and over again and something a bit left-of-field comes to mind that has not been explored: the characteristic of the lens. A lens (uncorrected) with known chromatic aberration will also have a relationship to image sharpness. I would imagine that the Mamiya lens is maybe not a highly corrected optic and it would have some definite chroma or a fairly nondescript refraction index (which would be further deranged by the use of a deep aperture)? So if a coloured filter is introduced, is that exaggerating the amount of chromatic aberration and thus resulting in a perceived greater loss of image sharpness? Colour theory and lens technology was explored in my uni days: the subject of aspherical, apochromatic and polyapochromatic theory and design. I have not for many years used an orange, blue or yellow filter on any of my lenses, most of which are apochromatic, but I do use red on my fairly bog-standard design 67 lenses (red, predominantly with ACROS 100) with absolutely no derangement of sharpness anywhere. So is the problem mechanical? Has the scene(s) been shot with another camera loaded with the same film, lens and filter? What was the result of the parallel test: unsharp or completely different in characteristics?
Au contraire, Poisson

When a lens has chromatic aberration, that means it is not focusing all colors at the same plane. Removing one or more of these colors (especially if you remove the one that is the most out-of-focus) actually improves the sharpness.

Usually it is blue that is the culprit (hence the blue color of many fringes on non-apo lenses) and using a red or orange filter would fix it right up...