[aurele]

there is one Pentax lens that is 2/1 which is the K85/2.2 Soft. see : K 85/2.2 Soft

[wildman]

Minimal (or lack of) internal reflections and minimum number of elements in the "signal" path.

Also Introduce an element into the light path to correct a given aberration and the additional element will always introduce aberrations of it's own. There is no free lunch in physics.

Can you explain that, please?

Sometimes, depending on the file, you have to correct for the uncorrected part of the light spectrum.

BTW, what lens/scope is that?

102 mm Wide View [item # 52270]
Note: I bought mine as a bare optical tube over ten years ago

The lens material has to be different on those two lenses then.

Crown and flint glass