On a Besseler Dual Mode Slide Duplicator, I use a
Black NASA F, equipped with a Brightscreen for an F3.
The F3 screen will fit the F, if you remove it from it's frame.
With this combination I have a choice of F2 6x mag
finder, or an original F folding prism, with built-in pop-up magnifier.
I have never witnessed any focusing problems at all.
Of course I've never focused to infinity !