Most of the focus issues I have encountered with fixed-focus screen 35mm SLRs are from the mirror. The mirror is a likely suspect because it moves with each exposure and needs to always come back to the exact same position that it was when it left the factory. The fixed-focus screen 35mm camera system I use has an easily adjustable (without camera disassembly) mirror for both angle and up-down.
Originally Posted by newcan1
In cameras with interchangeable focus screens, in addition to the mirror being off, there can be a host of additional focus issues relating to focus screen placement from tampering etc.
In cameras in which the mirror is not adjustable, sometimes it is easier to shim the focus screen. Sometimes you have to both adjust the mirror and shim the focus screen to avoid inducing parallax error between what the screen shows and what the film sees.