On my manual focus cameras like my RB67 or Yashica 124G when I get to the "critical focus spot" where it seems to be in focus...a tiny bit more and it is still in focus and then it starts to go OOF again. My question is about the tiny spot of critical focus where, for me, it is extremely difficult to tell the difference, even with the loupe. I know that for the exact focal length of a technical piece of equipment the is "the" spot where it is perfectly in focus. How do you decide when it is focused correctly? This may seem like a silly question...but???