Ian's question reveals the troubles of attempting a precise explanation without being careful about all the terms. Much pain and suffering on this forum has resulted from the lack of clarity (so to speak) between "in focus" and "acceptably sharp". (You could have said "apparent focus", which addresses a lot of issues.)
But then it's sort of a sport on this forum to out-precise the next guy.
Rick "guilty of that as anyone" Denney
Bookmarks