At 03:56 6/5/99 , Warren wrote:
>Sure if either the focusing screen isn't set into its frame correctly or if
>the stop where the mirror rests isn't adjusted correctly, the focus will be
>off. If the focus screen is resting on a bit of dirt or other object, it
>would force the screen closer to the lens and you would move the lens farther
>away from the plane of focus, which would in turn mean that the plane of
>focus in your photo would be closer than what you intended (or did I get that
>backwards??).
However, one must ask the question (and I don't have the answer) how
*critical* the alignment of these dimensions are in order to have a sharply
focused image on the focus screen produce a sharply focused image on the
film plane.
This is more logic than knowledge: There is a "depth of field" and
focusing is done with the aperture wide open unless you have manually
stopped down already (such as one of the shift lenses . . . elusive as an
"affordable" 24mm might be . . . I could only afford the 35mm). The faster
the lens (especially at longer focal lengths), and the more you have it
stopped open, the more critical the agreement between the two becomes. So,
I suppose if you have the mythical "faster than f/1.0" lens (yes, I know
Canon made an f/0.95 or thereabouts for its rangefinders) it becomes more
important.
Perhaps one of the group who has specific knowledge about the tolerances
required to keep things sharp for a *very* shallow depth of field (say
200mm or more at f/2) can tell us tolerance for correct alignment. If a
lens cannot rack in far enough for a sharp "infinity" focus through the
viewfinder, I will admit it would be frustrating even if not critical for a
sharp photograph.
I'm actually going to take some photographs today, not that there's much
hope it will produce anything useful (they won't be "pictures" and there
won't be any daffodils either).
-- John
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