priit@xxxxxxxxx wrote:
> Odd that I noticed quite a similar problem just last week. Got a film back
> from the lab and found that seemingly I had been completely unable to
> focus a 50/1.4 on a 2-4 screen wide open.
>
> I was worried about this, so I've done some more testing under controlled
> conditions, but I don't have any conclusive results as I haven't developed
> the film yet. It was obvious though that the actual distance measured with
> a tape measure and the lens distance scale did not agree. Also, got a
> totally different reading on the distance scale when focusing on a second
> body that has a split-image focusing aid screen.
>
> I immediately thought about the focusing screen being somehow shifted too,
> but I've been reluctant to do anything about it, as I don't have very
> steady hands and don't like poking around with the metal tweezers around
> the mirror and the screen.
>
As John pointed out, the cause may also be mirror alignment. The shorter
the fl and the shallower the DOF, like f1.4 produces, the more critical
it becomes.
Some time ago, I took a new to me 28-200 mm lens outside to try out just
looking through it. It soon became apparent that it would not hold focus
from tele to WA. Infinity focus at 200mm was fine, but it was off at
28mm. I thought at first I had gotten a bum lens, but it was in such
pristine shape and I couldn't visualize how that problem could occur
without something obviously loose. So I tried the lens on another body -
just fine!
Some further playing revealed that the mirror was off in the first body,
which was a fairly recent used addition to my menagerie. A little
thought led me to the conclusion that the greater problem at shorter fls
is a percentage thing. If the mirror is 2mm out of alignment at the
bottom, that's 7% of the focal length of 28mm, but only 1% of 200mm.
The other thing that should be true, but I never tested, is that the
focus should still be close at the top of the frame and furthest off at
the bottom, because of the way the mirror is hinged at one end.. So
that's something to check for on your test shots.
Moose
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