scfn.thpl.lib.fl.us> skrev i inlägg
>> ...

>> > I thought that the poor images I was obtaining with my 1250 mm lens
>> and my
>> > Minolta-370 was due to camera jitter introduced by the SLR shutter
>> > mechanism, so I did a series of experiments that showed me the
>> reason for
>> > the poorly focused images.
[most deleted...]

(I've jumped late into this, but a couple of things occurred
to me...)
A 1250 "lens" is maybe a spotting scope? And probably not
very sharp for photo use... (and probably without diaphragm
to stop down to improve the lens image - and for increasing
DOF for focus error covering). As to matte viewing screen
error, if you have noticed no error with other lenses, there
would be none with this one (a 1mm error would not be very
important to the focus of a very long lens, but it would
be disastrous to the focus of a wide angle, for
instance...) - or, an error large enough to be a problem
with a 1250mm lens on a 35mm camera would be a VERY large
problem when using a normal lens (the lens would not focus
correctly near infinity [according to its distance scale],
to say the least! ;-). Two other things - as another poster
pointed out, the motor wind (and mirror up-slap) would occur
AFTER the exposure; and, brightening condenser lenses in
newer compact camera viewfinders do introduce focus errors
away from the center of the focus screen with long
lenses - though, this should not introduce serious errors...
Mis-focusing is rather easy to do with long lenses, and it
can show often when the lens cannot be stopped down. Also,
even with a REALLY good tripod, it is difficult to make a
sharp photo with a very long lens, even if you can lock up
the mirror - and it is nearly impossible over long distances
(over land) due to air conditions. Over 300mm, photos with
long lenses take very special care in order to get sharp
images...