In article <4ecaea$a42@cello.hpl.hp.com>, jacobson@cello.hpl.hp.com says...
>In article <4e8e0r$n0m@newsstand.cit.cornell.edu>,
>Bob Neuman
>>In article
>> (most deleted)
>>>problem is that his camera needs to be collimated. With normal >>>lenses a slight error is likely to be not noticed, but with a
>>>300mm lens wide open the dof is small enough to magnify errors. (....)
>>I think this is very unlikely. Teles have less DOF, but greater
>>(visual) depth of focus (I know David Jacobson will holler about
>>this, and technically, any lens [of standard optical construction]
>>defocused by the same physical dimension will have equally large
>>circles of confusion at the same aperture, BUT, the viewed images
>>will differ, with the tele producing a more nearly sharp looking
>>image than the wide lens), so, if there had been a body problem
>>(on ALL of their bodies???), it would have been more obvious with
>>their shorter lenses, not the longer.
>I respect you enough to know that when you say something like this it
>has a good probability of being true. So does anyone have an
>explanation? (He's right about what I'm thinking, too.) (...)
I don't think this is an issue of ultimate lens sharpness, but it may
be related to something else. If I stand 10' from the faucet in my
darkroom, and use both a 200mm f4 lens and a 28mm f3.5 lens (nearly
the same speed) focused accurately on a point reflection on the faucet,
both lenses show a small point for the reflection, and a recognizeable
overall image in the rest of the field. If I carefully extend each lens 2mm with the focusing ring, and stand the same 10' away from the
faucet, the reflection is rendered as a disk by both lenses, and the
two lenses render the disks at about the same size. But, the defocused 200mm image of the area around the faucet is still recognizeable, whereas the image of the 28mm, defocused by the same 2mm, and which
has the same circle of confusion size, is completely out of focus,
with objects not recognizeable. (You can try this at infinity, also,
if you can find a useable point of light.) Also, by shifting the 200mm
lens 2mm, you are shifting the focus distance relatively slightly.
When you shift the 28mm lens the same 2mm, you are shifting the focus
distance relatively radically. Or, in the original post, if a 300mm
lens is shifted forward by, say, 1mm due to error in the
lens-mount-to-film spacing (unlikely to occur), the lens would still focus correctly everywhere but very near infinity. A 24mm lens,
however, would focus nowhere near infinity, and would look quite soft when used for infinity subjects.
This does not answer your post directly, but maybe it provides useful information for a future explanation.....
Hope This Helps