I had the same sort of problem on a nikon s compound trinocular when it was
fitted with a 1.2x objective. It was nothing to do with the scope. It was my
eyes. They were following the focus rather than staying fixed at infinity. I
found that I could force my eyes to focus the image into one but it was rather
uncomfortable. Answer for was to focus the images to one and to a position where
my eyes were relaxed.Not an easy thing to do as they tend to latch on to what
ever is there. Why this should happen at low mags is beyond me. I would be
interested to know why. Maybe it's because the na is very low and the depth of
field great. I have even had a similar problem focusing the image onto a
reticule on a monocular with a 3x objective. The eye tends to focus on what ever
is looked at. Recent info suggests that most people focus a scope to match their
vision at about 25ft rather than infinity. It's an interesting area. I can use
averted vision but the lack of resolution off centre isn't helpful.

If you have played with the prisms it could well be that you have messed things
up and the problem had nothing to do with the scope.

John


--- In Microscope@yahoogroups.com, "aent440" <andrew@...> wrote:
>
> I wonder if anyone can advise me as to the likely cause of a collimation
problem I have with an early 1970s Zeiss Jena SM 20 stereo microscope? The
`scope has a common main objective and a rotating barrel supporting 3 Galilean
telescopes which can be positioned to give 5 different magnifications from 0.6
to 4.0. At the highest magnification the collimation is absolutely perfect, but
as the magnification is lowered the left and right images move towards each
other until at 0.6 magnification the images are each offset to the centre by
several mm on stage. I can still form a stereo image but not without straining.
I think the root of the problems is that as the magnification is decreased from
maximum to minimum the working distance becomes shorter by about 20mm, although
the same is true of my Lomo MBS-10 that does not exhibit this collimation shift.
Fitting instead the binocular head from the Lomo MBS-10 the problem is not
evident, or at least not to any noticeable extent. Adjusting the binocular
prisms is easy (mechanically) as they sit on a layer of grease and have three
moveable wedges around the periphery.
>
> Andrew.
>