Re: Olympus SZ-60 focus

Tom Knight <tk@xxxxxxx> writes:
I've inherited an Olympus SZ-60 stereomicroscope, which works very
well for my applications, but has the annoying property of going out
of focus when I zoom in. Any thoughts on whether this can be
adjusted?

Maybe. Is there a "diopter adjust" collar on both of the eyepiece
tubes, or just one of them? If only one is adjustable, you can only use
it to make both of your eyes reach best focus at the same time.

But if both sides are adjustable, you can probably use the eyepiece
adjustments to make the image stay (approximately) in focus as you zoom.
Here is how: First, set both eyepiece adjustments to mid-range. Put
something with fine detail and good contrast on the stage. Zoom to the
highest power, and then adjust focus for best sharpness. Now zoom to
lowest power *without touching the focus adjustment*. Then individually
adjust the two eye tubes to bring each eye into best focus.

Now, if you adjusted the eye tubes very much, zoom back to high power
and readjust the focus adjustment again. Then zoom to low power and
readjust the eyepiece tubes again. This process should converge pretty
rapidly, so the amount of adjustment will quickly drop to nothing and
you can quit.

The theory behind it: The microscope zoom section was *probably*
designed to be parfocal - to keep the image in the same place as you
change magnification. But that only works if the object and image
planes are in the place the scope was designed for. If the eyepiece
adjusting tubes are both mis-set, you can still get the microscope to
focus, but the amount of focus change needed now varies with
magnification, and the microscope is no longer parfocal as you zoom.

To fix it, you start at high magnification, where the scope has least
depth of field (at the subject) and most depth of focus (at the
eyepiece) and adjust focus. This puts the objective in almost the
designed-for place, since at high power this position is relatively
insensitive to eyepiece position. Then you zoom to low power, where
the microscope has most depth of field and least depth of focus, and
use the eyepiece tube adjustments to give best sharpness. This puts
the eyepieces close to the designed place, the place to remain parfocal
with high power. Now, adjusting the eyepieces on low power may have a
small effect on focus at high power, so for best results you iterate the
process until the adjustment stops changing.

If you get it in focus at min and max power and it's *still* out of
focus at intermediate power, then perhaps the zoom just wasn't designed
to maintain focus during zoom.

Dave
.

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