Re: How to distinguish 160 vs 170 mm eypieces?

shiraz14 <shiraz14@xxxxxxxxxxxxxx> writes:

Firstly, thank you for your valuable input. Also, as a correction to
any possible misconceptions to my last post, NA was stated to be a
dimensionless constant as it was interpreted in the context of a
particular lens used to image a specific specimen under a stated
tecnique. Note however that NA is actually a variable (think those of
us who are relatively familiar in microscopy would know this ... this
is just for the clarification of any potential misinterpretation of
the information presented in the last post) ... factors influencing NA
include the optical technique used, mountant applied, objective,
condenser & eyepiece iris diaphragms (if any), etc.

Yes, it's a measurement of the angle of the actual light cone that forms
the image. The condenser iris is provided to explicitly adjust the NA
of the illuminating light, and it's generally adjusted to make that
equal to or slightly smaller than the NA that the objective was designed
for.

As for the clarifications, here they go:

"rear focal plane" (not "real focal plane") =3D conjugate image plane as
observed by the viewer during simple conoscopy.

I presume you obtained the information for the image plane being in
the eyepiece barrel from the Molecular Expressions website

That's a useful reference, but I also own a few microscopes and
telescopes, and I've done a small amount of optical experimentation on
my own.

- although
Molecular Expressions serves as a useful resource for microscopy, the
information provided there serves only as a preliminary guide to
microscopy fundamentals ... in reality, an infinite number of image &
focal planes exist (which may be sited at any point along the optic
train) - raising and lowering (including synthesis and elimination) of
focal (& image) planes may be achieved through the insertion of
apochromatic auxiliary lenses, etc, and any potential gain/loss in
magnification be compensated through the coupled use of a
corresponding reducing lens-pair system. For my purpose, I'd need to
have the rear focal plane lying directly below the prism assembly but
within the trinocular head, if this is not possible (due to space
constraints), then at least within an intermediate module sited
between the head and the IL axis.

Hold on just a minute. Although it is *possible* to create an optical
system that has many conjugate image planes, each such image plane
requires a well-corrected lens system to focus the image, plus there
needs to be a field lens or some equivalent that does not provide much
focusing power but which redirects the light diverging away from the
optical axis after one image plane back to parallel or converging so it
can be refocused at the next image plane (otherwise the illuminated
field will be small).

Thus, it takes quite a bit of extra optics to provide each "extra"
conjugate image plane beyond the one normally formed in the eyepiece.
The extra optics would cost money plus reduce brightness and contrast,
while providing no benefit for most microscopy, so microscopes generally
do not include them. Unless you modify the optics, there is just one
place where a real image of the subject is formed, and that's at the
field stop inside the eyepiece barrel.

Microscopes are often designed to take accessories like polarizers,
darkfield equipment and so on, but most of these want to be inserted
where the imaging light is parallel, *not* where it is focused to an
image. The light between illuminator and condenser is approximately
parallel, and the imaging light between the objective and tube lens in
an infinity-corrected microscope is exactly parallel, so that provides
the places to add these accessories. But no extra image plane is
provided. Unless your microscope is unusual, this will be true of it as
well.

The trinocular head is likely designed to have parallel light enter the
bottom end, go through a tube lens, and then be focused on the field
stop in the eyepieces, with a specific light path length through the
prisms and mechanics. Any change to this design will require mechanical
changes to the position of the eyepieces and other elements, and it may
also mess up optical corrections of those elements.

If you really need an extra conjugate image plane, you're probably best
to create one inside a module that is mechanically inserted between the
body tube of the microscope and the trinocular head, assuming the tube
lens is included in the trinoc head. The incoming and outgoing image
light is parallel, so you need a pair of well-corrected lens systems of
an appropriate focal length which are corrected for operation at
infinite conjugate ratio. You'll also need to trace some rays from
points near the edge of the image and add field lenses or aperture stops
as needed, to avoid losing brightness or losing contrast from stray
light. (I'm not an optical designer; you should talk to someone who
knows what they're doing for the details). This should be able to be
done without modifying the existing microscope parts.

Dave
.

Relevant Pages

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