Telescope Reviews: Pentax 20x60 PCF WP II (Prisms undersized)

Well, we can see several things in the exit pupil.

If we see an eliptical (lozenge?) shaped exit pupil, is not an indication of too small prisms. If we see an edge cut off on one side, it is an indication of small prisms. If we see a diamond shape it is an indication of the type of prism being used. So I'm not sure what Stephen had in mind, but at the very least, that statement is confusing.

Read the links I pointed to understand how to measure for a too small prism.

A circular image with a line cutting a chord across one side is an indication of a too small prism. Other than that you can have perfectly round prisms and they can be too small, or you can have diamond shaped exit pupil and it can be full size.

A diamond (lozenge?) shaped exit pupil is an indication of BK7 glass used in the prisms. It doesn't necessarily mean they are too small. It shows the nature of BK7 glass when used as a prism. The prism aperture could be full, and you would still see it as diamond shaped.

As I stated earlier seeing an eliptical shaped exit pupil, or as I sometimes refer to it as a cat's eye, is an indication of a tilted light path, not necessarily a too small prism. We see this sometimes when the prisms are being tilted to align images, but either the wrong prisms were tilted or there are other problems in the optical alignment requiring too much tilt that result in a not round exit pupil. But I will stress again, this is not an indication of a too small prism. You can get an eliptical exit pupil even in a binocular with prisms that are not too small.

The appropriate tests are outlined in the reference link above. One test is the direct reading of a scale with a loupe, but although that may show vignette is reducing aperture, it doesn't necessarily indicate the prisms are too small, so further investigation is necessary to determine whether the reduction is due to a baffle or the prism aperture.

Simply checking the size of the exit pupil does not necessarily give the answer. First magnification must be verified. Once magnification is verified, then you can use the multiple masks test to check exit pupil sizes and verify aperture. If there are no baffles interfering, then this will confirm a too small exit pupil.

So for the most part, other than the straight line chord mentioned above, there may be no visual indication that the prisms are too small. A proper investigation to reveal a too small prism involves checking baffles, prism aperture and magnifiaction. Without checking those, you cannot reveal if the prisms are too small. And even if they are too small, they can still produce a perfectly round exit pupil.

But now I'm repeating myself. All of this is explained in the link above. So please refer to that link for full explanations.

edz