Telescope Reviews: Undersized prisms

A too-small prism? Generally, no. It's almost always the use of a too-"fast" (short f/ratio) objective that's the culprit.

Let's consider only Porro prisms for now. In the great majority of binos, the front and rear prism apertures are the same. So if you use larger prisms so as to get larger openings, the optical path length through the prism cluster scales up by the same proportion. Therefore you gain nothing, because the larger opening is now moved up closer to the objective where the converging "cone" of light is wider.

I've written simple BASIC software to investigate this, and my conclusion is that for "standard" Porro prisms having same-sized front/rear apertures, the objective f/ratio should be no faster than about f/4 if the full objective aperture is to be utilized. But a *great many* binoculars have objectives faster than this (f/3.5 to f/3.8), and therefore suffer from undersized exit pupils, i.e., smaller effective apertures.

For example, an older 100mm binocular I examined had f/3.5 objectives, and therefore worked at an effective aperture of only 78mm (!!!) because the steep light cone could not be fully accommodated by the prisms. If the objectives were instead f/4, all would have been well.

All too often I hear that the "squaring-off" diamond-shaped pattern which causes exit pupil graying in the outer parts is evidence of too-small prisms. No. That's evidence of, again, a too-fast objective for the prisms' glass type. A prism must have an index of refraction high enough so that its critical angle of internal reflection is steep enough to accommodate the steep "cone" of light being focused by the objective. That's why BaK-4 prisms are more commonly used nowadays; they are of higher density, and hence higher index of refraction, than the ubiquitous BK-7.

The "diamond" pattern is the result of the prism cluster's four mutually perpendicular reflective surfaces, each of which fails to fully reflect light from its respective outer portion of the objective aperture. So the gray portion of the exit pupil represents that portion of the objective's aperture from which the light arrives at the appropriate prism's reflecting surface too steeply.

If one made a binocular with longer focal ratio objectives, good ol' BK-7 would serve just fine.

Whew! Hope this makes sense...

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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV

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