Telescope Reviews: Re: Undersized prisms

Glenn,

along with undersized prisms comes the ability of the manufacturer to build a smaller unit, so in fact it is not a myth it is a reality that binoculars are designed with undersized prisms. Obviously, some binoculars have been tested and proven to have properly sized prims. Is it any wonder that most of these units often have considerably more bulk then their undersized brethern?

Almost all binoculars I have tested that have too small prisms are too small by approx 10%. Increasing prism aperture by 10% (most are approx 20mm), would lengthen a prism by 4mm, probably lengthening prism pair light path by a total of about 8-10mm. If housing size were kept constant, that would require approx a change of 6mm in focal length, or 6mm change in lenghtening of binocular housing, and would require keeping the front prism aperture at the same position with respect to the objective. If housing size were allowed to vary, it would require no change to the focal length and the binocular housing could be allowed to get shorter.

Every minor adjustment in prism aperture that requires an adjustment in focal lenght requires another small iterative correction, or any change in prism size requires another change in housing size, but either way, obviously Nikon, Fujinon and a few others have got it all figured out.

Now I won't doubt that you can claim the error is not in the prism it is in the objective focal length. Since it is aperture that we use as a major criteria to buy, I choose to state that for the given aperture of the binocular, the rest of the system is underdesigned. Since you too, with a statement that it is not the prism, it is the focal lenght of the objective, would also require a redesign of the entire binocular housing, you really can't get away from the fact that the housing and the placement of its parts is imperfect.

In your example, where you stated that all would be well if the objective were f/4 rather than f/3.5, for a 70mm binocular to accomodate that with no change in prisms, you would be required to lengthen the binocular housing by 35mm, certainly no small mechanical redesign. You simply choose to redesign around the current prism, and I would choose to redesign around the current objective and change the housing by a tiny amount.

edz

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