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GlennLeDrew
professor emeritus
Reged: 06/18/08
Posts: 620
Loc: Ottawa, Ontario, Canada
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Re: Undersized prisms - the myth
06/18/08 09:28 PM
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Edz: you wrote, "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."
Then how is it that the very same prisms are often used in binos from 35mm up to 100mm? Again, if the f/ratios of their objectives were to be just that little bit greater (f/4), all would be well. That's what I was stressing.
The issue of prism size is important in one area only: that it allow a reasonably illuminated edge-of-field for the chosen ocular. A big field stop diameter, as for wide angle designs, requires a commensurately large prism. But the prism aperture can actually be a bit smaller than the field stop diameter and still deliver an image that has no detectable vignetting.
And the larger the space between the prism's rear opening and the field stop, the greater can be the difference between prism and field stop apertures... within the limits imposed by the objective focal ratio, of course!
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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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