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Old 07-03-2013
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Re: Binocular Conundrum: Field of View?

This difference between your hypothetical binos is the field stop of the eyepieces.

True Field of View

Eyepieces also determine the true field you see in the sky. To calculate the true field of view that you will see (in degrees), divide the eyepiece field stop diameter by the telescope's focal length and multiply the result by 57.3:

True field of view = eyepiece field stop diameter ÷ telescope focal length x 57.3

The Field Stop and Apparent Field of View

The field stop is the metal ring inside the eyepiece barrel that limits the field size. It's projected by the eyepiece so that it appears as a circle out in space when you look through the eyepiece. The angular diameter of this circle is called the apparent field of view (AFOV) and is a fixed property for each eyepiece design. For example, Plössl eyepieces have an AFOV of 50°, Radians have 60°, Panoptics have 68°, Delos have 72°, Naglers have 82° and Ethos eyepieces have 100° or 110°.

Tele Vue Optics: Choosing Eyepieces
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