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Given the rather huge price differences between eye pieces at the same focal length. How exactly does the AFOV affect the view seen through the eyepiece?

Are higher / lower AFOV better for certain situations? or is higher always better?

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AFOV = angular field of view –  JoeHobbit Jun 3 '11 at 0:15
Actually "AFOV" can stand for "apparent field of view" or "actual field of view," two quite different things. See my answer below. –  Geoff Gaherty Jun 18 '11 at 0:19

2 Answers 2

up vote 4 down vote accepted

Simply, a larger FOV provides a wider hunk of sky in the eyepiece. This is useful for capturing entire objects at higher magnifications than could be obtained with cheaper eye-pieces. It doesn't provide more light, just a wider view. As for better, if you've gone to the expense of buying one for a given focal length, it'll still work just as well as something cheaper so may as well use it.

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So from this answer I can infer two advantages from a larger FOV. 1) seeing entire DSOs at higher magnifications 2) for non-motorized scopes less frequent adjustments due to the object you're observing moving out of the FOV. –  Patrick Ritchie Jun 3 '11 at 2:33

There are actually two different fields of view to consider. The apparent field of view is the apparent view you see in the eyepiece, typically 35° to 110°. The actual field of view is how much of the sky you are actually seeing, typically from a few degrees to a few arc minutes. The actual field of view is the apparent field of view divided by the magnification.

The main advantage of a wide apparent field of view is the feeling of immersion in the view and presenting the object in context. Typically eyepieces with apparent fields of view in excess of 80° are described as giving a "moonwalk" experience. With these eyepieces you usually can't see the edge of the field of view, so it's like sticking your head out a window, rather than seeing the view framed in a window.

A secondary practical advantage of eyepieces with wide apparent field of view is that they are more versatile than eyepieces with a narrower view. A wide angle eyepiece typically takes the place of at least two narrower angle eyepieces, giving you a wide context and fine detail at the same time. I find that I can do most of my observing with only two or three wide angle eyepieces.

Eyepieces with narrow fields survive because they are less expensive, and also because their simpler optical design (fewer lens elements) allows more light throughput and better contrast. Serious planetary observers typcally use orthoscopic or monocentric eyepieces because of their high contrast. Planets are small in angular size, and don't need a wide field of view.

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