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exit pupil diameter

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exit pupil diameter

Postby dga on Fri Mar 31, 2006 2:33 am

as a follow up question i forgot to ask in the previous topic, i noticed when at a local sporting goods store that had several spotting scopes on a rack for viewing, giving the same objective lens (say 65mm) and at the same zoom (20x) i noticed some had a better field of view (i dont know if this is the correct terminology) and others you had to put your eye right on the lens cup to see the entire image. what is the term for this, and what makes some scopes have a larger image than others? a couple of less inexpensive scopes "seemed" to be a better image than the more expensive scopes, given the same parameters that i could adjust. the sales staff at the sporting good stores are much less knowledgable than the information i read from the support here. hence my question. thanks, dan.
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Postby on Fri Mar 31, 2006 10:19 am

Hi Dan

If two spotting scopes are set at the same magnification and both have the same objective (front lens) size, they will have the same exit pupil size, so I don't think this is what you are dealing with.

What you are experiencing is more likely an eye relief issue. The distance your eye can be from the lens and still see the entire field of view is a spec known as eye relief. An eyepiece with poor eye relief requires you to have your eye close to the lens to see the entire field of view. An eyepiece with long eye relief allows you to place your eye farther from the lens and still see the entire field of view. Eye relief in an eyepiece is a matter of the eyepiece design being used on a particular model.

A related issue is field of view. Field of view is the width of landscape you see when looking through the eyepiece. In general, as magnfication goes up, field of view goes down, but the specific field of view of any eyepiece is again a matter of eyepiece design. Zoom eyepieces typically offer a narrower field of view than a fixed power eyepiece at the same magnification. Field it's one of the features you sacrifice with a zoom to get the zoom function.

Let me know if you have other questions. Always glad to help.
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