What is eye relief?

[CJ]

Typical beginners question, I imagine. What is eye relief? I've seen it mentioned often and have a vague concept but would like to understand it exactly.
Thanks

[ColHut]

Very roughly it's the distance your eyeball has to be from the surface of the eyepiece in order to see the image properly. If it's too long it's hard hold your head in the right place, and if too short it may be uncomfortable or if you need to wear glasses impossible to see the whole image.

regards

[Merlin66]

Chris,
The light that exits the eyepiece forms an "Exit Pupil".
The diameter of this exit pupil is the aperture of the telescope divided by the magnification. For,say, a 200mm scope with x50 magnification the pupil will be 200/50 = 4mm diameter.
This same exit pupil is being formed across the whole angular field of view and at a specific distance they all "cross" behind the eyepiece. This distance is the "eye relief" of the eyepiece.
When the eye is positioned at this point you can see the whole field of view without vignetting, "kidney bean" or shadowing. The eye relief distance can vary with the design of the eyepiece, hence some eyepieces are more suited to observers who wear glasses.
Hope this helps.

[wasyoungonce]

Televue have a good primer on things like this, see here (http://www.televue.com/engine/TV3b_page.asp?id=79). Some of the links at the top of this page "advice"; "eyepiece calculator" are very good as well.

:thumbsup:

[Scorpius51]

Thanks Ken, Colin & Brendan

This is a question that I'd never asked. I've just found a good graphic to support that explaination, courtesy of Wikipedia, where

1. Real image
2. Field diaphragm
3. Eye relief
4. Exit pupil

Cheers
John

[Merlin66]

John,
Good illustration, but the image doesn't open for me.....

[CJ]

Thanks. Great info. A picture paints a thousand words. Often far more.

Next question! I've often seen the term "comfortable eye relief". Indeed, it's mentioned in the Barlow section on the Televue page referred to in Brendan's link. Is this purely a subjective thing or some mathmatical value?

[Scorpius51]

It opens for me, but I have just changed it to a JPEG format - it was PNG. Maybe that was the problem?

John

[Merlin66]

Hmmm
The one click open.....doesn't work for me....