Eye relief: Hold an eyepiece up to a light (other than the Sun) and bring
it close to your eye. The distance between your eye and the eyepiece at the point
where you first see the entire black circular field stop (allowing you to see the
maximum field of view) is the eye relief.
Eye relief should be fairly long for comfortable viewing, particularly if you must
wear eyeglasses, where you will need a minimum of 15mm of eye relief to see
the entire field of view. Even if you don’t wear glasses, very short eye relief
can allow your eyelashes to touch the eyepiece, transferring oils from your lashes
to the lens. This can eventually damage eyepiece coatings, so examine your eyepieces
frequently and clean them as needed.
Eye relief usually decreases as eyepiece focal lengths get shorter, so if you wear
glasses you may want to use a Barlow lens in conjunction with a longer focal length
eyepiece to get both satisfactory eye relief and high power. In any case,
if you wear glasses, your inability to get your eye close to the eyepiece will not
result in out-of-focus images, merely a narrowing of the field of view.
Manufacturers measure eye relief from the last surface of the eyepiece eye lens
(the lens closest to your eye) to where the image is formed. While this is the technically
accurate way to measure eye relief, it can often lead to misleadingly long eye relief
figures. Since the eye lens is usually recessed within the eyepiece body to protect
the lens from damage, you can’t use that portion of the eye relief that’s recessed
within the eyepiece body. To calculate the usable
eye relief, we measure the eye relief from the rim of the eyepiece or eyeguard (where
an observer's eye or eyeglasses touch, which limits how close you can get to the
eye lens) to where the image forms. Accordingly, our eye relief figures are usually
shorter than the manufacturers' technically correct measurements, but we feel our
figures are more realistic.