If you wear eyeglasses for near or far-sightedness, you won’t need them to look through a telescope. You just need to slightly tweak the focuser of your scope to compensate for your eyes. But if you wear eyeglasses for astigmatism, a condition which often afflicts older observers, then you may need to keep your glasses at the telescope. This makes it hard to get your eye close enough to the eyepiece to take in the whole field of view unless you choose an eyepiece specially designed with long eye relief.
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Eye relief is simply the distance (in millimetres) you need to hold your eye from the outer lens of an eyepiece to see its full field of view. Short eye relief means you have to jam your eye up close to the lens. This is always a problem with lower-cost eyepieces like Plossls, especially when they have short focal length.
Without glasses, 10-20 mm of eye relief is fairly comfortable. But if you need glasses at the eyepiece, look for eyepieces with at least 17-20 mm of eye relief.
Even if you don’t were glasses, your eyelashes sometimes brush against the the top lens of an eyepiece and they can leave streaks of eyelash oils that have to be cleaned off regularly. So eye relief is always a specification to keep in mind.
Fortunately, there are some good-quality long-eye-relief eyepieces on the market. The sub-$100 hardware from Orion/Skywatcher and AstroTech are worth considering, though they trade good eye relief for a modest field of view. The new Televue Delos eyepieces make no such tradeoff. You get 20 mm of eye relief with a 72 degree apparent field of view and good field flatness to the edge. The catch? These eyepieces are more than $350 each. Though they hold their value well.
If you’re new to the world of eyepieces, which are harder to choose than a telescope and just as important, here are some past articles to consider…