Eye relief is the fixed distance from the curved surface of a telescope eyepiece’s outermost lens – the eye lens – to the point at which you can observe the whole field of view without vignetting – the exit pupil.
In a nutshell, eye relief refers to how far your eye must be from the eyepiece in order for you to see the entire field of view.
The longer the eye relief, the more comfortable the observing experience becomes.
If you suffer from astigmatism, wearing corrected spectacles while observing can generally be beneficial – but wearing spectacles forces your eye further from the eye lens, so spectacle wearers in particular should choose eyepieces with a long eye relief.
A bigger distance (called longer eye relief) is useful if you wear glasses.
Find out more in our beginner's guide to eyepieces.
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