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I have to wear glasses while observing with longer focal lengths (15mm+) due to astigmatism. Also I wear a transitional glasses for long vision and reading. They are almost unusable for observing so I have to switch to non-transitional glasses. And because of glasses I need at least 16mm of usable eye relief on a eyepiece. "Usable eye relief" = from the very top of the rim and not from the eye lens (this is the one makers specify). So choosing eyepiece is difficult.
If you don't have astigmatism or it's very minor then you might not need eyeglasses with eyepieces.
To identify what max exit pupil you can use for observing without being affected by astigmatism you need to use the following formula:
Max Exit Pupil = SQRT (2.3 / Cylinder)
Where Cylinder is a positive value of your "Cylinders" in diopters from your eyeglass prescription.
In my case:
My cylinders: -0.75D (while my eyesight is -6.5D)
Max Exit Pupil = 1.75mm = SQRT (2.3/0.75)
If I have 100ED f9 then it's 15.8mm eyepiece. Which is pretty much true. I need glasses if I'm using any eyepieces longer than 15mm and don't need glasses if shorter than 15mm.
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