What’s the difference between two Plössl eyepieces?

I bought exactly the same Plössl eyepiece that a buddy has, but mine doesn’t work as well as his. Did I get a lemon?

That’s possible, but very unlikely. Our guess is that your telescope has a short focal ratio, perhaps f/5 or less, while your friend may have an f/10. Plössl eyepieces are good all-around performers, producing sharp images at the center of the field, but they have only four lens elements. Better edge correction with a short-focus telescope is one of the things you pay extra money for, and sophisticated eyepiece designs have as many as eight elements. An eyepiece that performs superbly on an f/10 Schmidt-Cassegrain telescope might not be such a winner on an f/4.5 Dobsonian reflector. As always, your best bet is to try an eyepiece on your own scope before purchasing it.

— Gary Seronik