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Yes, that's the tricky bit. If you have an astigmatism, or if one eye is worse than the other, it gets more difficult.

The one eye vs. the other is not a big deal... you just cover one eye with a patch or your hand and look through individual lenses with the other....

However, the astigmatism part... hmmm...

Wonder if I could invent a home eye-test machine? :D

For the most part, they're not that complicated in the optician's, just a higher tech looking version of covering one eye and trying the different lens with the other. Though I admit, I don't know how they test for astigmatism.