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To get a good idea, this is generally the width of standard lenses.

Position the side of the card on the center of your nose and ask: where does the other side of the card end?

A. If the card’s side is even with the end of your eye, you wear a medium.

B. If the card’s side goes past the end of your eye, you wear a small.

C. If the card’s side does not reach the end of your eye, you wear a large.