Invisible Lenses is a technique often used by animators when drawing eyeglasses which involves only drawing the frames of the glasses and not the lenses. This is most easily seen with partial-rimmed glasses, but can also be seen on glasses with full rims if no reflection is drawn on the lenses. It is much rarer on rimless glasses because it is impossible to show the shape of the lenses if there is no outline at all.
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Lenses drawn with minor reflections but no outline
Full-rimmed glasses with no reflection
Lenses with no outline or reflection