cheap shades have inferior optical quality. The acuity of good lenses requires careful manufacturing control that includes "decentering" or tapering the lenses. Inexpensive plastic lenses will strain your eyes. You might not notice it at first but after a while, subtle fatigue or even severe eyestrain and headaches will set in.If your cheap sunglasses are polarized, they use a polarizing filter that is either glued or sprayed onto the lens. The filter is certain to detach, scratch, or delaminate. And the quality of the polarization will be poor anyway.Quality sunglasses provide all the protection you need, plus they have superb visual acuity. Wearing them is a pleasure. They're worth seeking out.