Following are the steps to check if lenses of nonprescription sunglasses are made properly :
1 Look at something with a rectangular pattern, like a tiled floor
2 Hold the glasses at a comfortable distance from your face and cover one eye.
Move the glasses slowly from side to side, then up and down, looking through
the lenses.
3 If the rectangular lines stay straight, the lenses are fine. If the lines are wavy or wiggle (especially in the center of the lens), try another pair.
