How to Find Magnifying Lens Strength
Magnifying glasses work because they have a curved lens which bends the light between a viewer and an object. The degree to which the light is bent, referred to as the optical power, directly affects the strength of magnification. The magnification power therefore has a direct relation to the curvature of the lens and an indirect relation to its size. If you are unsure of the strength of your magnifying glass, with a few quick measurements and calculations you can figure it out.
Choose any word from the newspaper and hold your magnifying lens above it so that you can see it through the lens, keeping the lens at the closest distance to your eye at which you can still see clearly and comfortably.
Find the focal length by moving your magnifying glass various distances from the newspaper word until you find the distance at which the word appears the clearest. Measure the distance between the lens center and the newspaper word.
Measure how far you kept the magnifying glass from your eyes.
Plug the numbers into the following equation, which uses f as the focal length, D as the distance between your eyes and the lens, and MP to represent magnifying power. MP = D/f