Diopter:
The diopter is the unit used to express the power of magnifying glasses. The power of a lens in diopters is the reciprocal of its focal length in metres, i.e., a 1 diopter lens has f=1m, a 2 diopter lens has f=0.5m, a 3 diopter lens has f=0.333m, and so on. The neat thing about diopters is that if you stack several lenses, the new magnifying power can be obtained simply by adding together the diopters of the individual lenses; i.e.,

D = D1 + D2 + ....... etc.

This formula holds good only if the lenses are close together, i.e., well inside each other's focal distance.

Thus, if you stack a 1 diopter lens with a 2 diopter lens, you get a 3 diopter lens; but that doesn't mean it's a good idea. Stacking lenses like this is a useful practice while you're trying to determine the magnification power needed for a particular job; but if you want best corner to corner image sharpness, you should then obtain a single lens of the right power.

Lenses which are overall convex (bulgeing) have positive diopters. Lenses which are overall concave (indented) have negative diopters. The low-cost close-up lenses designed to screw into the camera filter ring are usually convex on one side and concave on the other, but the convex part is more curved than the concave part, so overall, it's a magnifying glass. This concave-covex structure is called a `meniscus' lens, and is used because it gives the best minimisation of chromatic aberration for a lens constructed from a single piece of glass. Needless to say, adding such a lens to your camera causes a reduction in optical resolution. Far superior results can be had, at a price, by using two-element achromatic close-up lenses, such as those manufactured by Nikon and Canon.