We run into the term "diopter" in various areas of our work, often (I'm afraid) applied inappropriately. I thought it might be good to talk about this term.
The diopter is the unit of lens power, where power is the reciprocal of focal length. The power in diopters is the reciprocal of the focal length in meters.
Don't try to confuse power with magnification or anything similar. It is mainly a concept useful in optical design and in ophthalmology (vision correction).
We can of course state the power of an ordinary camera lens (50 mm focal length = 20 diopters power) but there is rarely any point to doing so.
Supplemental closeup lenses
Supplemental closeup lenses are rated in diopters. Typical ratings are +1, +2, +4, etc. diopters. These lenses, added to the front of our "main" lens, provide for an increase in the maximum available magnification. They basically do this by providing for focusing at a closer minimum distance. (They also decrease the focal length of the lens combination.)
It is unfortunately all too common to call these lenses "diopters". That's a little bit like calling boxes of corn flakes "ounces". The simpler kinds of such lenses, with a single element, are also often called "closeup filters", presumably since they look a bit like a filter, but of course they aren't.
Note that for the Canon supplemental closeup lenses (250D, 500, and 500D), the numerical part of the model designation is the focal length of the lens in mm; Thus the 250D is a 4-diopter lens, and the 500 and 500D are 2-diopter lenses.
Vision correction for viewfinders
Eyeglass lenses are specified in terms of their power in diopters. (That is what the numbers are on your eyeglass prescription). Farsighted people need a converging (net convex) lens to help them focus on near objects. Such lenses have a positive power. Nearsighted people need a diverging (net concave) lens to help them focus on distant objects. Such lenses have a negative power.
Often persons using eyeglasses would rather not wear them when looking into a camera viewfinder eyepiece. For that reason, many cameras have viewfinders with a "vision adjustment" control. Moving this control has the effect of placing a corrective lens (of adjustable power, including the sign of the power) between the "ideal" viewfinder and the viewer. This takes the place of the viewer's eyeglass lens.
The range of this correction is specified in diopters, perhaps "from -3 to +1 diopter".
What do I mean "ideal" viewfinder? In such a viewfinder, the image would appear to be (from the standpoint of the focusing of the viewer's eye) at infinity.
Typically, the normal, or default, setting of the vision correction system is about -1 diopter. Why is that?
In fact, at this setting, the finder image appears to lie not at infinity but rather at a distance of 1 meter (about 39 inches). Does this mean that the camera designers assume the "normal" user is rather nearsighted? Not quite.
Even for viewers with "normal" vision, the eye's focusing mechanism is more relaxed somewhere in the middle of its focusing range - not at infinity. Thus, if the default setting of the viewfinder vision correction system produces an image closer than infinity, even the viewer with "normal" vision will have a more comfortable experience.
Unfortunately, the viewfinder vision correction adjustment is often called the "diopter" setting, or just the "diopter", as in: "I had trouble seeing clearly in the the camera viewfinder all morning, until I realized that while my wife was using the camera she must have moved the diopter."
Well, that's it for now. Carla tells me that my ounces are ready.