Diopters are optical elements that can be placed in front of a prime or zoom lens, offering a fast and convenient way to increase the lens' magnification. In practical terms they move both the close and far focus distance of a lens closer to the film plane; on one hand the lens can focus closer than its native close focus distance, but on the other the far focus moves from infinity to a closer value. The strength or magnification of a Diopter is expressed in the 'diopter power' number. Common diopter powers are +0.5, +1 and +2.

There are many uses for diopters, the most popular being: When a static shot of something small is needed, but the close focus distance of the lens is insufficient. For best optical quality at the highest magnification, use a single low power diopter with a long focal length prime lens. This will also leave ample room for lighting. Using a diopter on a wide angle lens reduces the close focus distance by only a small amount but throws the background substantially out of focus, thus achieving a unique wide angle look with shallow depth of field. A shot where the focus moves from something very close to the lens to a medium distance or vice versa. Special effects work.

With a diopter attached, the regular focus scale of that lens is not valid anymore, and focusing has to be done through the viewfinder.

Having a set of diopters is standard procedure on many projects. They are used instead of macro lenses because they are less expensive and can thus be rented for the whole show. If an extreme close up is planned, a macro lens is rented, but often the situation arises where an unplanned close up is needed, and that is when a diopter can save the day. One cinematographer interviewed said: "A craftsman never knows when he needs a hammer, but he always carries one. It is the same with diopters."

Some cinematographers like diopters because they match the look of the prime lens more than the traditional macros in terms of color balance and in terms of focal length. Also, traditional macros need more light than regular primes, and thus a scene would have to be re-lighted for a macro shot, which costs time and money and again results in a different look. Diopters need no exposure compensation, so a T1.3 lens stays a T1.3 lens.

The use of diopters is subject to local shooting styles just like the use of all other film equipment. Cinematographers on one continent, for instance, prefer to use them with long primes, as that gives a higher magnification ratio and allows for more space between subject and lens. Another continent is currently going through a phase where "close and wide" is popular, so they use diopters with wide angle primes and short zooms.

Diopters come in two basic types: regular Diopters that consist of one simple glass element, and achromatic diopters that consist of two or more glass elements. The optical quality of regular Diopters is poor, mostly due to chromatic aberration, also known as color fringing. They also show spherical distortion, geometric distortion, flare, poor contrast and poor edge resolution. Achromatic Diopters consist of at least two glass elements and are much higher in quality.