The unit of measure in optics
A dioptre measures the optical power of a lens. The measurement is equal to 1/[lens focal length in meters]. Thus, the dioptre is a unit of reciprocal length. For example a three dioptre lens focusses at one third of a metre. A four dioptre lens focusses at quarter of a metre. In calculations the dioptres roughly add together. A two dioptre lens and a three dioptre lens makes a five dioptre lens.
The dioptre can also be used in the calculation of the curvature of a lens although this use is unusual in discussion of lenses in a photographic context.
The dioptre is not included in the Standard International SI-metric system of measurements. As such, the measurement has no official international designation, abbreviation or name. The unit is however recognised by name as a dioptre or diopter and used widely since it is easy and effective to use in general calculations.
The Dioptre dial on a camera
As part of the DSLR camera viewfinder there is a small adjustable dial. It is normally found incorporated into the viewfinder fitting. This is the dioptre. It adjusts the viewfinder sharpness for your eyesight. The dioptre controls the viewfinder focus. It’s function is to ensure you adjust the focus of the viewfinder so you can see through it properly. Adjustment does not affect the photographic lens focus or any other aspect of the final image.
The camera dioptre is limited in its ability to compensate for eyesight problems. It allows for a small amount of focus control for normal eyesight or for people who normally wear average prescription glasses. However, it cannot compensate for severe sight problems, rare conditions, complex problems or eyesight distortions. However, when used in conjunction with your prescription glasses it may help you match your eyesight and glasses to the focus for the viewfinder to help you see through it properly.