With Diopter adjustment is referred to the possibility of optical devices to compensate for ametropia of the eye with the use of eyepieces by correcting the refractive power.
The correction of the refractive power is usually achieved by a mechanical adjustment of the focus at the eyepiece.
The amount of correction is usually just as the power in the unit of measure specified diopters and is usually between -3 and 3 diopters. A dioptric adjustment of zero means that a human eye with normal power provides a sharp image. Positive numbers increase the diopter power for far-sighted, and negative diopter numbers reduce the power for near-sighted.
A typical problem arises with binoculars, such as binoculars or stereoscopes, if both eyes have different refractive power. Here an eyepiece is often provided with a dioptric adjustment so that the difference for the corresponding fuzzy seeing eye can be compensated, if it has been in focus for the other eye.
Eyepieces with dioptric adjustment but are also used in some monoculars, such as that of higher quality viewfinder cameras when here, the optical image of the lens can not be set for the observing eye, but for a photographic film or an image sensor. With SLR cameras, the photographer looks through the eyepiece, for example via a pentaprism on a built- dial. Depending on the refractive power of the eye can be brought into focus with the diopter compensation the viewfinder for the photographer on the eyepiece without the lens or the focusing screen must be adjusted.
For devices with an electronic viewfinder, such as mirrorless system cameras, the image of the lens is not imaged by an optical beam path in the viewfinder, but comes only indirectly from a small electronic screen. In this case, the viewfinder image will be in focus, for example, by varying the distance between the screen and the eyepiece.
By using the diopter, it is also many eyeglass wearers possible to use without glasses and without any loss of image sharpness or the visible angle of view eyepiece.
For optical devices with high-resolution electronic viewfinder the many benefits when live view or as servicing adjustments can be accomplished even without glasses using the viewfinder.