|Variable diopter adjustment by wheel|
|Variable diopter adjustment by slider|
On a D-SLR, the diopter is typically built in to the viewfinder and can be changed by rotating the little wheel (or slider switch) next to the viewfinder (variable diopter adjustment). While this adjustment doesn't actually affect the focus of a camera, it will affect the focus if the diopter is not adjusted properly and you are manually focusing because it changes what you think is in focus. On a film camera, there could be a similar piece in the viewfinder like the D-SLR wheel or slider, or it could be an attachment that screws or slides on to the viewfinder eyepiece.
|Eyepiece that requires a slide-on diopter (rectangular)|
The diopter allows for fine focusing on the viewfinder, allowing the photographer a more accurate image before exposure. It can also allow the shooter to compensate for their need to wear glasses. For cameras with interchangeable diopter viewfinders, it is recommended to first determine if you will be shooting with or without your glasses, and which eye you put up to the viewfinder. Then, check with your optometrist or ophthalmologist for what the correct diopter is based on your eyesight. Ask them to determine your diopter from your corrective prescription when viewing an object at a distance of approximately one meter.
|L- A threaded screw-on diopter, R- a slide-on diopter|
Eyepiece diopters are specifically designed for different camera models (so one will not fit all) and are identified by a plus (farsighted), a minus (nearsighted), or a neutral numeric diopter value. The starting point, or base diopter value for most cameras is -1, which is the correct diopter for someone with normal vision. Other cameras, like many Bronica models for example, a -1.5 is the standard starting point. For Hasselblad cameras, it changes depending on which prism you are using, so double check on which starting diopter your camera has. If you need a +2 diopter, then you should just remove the standard or supplied (like the -1), and attach a +2.
|A -1.5 (standard) diopter on a Mamiya RZ waistlevel|
Neutral correction eyepieces do not replace the supplied standard eyepiece. The use of a neutral correction eyepiece will change the diopter value of the camera from -1 to a zero. Other common diopters that are available are: -5, -4, -3, -2, +0.5, +1, +2, +3.
There are a few other eyepiece accessories that you may want to consider using also. If you will be wearing your glasses while shooting, you may want to choose an eyepiece with a rubber coating, which is best because they are designed to lessen scratches to a photographer's eyeglasses. Another optional diopter accessory is for “anti-fog”, which prevents the viewfinder from fogging up and can also be useful.
To find diopters on keh.com, look under the category and brand of camera you have, then “Camera Accessories”, then “Prism and Viewfinder Accessories”.