Thursday, April 19, 2012

Diopters

What is a Diopter? The technical definition is “A unit of measurement of the refractive power of lenses equal to the reciprocal of the focal length measured in meters”. The term diopter is used mainly in optometry, as a term for a corrective lens. It is also sometimes used as a measurement of curvature, and in the relation to magnifying power. But what is a diopter when we talk about them in photography? The term can refer to two different things, but for either application, it is essentially a piece of glass used in focusing. The most common diopter on a camera is part of the eyepiece on a viewfinder of a camera that adjusts the focus based on the users eyesight. The other type of a diopter in photography is also sometimes referred to as a close up lens, which is something that allows you to focus at a closer distance, almost like a macro lens. For the purposes of this article, we will be talking about the diopters used in conjunction with viewfinders.

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 screw-on diopter (circular)

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”.