ark@alice.UUCP (Andrew Koenig) (07/10/85)
> The Olympus OM-4 (and OM-3?) have built-in diopter correction for > eyeglass wearers so the camera can be used without glasses. When you look into the finder of a camera, you are looking at a "virtual image." This image APPEARS as if it is located some distance from your eye. This distance is chosen so as to enable most people, even if they wear glasses, to focus on it reasonably easily. For most cameras, it is about 1 meter. Diopters are reciprocal meters. In other words, if you look through a 1-diopter lens at an object one meter away, it appears to be infinitely far away. If you look through a -1 diopter lens at something at infinity, it appears to be one meter away. For +/-2 diopters, it's half a meter, and so on. The reason lenses are sometimes measured in diopters is that if you put several (thin) lenses right next to each other, you can calculate the effect of the result by adding the diopter numbers (more or less). Anyway, the typical camera presents a virtual image one meter away, so you can think of the eyepiece as being -1 diopter with an image at infinity. Thus, if you can see things clearly at great distances with an eyeglass prescription greater than -1 diopter (0 is greater than -1), you should be able to use most cameras without glasses. Most cameras also have auxiliary correction lenses available too. I have a friend with very thick glasses who uses a -3 correction lens in her camera eyepiece. She therefore takes her glasses off to use the camera. If I try to use that camera without first taking the correction lens off, instant vertigo results. The advantage of using a correction lens is that you don't need to worry about scratching your glasses on the camera eyepiece. The disadvantage is that you have to take the glasses off.