What is diopter and why is it important? How do I focus my binoculars?

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	What is diopter and why is it important? How do I focus my binoculars?

Answer Diopter is a measure of the optical power of a lens, which is determined by lens focal length. It’s widely used for eyeglasses, where it is defined as the inverse focal length of the lens in meters. A higher diopter is a stronger lens and correction.

Each eye is slightly different. Usually this isn’t noticeable, as the brain’s fusion of sight from each eye compensates for slight differences. When using binoculars, each eye looks through a different optical system and the magnification of the optics exaggerates the differences. Any differences there are may lead to eyestrain.

Many binoculars will have an individually focusing eyepiece (usually the right side eyepiece) that is marked + / -. This is called the diopter adjustment (or diopter) for the binocular and can be used to adjust for differences between your eyes for greater viewing comfort and acuity.

To use the diopter adjustment, first set it at zero by placing the index mark on the focusing eyepiece opposite the zero or middle on the scale. Using the center focus knob that moves both eyepieces, focus on a star. Be sure it’s sharp in the fixed eyepiece. Then turn the diopter adjustment until the other eye, too, has a sharp view.

The number on the scale is the diopter correction for that eye relative to your other eye. It’s probably close to what an eye doctor may have measured for your eyes.

Usually this is the only adjustment you’ll need to make, meaning you can just use the center focus after making your diopter adjustment. However, dark-adapted eyes tend be more near-sighted than daytime sight. You may need to change the diopter slightly if you use your binoculars for both astronomy and general purposes. Since everyone’s sight differs, anyone else using your pair of binoculars will have to make their own diopter adjustment.

Article Details

Article ID:

2084

Created On:

Nov 18 2008 01:56 PM