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

Diopter is a measure of a lens's optical power, which is determined by its focal length. Diopter is widely used for eyeglasses, where it is defined as the inverse focal length of the lens in meters. A higher diopter means 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 eyepiece (usually the right side eyepiece) marked with +/- that can be individually focused. This is the diopter adjustment (or simply diopter) for the binocular and can be used to adjust for differences between your eyes for greater viewing comfort and acuity. To use your binoculars' diopter:

  1. Set it at zero by placing the index mark on the focusing eyepiece opposite the zero or middle on the scale.
  2. Using the central focus knob that moves both eyepieces, focus on a distant object such as a leaf, rock, or star. Be sure it’s sharp in the fixed eyepiece.
  3. 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 time you'll have to adjust the diopter, and you can just use the center focus from now on. However, dark-adapted eyes tend be more near-sighted than daytime sight, so you may need to change the diopter slightly if you use your binoculars for both celestial and terrestrial viewing. Also, since everyone’s sight differs, anyone else using your pair of binoculars will have to make their own individual diopter adjustment.

Updated 10/21/13