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The Space Between

Setting Interpupillary Distance

by Michael and Diane Porter

Birder

Here's a tip on customizing your binocular for your own eyes.

But before you even look through your new binocular, first adjust the eyecups to suit whether or not you wear glasses.

If you wear glasses, put the eyecups down, in the short position.

If you do not wear glasses, pull or twist the eyecups out, so that they are in the long, extended position.

Now you're ready to set the proper distance between the two barrels of the binocular. The goal is to have the center of the left and right eyepieces directly in front of the pupil of your left and right eyes.

But people's eyes differ. Some are wide spaced and others are closer together. So binoculars let you adjust the interpupillary distance to match the distance between the pupils of your eyes.

Here's how to do it. Don't worry about focus yet. We're just setting the distance between the two barrels.

 

In the movies, when they show what someone sees through a binocular, they almost always get it wrong. They show a sort of dumbell shape, like the one below.

Binocular View - Wrong
WRONG

 

Hold the binocular up to your eyes and move the barrels closer together or farther apart until the two images become one circle. 

What you see should look something like the circle below. One image. One circle.

Correct Binocular View
CORRECT

When you see one circle, you have the interpupillary distance set correctly. That's all there is to it.

Now you're ready to focus your binocular. If you find that you can't focus both eyes at the same time, you may need to set your diopter

Text and photos copyright 2007 by Michael and Diane Porter.

 
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