Binocular Diopter Adjustment
by Diane and Michael Porter
The diopter adjustment is a control knob on your binocular. It is designed to let you compensate for differences between your own two eyes. Once you set the diopter, then the two barrels should stay in proper relation. From then on you can focus just by turning the central focusing knob.
Sometimes people don't realize that their binocular has a diopter adjustment. If it gets turned, it can make the binocular unusable, as the two eyes can never focus at the same distance at the same time. (Sometimes a person thinks there's something wrong with the binocular, when all it needs it to have the diopter adjustment set properly.)
Diopter Setting Procedure
It's best to do this with both eyes open. Holding one eye shut will slightly distort the shape of the eyeball and temporarily affect its focus. To avoid this, use a lens cap to block one of the images.
1. Put the lens cap on the right barrel, or whichever barrel is the one affected by the diopter adjustment.
2. Use the center focus knob to bring a mid-distance object into sharp focus.
3. Switch the lens cap to cover the other barrel.
4. Without moving the main focus knob, turn the diopter adjustment so that the same mid-distance object is in sharp focus.
5. Look at the diopter setting scale, and note the setting.
6. Repeat steps 1-5 a couple of times to confirm the accuracy of your setting.
7. If they are your binoculars and they don't have a detailed scale, you might want to mark the setting with a Sharpie so you can re-set them again quickly.