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The Binocular Advisor

Binocular Tuneup
The Diopter Adjustment

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.)

Here is how to set your binocular's diopter adjustment.

Diopter adjustment on the right eyepiece

You use the binocular's central focusing knob to focus both barrels at the same time. Then, to adjust for differences between your eyes, you use the diopter adjustment one time to fine-tune the focus for the right barrel only. From then on the two sides will stay focused together, whether you're looking at objects near or far.

Start by placing the diopter setting at the zero, or center, position. On some binoculars, you'll see a zero to show the center of the diopter adjustment scale. On others there may be a different symbol to indicate the central position. You can turn the ring to the left or the right of the center position.
Diopter Setting
Cover the barrel which the diopter adjustment affects, (which is usually the right side).  It's best to use a lens cap. If you haven't got a lens cap, you can tape a dark card over the lens, or ask a friend to cover it with his or her hand.
Lens cap
Look at an object in the middle distance (about as far away as the house across the street).  Pick something that won't move, such as ceramic turtle. Keep both eyes open and focus the binoculars, using the regular center-focus knob, until the image is clear and sharp.
Left eye looking
Don’t just close one eye and squint!  When the eye is squeezed closed, the pressure on your eyeball temporarily changes its shape and makes it focus differently.  That can throw your adjustment completely off. Keep both eyes open. Focus
Once you have the focus correct for the left eye, you can use the diopter adjustment to focus the other eye. Be careful to keep the central focus wheel in the same position you just set. Now switch the lens cap to the other side, so that you will be able to see through the other barrel.
Change the cap
Stay in the same place. Look at the same object as before. Although you're keeping both eyes open. you're seeing with your right eye only.
Right eye looking
To fine-tune the focus for the right eye, turn just the diopter setting back and forth, until the object is in sharpest focus.  Don't turn the center focus wheel at all.
Focus diopter

When the image through the right eye is sharp, remove the lens cap and look through both eyepieces at once.  The image should be in sharp focus in both eyes, and your eyes should feel comfortable while looking at it.

Both eyes looking

Diopter adjustment on the center column

Some binoculars have the diopter adjustment on the center column. The technique for customizing the diopter adjustment is essentially the same:

  • Keep both eyes open but cover the right lens with a cap.
  • Focus on a middle-ground object, using the central focusing ring.
  • Change the lens cap so that you see through your right eye.
  • With both eyes open, and staying in the same position, focus on the same object by using the diopter adjustment on the central column.
  • Remove the lens cap and enjoy the matching view through both eyes at once.

Equipped with this understanding of the diopter adjustment, you can allow your binocular to do its best for you.

Diopter locking mechanisms

Some binoculars (especially high quality, expensive ones) have locking mechanisms to prevent the diopter setting from getting turned accidentally.

Copyright 2006 Michael and Diane Porter

Other optics articles of interest:

Birding Optics

10 Myths of Birding Optics

Birding Binoculars and How They Work

The Binocular Advisor

Binocular Picks for Every Price Range

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