How can I adjust my binoculars so I see a good image without blacked-out areas?

April 1, 2009
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Considering how expensive binoculars can be, it’s odd that most companies don’t include operating instructions in the package. Using binoculars is like riding a bike —wonderfully easy, once you have the hang of it.

Before you try to see birds through your binoculars, you need to make a few adjustments. Virtually all binoculars have several helpful features that allow them to be tailored to different users. The eyecups hold the ocular lenses (the lenses you look through) exactly the right distance from your eyes (this distance is called eye relief), to optimize magnification and cut out peripheral light, making the image clearer and brighter. Extend the eyecups if you don’t wear eyeglasses. Since eyeglasses hold binoculars away from the eyes and let in peripheral light anyway, retract the eyecups if you do wear glasses.

Next, set the barrels of the binoculars to match the distance between your eyes. Looking through them, adjust the barrels until you have a solid image through both eyes. If the width isn’t set properly, your image will black out.

Virtually all binoculars on the market have center focusing, in which a single knob or lever controls the focus for both eyepieces simultaneously. Our eyes are seldom precisely matched, so to accommodate the difference between our two eyes, binoculars also have a diopter adjustment near the optical lens on one side or the other, or as part of the center focus knob. Diopter adjustments are normally numbered from +2 to –2. Here’s how to adjust the diopter so you can use your binoculars without eyestrain:

  1. First find the diopter adjustment and set it at zero.
  2. Find something a good distance away that has clean lines. A sign or something else with letters or numbers is often a good choice.
  3. Cover the objective lens (the large outside lens of the binoculars) with the lens cap or your hand on the side controlled by the diopter adjustment, and then focus on the sign using the center focus knob. Try to keep both eyes open as you do this.
  4. Switch hands, uncovering the lens with the diopter adjustment and covering the other lens. Focus again, this time using the diopter adjustment, not the center focus.
  5. Repeat a couple of times to make sure. After you’re done, your sign should be crisply focused through both eyes.
  6. Notice the number setting on the diopter adjustment. Sometimes during normal use, the adjustment knob may get shifted, so every now and then when you start using them, check to make sure it’s set where it should be for your eyes.

Finally, make the neck strap as short as it can be while still allowing you to use the binoculars comfortably and put them over your head easily. The longer the strap is, the more the binoculars will bounce, and the greater the chance you may bonk them against rocks, tables, and other objects whenever you bend down.

If you’re new to birding, watch our free how-to video series, Inside Birding, to get started on identifying birds with confidence.

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