How to set the Diopter adjustment on binoculars

Get the best from your binoculars!

Agent 74 gives a step by step guide to setting the diopter adjustment on your binoculars.

On all binoculars of reasonable quality there will be a diopter adjustment, this adjustment is to compensate for any difference between the users two eyes. It is important to set the diopter correctly both for the benefit of achieving the best image possible and more importantly to prevent eyestrain and discomfort. In most cases the diopter adjustment can be found on the right eyepiece, but occasionally it will be on the left. Once set correctly, the diopter will need no further adjustment and all focusing will thereafter be done using the centre focus wheel.

Should the diopter become adjusted incorrectly without the user realising, it can make the binoculars appear faulty, because the two eyes will never be able to focus at the same distance simultaneously. Obviously if two people are sharing a pair of binoculars then the diopter should ideally be reset for each user.

How do I set the binoculars diopter adjustment?

(The following instructions are for binoculars with the diopter adjustment on the right lens)

  1. Start with the diopter adjustment in its central position (sometimes there will be a zero or an identifying mark to show where the central position is)
  2. Cover the right barrel. It’s best to use a lens cap. If you haven’t got the lens cap, you could tape a piece of card over the lens, or cover it with your hand.
  3. Look at an object in the middle distance (about 50 to 100 metres away). Pick something that is stationary and with some detail, and whilst keeping both eyes open and using the centre-focus wheel, focus the binoculars until the image is clear and sharp. Once the focus is correct for the left eye, you need to focus the right eye using the diopter adjustment.
  4. Making sure you do not alter the central focus wheel, swap the lens cap to the left side, so that you can see through the other barrel. Keeping the same position observe the same object as before. If the view is not as clear and sharp as it was previously you need to fine-tune the focus for the right eye using the diopter adjuster back and forth, until the object is in sharpest focus. Do not adjust the centre focus wheel.
  5. When the image through the right eye is sharp, remove the lens cap and look through both eyepieces simultaneously. The image should be clear and sharp and in focus for both eyes, and you should feel no discomfort while viewing. Your diopter is now set and the binoculars are ready for use.

NB. If the diopter adjustment is on the left eyepiece then follow the steps above but cover the left eyepiece first and adjust the right with the centre focus wheel. Then cover the right and set the left using the diopter.

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