About Binocular Focus
While a simple, inexpensive pair of binoculars may only have one focus adjustment to blur or sharpen both barrels at once, most binoculars have both a dual adjustment and a diopter adjustment ring that will focus a single barrel. This is essential for the clearest images, because even a birder with excellent vision will have different visual acuity in the right and left eyes. The diopter adjustment, then, allows for focusing with a single eye to calibrate the binoculars for that difference in acuity. After they are properly adjusted, the central focus wheel will adjust both barrels simultaneously to maintain that optimum adjustment for viewing at different distances.
Focusing Your Diopter Adjustment
For the best possible images with your binoculars that will give you the sharpest looks at birds and the tiny details necessary for proper identification, it is essential to focus the optics’ diopter. While different binocular models may have slightly different focusing methods, the basic technique is the same for all binns.
Find the diopter adjustment. For most binoculars, the adjuster knob is integrated into the eyepiece of one barrel, most commonly for the right eye. If the diopter adjustment knob is on the center focus wheel, it may snap in or out to make the adjustment. For a central diopter, check the binoculars’ instructions to learn which barrel is affected as this adjustment is made.Set the diopter adjustment to zero before beginning to focus. This will ensure the best possible focus result that is tuned for your eyes and your vision needs.Close the eye or cover the barrel that includes the diopter adjustment knob. Look only through the other barrel with the appropriate eye, as this adjustment will not work if you use the right eye to look through the left barrel or vice versa. Covering the barrel may be easier if you have trouble closing just one eye, as it is important to avoid squinting that would change the shape of your eyeball and affect the necessary focus.Use the center focus wheel to get a clean, crisp image, focusing on an object 20-30 feet away. Choose an unmoving object in good light to ensure the best focus.Uncover the diopter adjustment barrel (or open your eye) and cover the opposite barrel or close the opposite eye. Looking through the appropriate barrel with the appropriate eye, use the diopter adjustment knob only to change the image focus until it is as sharp and clear as possible for that eye.Uncover both barrels and use both eyes to check the overall focus. Use the center focus wheel only and check objects at different distances and in different types of light to verify the image clarity. If necessary, repeat the previous steps to improve the total focus.Lock the diopter adjustment knob, if possible. Expensive binoculars may have a locking mechanism on the knob, while other models may just have the knob snap back into place. If the knob does not lock, you may want to make a small mark with a permanent marker (or use a bit of white paint or pale ink on black or gray binoculars) to show where the optimal adjustment position is to refocus the binoculars quickly if they get misaligned.
Are Your Binoculars Still Blurry?
If you have carefully aligned your diopter for the best image but still have trouble with blurry images, there may be other ways to improve the focus of your binoculars.
Check the distance between the eyepiece and your eyes. If you wear glasses, you will want to twist or fold any eye cups down to bring your eyes closer, and if you do not wear glasses, verify that those eye cups are raised. Check the distance between the eyepieces to align the barrels with the distance between your eyes. Some binoculars “fold” to adjust this distance, but do not try to force motion; instead, consult the instructions for your optics to make this change if you have trouble seeing a full image with both eyes. Study the light conditions while you are viewing birds. Shadows, rapidly changing light at sunrise and sunset, or poor weather such as mist or fog can all affect how you see birds. The image may appear to be blurry to you, but the conditions themselves are blurry, not the optics. Inspect your equipment for any smudges, dust, dirt, or fogging on the lenses or eyepieces, and clean the binoculars if necessary. A tiny smudge will not account for a very blurry image, but dirty binoculars cannot present the best possible images. Check the distance to the bird. If the bird is only a few feet away, the close range of your binoculars may not be suitable to focus correctly. Similarly, if the bird is at a great distance, it may be too far out of range without a greater magnification. Consider changes in your eyes. As we age, our visual acuity changes and we may have more difficulties seeing objects at different distances or with the same amount of detail that we’re accustomed to seeing. If you use vision correction such as glasses or contact lenses, readjust your binocular focus whenever your prescription has changed to ensure the best possible bird views.
Birding binoculars are expensive, and knowing how to use them properly begins with adjusting the focus to suit your eyes. If you have the diopter adjusted correctly and there is no other explanation for a blurry image, contact the optics’ manufacturer about the possibility of a repair or replacement.
