The intrinsic value of a fine binocular lies in its ability to produce bright, sharp, aberration-free images of viewed objects, which can be seen and fully appreciated from a distance only when the binocular’s two optical systems are correctly focused. This process is complicated by the fact that, besides making allowances for viewing distance, the binocular must also adjust for abnormalities that might exist in the viewer’s eyesight.
Interestingly, about 90 percent of the people who own binoculars don’t know how to focus them properly. This is particularly true of center-focus models. Without proper focusing, the best binoculars have no advantage over the worst. The following information will help you get the most from your hunting binocular.
1. Set Eyepiece Spacing
The distances between the pupils of human eyes, called interpupillary distance (IPD), can vary greatly, ranging from about 55mm to 75mm, with the average being 66mm. Begin by holding the binocular in a normal viewing position. Then, while grasping the barrels firmly with your hands, move them either closer together or farther apart until the images seen by your eyes form a single circular field of view. If the binocular has an IPD scale, note the setting for future reference. In any event, you’ll need to readjust when glassing nearby objects. With practice, you’ll be doing it automatically in a couple of seconds.
2. Choose Eyecup Height
Adjustable eyecups, which are common on most modern binoculars, serve the dual purpose of excluding extraneous sidelight and positioning the eye pupils the correct distance from the eyepieces so as to see the instrument’s full field of view. The commonly used types of eyecups include rubber roll-down, pop-up or twist-up. Normally, the eyecups should be in the “up” position for non-eyeglass wearers and in the “down” position for eyeglass wearers. Conformity aside, they can be positioned to fit your needs, which is especially easy with modern eyecups that have intermediate positions.
3. Focus the Lenses
There are three major types of focusing systems: 1) center focus with diopter adjustment; 2) individual focus, in which each eyepiece is focused separately; and 3) fixed focus, which is permanently prefocused for a specific viewing distance. Center-focusing binoculars are the most common type. Beside the center focusing wheel, there usually is a separate diopter adjustment that serves to compensate for unequal vision that might exist between the viewer’s two eyes. The diopter adjustment, which often has a scale showing plus (+) and minus (-) diopter settings on opposite sides of a zero (0) marker, might consist of a focusing ring located on the right eyepiece or it might exist as a cleverly disguised but separate function of the central-focusing system.
Diopters in these positions will adjust only the right optical system. However, on some models, such as Steiner’s center-focus binocular and the majority of zoom-type binoculars, the diopter setting is located on the left eyepiece, where it will adjust only the left optical system.
[pagebreak] 4. Focus Non-Diopter Side
Using a lens cover or your hand, cover the objective (front) lens on the side of the binocular that has the diopter setting. Then, using the center-focus wheel, focus the side without the diopter setting on a distant object with fine detail, such as tree branches.
5. Adjust Diopter Setting
While taking care not to change the setting of the center-focus wheel, uncover the objective lens on the diopter side, cover the opposite objective lens, and view the same tree with your other eye. Then, using the diopter setting, focus on the tree branches until the details sharpen again. At this point, it should appear sharp when viewed with both eyes. If not, you’ll need to repeat the process until you get it right. OOnce you’ve made the proper adjustments, you can focus on targets at any distance simply by turning the center-focus wheel. If there is a diopter scale, note the setting for future reference. With practice, this procedure will become second nature and you’ll be doing it from scratch in less than a minute. (Note: If you reverse the procedure and begin by using the diopter setting, you’ll never get the focus exactly right.)
6. Focus Both Sides
Because of their ruggedness, simplicity and imperviousness to water, dust and other atmospheric contaminants, individual-focus binoculars have long been favored by military forces around the world. As the name implies, this focusing system is characterized by the fact that each optical system must be adjusted separately by rotating diopter adjustments located on the eyepieces. This can be a nuisance, especially when the user is viewing objects at varying distances.
The focusing procedure is essentially the same as for the diopter adjustment on center-focus binoculars (see step 5), except that it must be done individually for both barrels.
Another difference is that, while experienced users of center-focus binoculars tend to refocus more or less continually, the users of individual- focus binoculars tend to adjust them for either intermediate or long ranges and refocus only when it is necessary.
As the name implies, fixed-focus binoculars have no focusing system. Instead, the optics have been permanently pre-focused by the manufacturer at infinity for people having 20/20 vision. Though obviously not for everyone, these binoculars work surprisingly well for young people with normal vision in both eyes or people whose sight has been aided by eyeglasses or contact lenses.
Some owners of fixed-focus binoculars believe that they focus automatically. Rather, it is the focusing ability of the viewer’s eyes that causes objects at varying distances to appear sharp.
If you’re like most hunters, you own binoculars that must be manually focused. To do what you require of them, they’re the best type.