Read about Binoculars

© Sonoma Outfitters 1978 - 2013

Binoculars and Scopes

Binoculars are a fairly simple instrument that brings an object "up close and personal". The binocular characteristics that you choose will be influenced by your usage needs.

• What will you be viewing? Birders like wide-angle, close-focusing binoculars regardless of size, while backpackers tend to prefer compact models.
• Will you use them in normal lighting or during dusk, or deep in a forest?
• Will they be exposed to elements or relatively protected?

Just like any product, quality makes the difference in price, but better quality usually means better performance, less eye strain, and a longer binocular life.

PRISMS By bending light within the binoculars' barrel, prisms increase power without increasing size. PORRO prisms have eyepieces off set from the front lenses and a "hippy" shape. Though bulkier and heavier, they are generally brighter with better depth perception and less expensive to manufacture or repair.

ROOF prism binoculars have the lenses positioned in line and are more slim and trim. Though slightly more expensive, they are still the most popular. Compact models can easily be carried in a pocket, purse, or backpack. The trade off for this convenience is that in low light situations compact models give you about 30% less brightness than standard size models. (This is due to the smaller objective lens.)

POWER The ocular lens (closest to your eye) magnifies the image and brings it to the eye. In a 7x35 binocular, 7 is the magnifying power, meaning that you see an object 7 times larger than with the naked eye. ZOOM binoculars have two power numbers which represent the range available, for instance 8x-24x35. With the flick of the zoom power lever, you can focus on anything at any power between 8 and 24. To accomplish this the binocular has an additional set of lenses but, because the light must now bounce off these additional surfaces, some of the light gathered will be lost. Generally, binoculars with lower magnifications offer greater depth of field, and a wider field of view than those with higher magnification. Binoculars greater than 10 power are not recommended because the greater power magnifies hand movements and makes what you see too "jumpy" (unless used with a tripod).

LIGHT TRANSMISSION The objective lens (furthest from your eye) gathers and focuses the light rays. The number 35, in a 7x35 binocular, indicates the size of the objective lens in millimeters. The larger the number, the more light gathered and the brighter the image. A brighter the image means clearer vision, an important feature in lower light conditions (such as at night or in foggy overcast weather).

EXIT PUPIL This is the column of light that comes through the binocular to your eye. The larger the exit pupil, the clearer the image will be in low light conditions. The general rule is:

• 2.5 - 3.5mm for normal daylight
• 3.5 - 5.0mm for low light, very overcast
• 5.0 - 7.0mm for extreme low light, night.

To calculate the exit pupil, divide the objective lens diameter by the power of the ocular lens. Using this formula, a 7x35 binocular has an exit pupil of 5mm (35  7). Anything larger than 7mm is of no benefit because the pupil of the human eye never gets larger than that.

IELD OF VIEW The width of the area you can see at 1,000 yards is indicated on the binocular in either feet or degrees. For example, 420 feet at 1,000 yards means that if you were looking at the Great Wall of China from 1,000 yards away, you would be able to see a 420 foot section of it through your binoculars. When converting degrees into feet 1 = 52.5 feet so, for example, an 8 field = 420 feet at 1,000 yards (8 x 52.5). WIDE ANGLE binoculars increase your field of view without sacrificing power. They have special optics which increase your field of view by approximately 35% in the case of wide angle, and 65% for extra wide angle. A wide field of view is better for following fast moving action or scanning for wildlife

. DEPTH OF FIELD This is the distance that can be viewed, in focus, from near to far. A generous depth of field is helpful when trying to locate close-at-hand subjects, such as a bird in a maze of branches.

FOCUS The ability to distinguish fine detail, is mostly dependent on the quality of the lenses and prisms. There are three focus methods. Fixed is convenient, but loses in overall image quality and versatility. Individual focusing is time consuming but accurate. Center focusing adjusts both eyepieces at once and is usually the preferred option.

DIOPTERS are a mechanism, on one eyepiece of center focus binoculars, that adjusts to compensate for variances in vision. To set, focus the binocular while covering the right front lens. Then focus on the same subject using the diopter control while covering the left front lens. Reverse this if diopter is on the left.

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