Posted to Binoculars Articles 2009-03-07
Roof prism, Porro Prism, BAK 4...how many types of binouculars are there?
Binoculars are differentiated mainly between the image brightness, the magnification they provide, the field of view and the quality of the glass used in their optics. As binoculars have become less expensive, better materials have become more or less standard allowing consumers to purchase high-quality binoculars at reasonable prices. Choosing the binoculars you need, will depend largely on the distances you intend to view, the lighting conditions under which you'll be viewing and the weight of the binoculars.
Binoculars, like all optics, have to rectify the image before it meets the viewer's eye. Higher quality binoculars use a roof prism to accomplish this. The prism is shaped something like the roof on a house. The upside down image is flipped so that when it's viewed through the eyepiece it appears normal. A pentaprism accomplishes the same task and sometimes allows for a brighter image. Image brightness in binoculars will not be determined by one element alone, however.
Binoculars are stamped with two numbers, such as 8x50. The first number designates the magnification factor of the lenses. In the above example, the binoculars would magnify images so that they appeared 8 times as large as they do to the naked eye at a given distance. The second number is the diameter of the front lenses, measured in millimeters. Larger front lenses produce brighter images as the lenses are able to gather more light. Absent image-stabilizing technology, very high magnification binoculars will react to a user shaking more than will lower magnifications. Therefore is will be more difficult to hand hold larger magnification binoculars.
Another important measurement of binoculars is the field of view. The larger binocular magnification equals the narrower the field of view at a given distance. This can be compensated for with wide-angle eyepieces, though models with such eyepieces are generally expensive.
The relative brightness of a binocular image is very important, and often overlooked. This can be estimated by squaring the exit pupil value of the binoculars. In general, exit pupils of about 3mm provide adequate brightness for daytime viewing. For nighttime viewing, a minimum of 5mm is recommended. A narrower exit pupil means that the image will be more difficult to keep aligned with the viewer's eyes. If you plan to use your binoculars in applications where some movement is unavoidable, such as boating, choose a wide exit pupil.
The optical quality of the glass used to make the components affects all elements of binocular performance. Bak-4 is generally the best commercially available rating for binocular components. Cheap binoculars generally have prisms that are rated BK7, which offers significantly compromised performance.
If one is planning on using binoculars for hiking, hunting or camping, lower magnifications are usually more compact and lightweight.
Here are some models to check out:
Bushnell Elite e2 8x42 :
Roof prism optics, close focus to 8ft. / 2.4 m, water / fog proof
Nikon 10x36 Monarch ATB :
fully multi-coated, 100% water and fog proof, ultra rugged, rubber armored design
Pentax 8x42 DCF HRc :
waterproof and nitrogen filled (JIS Class 6), Phase coated roof prisms, fully-multi-coated optics
