Binocular Information

I will include some basic information on binoculars here, including a list of books.

Glossary

AFOV: Apparent Field of View.  The apparent angle of view the binocular optics allow, mainly determined by the eyepiece design.  For most binoculars, this is in the range of 40 to 90 degrees.  A moderate AFOV for a binocular is around 50 degrees.  Wide angle binoculars typically have more than 60 degrees AFOV.

Exit Pupil:  The image formed by the binocular optics near the eyepiece, normally a circular disk.  This would be approximately where you place your eye for viewing.  In general, you want a larger exit pupil of around 5 to 7mm for night viewing because it gives you a brighter image.  A smaller exit pupil will usually suffice for daytime viewing.  The diameter of the exit pupil can be calculated by dividing the diameter of the objective lens by the power of the binocular.  For example, a 7x50 binocular has an exit pupil of about 50mm/7 = 7mm.

Eye Relief: The distance from the eye lens of the eyepiece to the exit pupil, approximately the distance you can hold your eyes from the eyepiece for comfortable viewing, though this can be reduced by the design of the eyecup and if you wear glasses.  I would recommend at least 20mm of eye relief if you are using glasses.  Unfortunately, few binoculars have this much eye relief.

Prism Index: The index of refraction of the glass type used in binocular prisms affects the quality of the image formed.  The difference in the index of refraction of glass and the air allows the total reflection of the light path in most binocular prisms (note that in some designs metallic reflective surfaces are used).   With low index prisms, the steeper light rays towards one side of each reflective surface may not be totally reflected (depending on the f/ ratio), they pass through the prism (this can be seen with a simple ray trace diagram).   This causes squared off exit pupils, easily seen when you hold the binocular up to a bright field (sometimes only two sides are squared off).  This results in reduced illumination towards the edge but in practice the eyes normally focus on the center of field so this drop off is not really noticed.   Most high index prisms are advertised as "BaK4" but there are other types of high index glass used in prisms.  Most inexpensive binoculars use low index BK7 glass prisms.  See below for an example of a squared off exit pupil (from a Sears 6281).

Prism Leaks: This is a term I use for unwanted sources of light leakage in the view caused by poor prism design (low index glass, insufficient shielding or baffling).  These can lower the contrast and in some cases cause secondary images (parasitic or ghost images).  Prism leaks are fairly easy to look for using a similar procedure to checking the exit pupil.  Just hold up the binocular to a bright background with your eyes several inches away.  You will see the round exit pupil of the desired image (sometimes squared off).  Some recommend using a magnifier lens to check the exit pupil but I do not find that necessary.  You will often see other sources of light in or around the exit pupil.  Many of these are caused by prism leaks.  See the images below for examples.  The first two are from a Sears 6281, with the first image slightly overexposed to exaggerate the leaks.  The second image is more indicative of one might see viewing a typical bright scene.  Note the reddish reflection in the lower left hand corner due to the use of a copper prism shield (blackened shields should have been used).  The leak in the far upper left corner is from an opening in the prism cluster assembly which allows light to get to the eyepiece directly from the objective.  The third image is from a Kershaw 12x50 with three obvious leaks (this model uses prisms in the Porro-II arrangement).

Real Field of View: This is the actual field of view seen through the binoculars, expressed either in degrees or a width at a certain distance (for example, 372 feet at 1000 yards).  As approximation, the real field of view is the apparent field of view divided by the power of the binoculars.  Another approximation: real field of view in degrees = number of feet at 1000 yards/52.5.  For example, 372 feet/52.5 = 7 degrees.