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| Behind the numbers. |
- Exit Pupil & Brightness Index
- The exit-pupil diametre is a figure generally used as a measure of the brightness of an image delivered through the eyepiece.(It is the diametre of the shaft of light you see if you pull the binoculars away from your face.) To obtain the exit pupil diametre, divide the objective lens diametre (the 50 in 10x50) by the magnification power (in this example: 10). Thus, 50/10 equals 5mm.
 In dark viewing situations, the pupil of the human eye opens to receive more light. Thus, nighttime binocular use requires a larger diametre of light from the eyepiece to match the expansion of the pupils - usually 5-7mm.
 In bright light situations, the pupil in the human eye constricts. So, for most daylight situations, it is not necessary for binoculars to transmit great quantities of light. Thus, 2-3mm is usually sufficient.
- Angle Of View
- When looking through a pair of binoculars, the width of the area you see is your angle of view. This figure is noted in the model specification in degrees, such as in the example below: 6.5°. Naturally, the wider your angle of view (the greater the angle in degrees) the easier it will be to spot or locate a subject, particularly when it is in motion. A binocular having an Apparent Angle of View of 65°or greater is considered a wide angle binocular, regardless of the magnification power. To calculate the Apparent Angle of View, multiply the Angle of View (in this case 6.5° by the magnification (here, 10). Thus, 6.5x10 equals 65.
- Magnification
- Magnification is simply the power of your binoculars to increase the size of an image. It is indicated by the first number in the model specification. For instance, 10x50 would indicate that this model multiplies an image ten times. Hence, 10x. Thus, an object at a distance of 1,000 metres(1,000 yards) would appear to be as near as 100 metres(100 yards): 10 times closer. However, as magnification power increases, brightness is dissipated, the field of view is decreased and blurring from unstable mounting, like shaky hands, becomes increasingly more pronounced.
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