Magnification is the degree to which the object is enlarged. With a 10x40 binocular, the object will appear to be ten times closer than without the aid of a binocular. The number immediately following the "x" is the diameter in millimeters of the objective (front) lens. The larger the front lenses the more light that is allowed to enter the binocular. At 7x35 binocular has 7 times magnification with a 35mm objective lens.
Field of View / Angle of View
The field of View is the width, measured in degrees or meters, of the viewing area you would see at 1000 meters. For example: if you were looking through a pair of binoculars that had a field of view of 100 meters at 1.000 meters, and the subject that you were looking at was 1.000 meters away, then you would be able to see 50 meters to either side of the object. (assuming the object was in the center of the sight picture). The Field of View may be expressed in meters at 1000 meters or degree of angle (Angle of View). Each degree eguals 17.5 meters of width at 1000 meters. So a binocular that indicates an eight ÷ degree angle of view has a 140 meters field of view at 1000 meters (8 X 17.5= 140) Note: As magnification increases , field of view decreases
The exit pupil is the diameter of the Beam of Light that passes through the Binoculars into your eyes. The larger the exit pupil, the brighter the image will appear. The exit pupil is an important indicator of the binocular“s low light performance. Exit pupil is expressed in millimeters and is calculated by dividing the diameter of the objective tens by the magnification. An 10X42 binocular has an exit pupil of 4.2mm. Note: If you will be glassing early in the mornings and late in the evenings, a large exit ÷ pupil will be needed.
Eye Relief is the distance a binocular or teleshope can be held away from the eye and still present the full Field of View. Note: Extended eye÷relief is one of the three most critical performance factors, along with magnification and Field of View, especially for those who wear eye glasses in the field.
Prism Systems There are two types of prism systems used in binoculars, Porro and Roof. Roof prism binoculars have the lenses positioned in line and are more compact. Porro prism binoculars have eyepieces offset from the front lenses. Note. In general, Porro prisms give the viewer better depth perception. On the other hand, roof prisms help eliminate the mass of the standard binocular.