By PJ Williams
There are a few factors that should be considered when purchasing binoculars. Aperture is the diameter of the lens, and is expressed as the second number in 7x50, where 7 is the magnification and 50 is the diameter of the object lens in millimeters. For daytime use, 35 or 40 mm lenses are sufficient, but for nighttime a minimum of 50 is recommended. In general, 7x50 binoculars are a good all purpose choice. Read more...
Magnification ranges from 6x to 20x. The higher the magnification of the binoculars, the heavier they are and more difficult to hold steady. Also, as magnification numbers go up, the viewing field decreases. Marine use requires a wide viewing field. Numbers from 6x to 11x are considered best.
Exit pupil is the diameter of the ray of light transmitted to the eye by the lenses used in the binoculars. Most human pupils open no wider than 7mm when fully night adapted. The idea is to use all the available light projected by the binoculars optics, therefore you should choose a pair of binoculars to match your eyes. Divide the magnification by the aperture and you get exit pupil. 7x50 gives an exit pupil of 7mm, as does 8x56.
As people age, their eyes loose the ability to open as wide. After 40, the pupil opens between 5-6mm when night adapted. Binoculars are designed with this fact in mind, and are available in 7x42, 8x42, and 10x50 models.
Field of view is expressed as the width of the entire viewing field at 1,000 yards. A common number is 367 feet at 1,000 yards. This means that at 1,000 yards, you are viewing an area 367 feet wide when the binoculars are held stationary. Wide angle eye pieces are available but tend to distort the images at the edges of the field of view.
Eye relief is the distance your eyes need to be from the eyepiece in order to see the entire field. 10 to 15mm is normal. Wide angle binoculars require your eyes to be closer than this and may be uncomfortable as a boat pitches with your binoculars pressed tightly to your eyes. Another factor is whether the user wears eyeglasses when viewing objects with binoculars. There are models with eye relief of 20mm or greater which are ideal for those who wear glasses.
Optical coatings enhance light transmission and contrast. Uncoated lenses will reflect white spots of light when held upside down and viewed from above. Coated lenses will appear deep green, blue or purple. Coated lenses are best but generally more expensive.
Prisms allow binoculars to be compact. There are two types; porro-prisms and roof prisms. In general, roof prism binoculars are more costly to make, but allow for more compact binoculars. Porro-prism binoculars use two different types of glass. BK7 are more economical than Bak4 prisms, but BaK4 prisms enhance light transmission. Porro-prisms regardless of price tend to out perform all but the most expensive roof prisms. Source: Astronomy Magazine