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What is the diameter (objective size) of a pair of binoculars?

The objective lenses of binoculars are the front lenses. The diameter of one of these lenses, given in millimeters, will be the second number describing a particular binocular. Hence, a 7x42 binocular has an objective lens of 42mm.
magnification marking
Objective size  (diameter) is
marked on many pairs of binoculars.  

The diameter of the lens determines the light gathering ability of the instrument, with the greater light gathering ability of a larger lens translating into greater detail and image clarity. This is especially useful in low light conditions and at night.

Doubling the size of the objective lenses quadruples the light gathering ability of the binocular. For instance, a 7x50 binocular has almost twice the light gathering ability of a 7x35 binocular and four times the light gathering ability of a 7x25 binocular.

This might lead you to assume that bigger is better when it comes to the diameter size of the objective lenses, but in reality the size of the lens must be considered along with exit pupil and intended usage to determine the best binocular for you.

What is the eye relief for binoculars?

Eye relief is the distance (in millimeters) that a binocular can be held from the eye and the full field of view can still be comfortably observed. It measures the spacing from the last surface of the eye lens of an eyepiece to the plane behind the eyepiece where all the light rays of the exit pupil come to a focus and the image is formed. Your eye should be positioned here to see the full field of view of the eyepiece.

Eye relief should be at least 10mm, 15mm for the best comfort, maybe more if you wear eyeglasses.

Hold your binoculars so they are just in front of your eyes to take advantage of its eye relief. You’ll lose field of view if you place your eye farther away and may even move your eye out of the beam of light from the eyepieces. Getting too close will prevent you from blinking and may also cause a black ring to appear around the field of view.

What is the field of view of a pair of binoculars?

Your binoculars field of view is the width of the area you can see. It’s usually described in two ways: angular field of view and linear field of view.

Angular field of view is the true angle seen through the optics and is usually measured in degrees.  The linear field of view is the width of the area seen and it’s given in feet observed at 1000 yards. A larger number for either angular or linear field of view means you see a larger area.

Binocular field of view
Angular and linear field of view
for binoculars.

Angular field of view can be used to calculate the linear field of view. Just multiply the angular field by 52.5. For example, if the angular field of a particular binocular is 8 degrees, then the linear field at 1000 yards will be 420 feet (8 x 52.5).

Field of view is related to magnification. In general, a greater magnification yields a smaller field of view. A large field of view is especially desirable in situations involving motion – birds in flight or you are on a boat or in a car.

I want to know: what determines the brightness of the image in my binoculars?

Brightness is important to consider when comparing different pairs of binoculars. It impacts your overall ability to see things (sky objects, animals and landscapes) in low light. The quality of the image contrast and color also depend on brightness.

The physical factors determining a given pair’s image brightness are the objective diameter or front lens size, the light transmission of the optics and coatings used on the lens surfaces and prisms throughout the binocular and the magnification. A larger objective will gather more light. Modern coatings allow more light to pass through the optics. When comparing two pairs of the same objective size a lower magnification will have greater brightness (a 7x50 binocular will have a brighter image than a pair of 15x50s).

If you want to compare numbers before looking through actual binoculars, there are several calculations used for brightness or the effectiveness of binoculars in low light. The first is twilight factor, found by multiplying objective size by magnification and taking the square root of the resulting product. (The twilight factor for a pair of 7x50s is 19.) The larger the twilight factor, the easier it is to see detail in low light.  A second measure is the relative brightness index, RBI, which is the square of the exit pupil (exit pupil is the objective size divided by the magnification). Those same 7x50s would have an exit pupil of 7 and a RBI of 51. Finally, RLE is used when comparing coated to uncoated (vintage) binoculars. Just increase the value of the RBI by 50 percent. The RLE for coated 7x50s is 77. Binoculars with higher RBI and RLE numbers deliver brighter images.


Every binocular is identified by two numbers, for example 8x21, 10x50 and so on.

The first number before the "x" indicates its magnifications. A 8x21 binocular brings objects 8 times closer. A lOx5Omm binocular brings objects 10 times closer.

The second number after the "x", indicates the measurement of the diameter in millimeters of the objective lens (the lens furthest from the eye). This size determines the amount of light which reaches the eye and the brightness and definition of what you see. The greater the objective size, the greater the brightness.

 The field of view is a measurement of the width in feet or meters that can be seen through the binocular, at 1000 yards or meters.

Coated, fully-coated, multicoated, fully-multicoated, phase-coated binocular optics. Why are these important?

Optical coatings reduce internal light loss and glare and ensure even light transmission, resulting in greater image sharpness and contrast. Binoculars may have 10-18 glass surfaces, each one contributing to scattered light, so coatings make a big difference in what you see. Coated optics will have a less shiny, even dark appearance when looking into the barrel or tube. You may see a greenish, bluish or brownish tint as well. Most coatings are magnesium fluoride or calcium fluoride and work by destructively interfering with certain colors or wavelengths of light, eliminating their reflection. More light gets into your binoculars and more light is able to pass through to your eyes.  Almost all modern consumer optics have some kind of coating on most of the optical elements. However, there are different levels and qualities of coatings.

Coated optics means that at least one of the major optical elements has a coating on at least one surface. Fully-coated means that all lenses and glass surfaces have a coating layer. Multicoated means that at least one of the major optical elements in a fully-coated binocular has multiple coatings of antireflective compounds on at least one surface. Fully-multicoated means all glass surfaces have multiple coatings and it is the best kind, resulting in light transmission of 90-95 percent, bright, sharp and contrasty images.

Some roof prism binoculars feature phase coatings. Because the light travelling through the prisms in these binoculars takes a slightly different path in one prism, it's shifted by one-half its wavelength and contrast is lowered. By applying a special coating that compensates for this shift, the contrast loss is eliminated, giving phase-coated binoculars brighter and more vivid images.
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