About Binoculars
Binoculars are usually described with a set of numbers such as 7x26, 10x25, etc. that are often printed somewhere on the body of the binoculars. The first number refers to the amount of magnification provided by the two small lenses that you put up to your eyes. These are called ocular lenses. The second number indicates the diameter of the two large lenses at the other end that are called objective lenses. These control the light gathering capabilities of the binoculars.
Magnification:
Magnification refers to how big an object will appear when viewed through the binocular. A 10x binocular makes objects appear 10 times closer than they would when viewed with the naked eye. The amount of magnification can impact the stability of the viewed image. Larger magnification amplifies any movement of your hands. When exceeding 10x magnification it is often the case that what is gained in further magnification is lost in the stability of the image. It is not recommended that you use binoculars with a magnification higher than 10x for hand-held use. When going over 10x, the use of a tripod is recommended. The lower the magnification the wider the field of view is and the easier it is to hold a steady image. High magnification is useful for birding and star gazing but a wide, steady field of view is more useful for hunting and general use
Objective Lenses:
These are the larger of the two outer lenses in the binocular. These lenses are what gathers all the light that you see- the bigger the lens the more light allowed in. Also, a larger objective lens generally provides better resolution and more detail than a smaller one. The sizes range from a tiny 15mm for theater glasses to 50mm and larger for low-light and spotting binoculars. The larger the lens the more light allowed in but also the heavier the binocluar is.
Brightness of Images – Exit Pupil:
One way to measure the relationship between magnification and objective lenses is to calculate what is known as the Exit Pupil. The exit pupil is an expression of the maximum amount of light that can be transmitted to the human eye. Exit pupil is determined by dividing the size of the objective lens by the magnification. For example, a 10x40 binocular produces an exit pupil of 4.0, as would a 8x32 binocular. A 10x25 binocular has an exit pupil of 2.5. Larger exit pupils transmit more light to the user’s eyes. A general rule of thumb is to have the exit pupil greater than 3.5 for full-size binoculars, greater than 3.0 for mid-size binoculars, and greater than 2.5 for compact binoculars.
A binocular works best when the exit pupil diameter matches your eye's pupil diameter. A smaller exit pupil is good for binoculars for bright daytime use. Larger exit pupils are good for low light conditions and star gazing.