The numbers on a binocular describe its configuration. Let’s take 8×42 binoculars as an example. The first number (8) refers to the magnification, or, how many times closer an object will appear when looking through the binocular compared to looking at the same object with the naked eye. The second number (42) indicates the diameter of the objective lens (the light-gathering lens) in millimeters.
Consider this general rule about optics: low magnification provides a wide field of view and is easy to hold steady. So although a magnification of 10, 12 or higher is available, 6, 7, and 8-power binoculars are easier to use and in many cases more practical. For a better understanding of this, please watch the video below, Understanding Optics: Magnification:
In regard to the objective (front) lens size: the bigger the lens, the heavier the binocular. While a large lens may provide a brighter image (all other things being equal), the compromise is increased size and weight. On the other hand, a compact binocular fits in your pocket quite nicely, but because of its small objective lens size, it will not be as bright, especially in low light conditions such as dawn, dusk, or in the woods. Our video, Understanding Binoculars: Aperture, will help illustrate this:
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