The first of these numbers refers to the magnification or power offered by the binocular. Magnification is why most people buy a pair of binoculars. In the examples above, '7x' means the binocular makes whatever you look at appear seven times closer than it does to the unaided human eye. '8x' means the binocular makes whatever you look at eight times closer than the unaided human eye. '10x' makes things look ten times closer, and so on. The first number used to describe binoculars always refers to their magnification. Common binocular magnifications are 6x, 7x, 8x, 9x, and 10x.
There are also variable power zoom binoculars, such as 7-21x50. These almost always perform much better at the low power setting than they do at the higher settings. This is because the front objective lens cannot enlarge to let in more light as the power is increased, so the view gets dimmer. At 7x, the 50mm front objective lens provides a 7.1mm exit pupil, but at 21x, the same front objective lens provides only a 2.38mm exit pupil. Also, the optical quality of a zoom binocular at any given power is inferior to that of a fixed power binocular of that power. In general, zoom binoculars are not the bargain they seem to be.
The second number most commonly associated with binoculars refers to the diameter of the front objective lens in millimeters. Thus in '7x50,' the '50' means that the front lenses of the binoculars are 50mm in diameter, and that would be good for dimmer light conditions. They are sometimes referred to as night vision binoculars. This size is large for hand held binoculars and may be awkward to handle without a tripod or something to rest them on.
Power affects brightness, so the higher the power, the dimmer the view. And power also affects the field of view of binoculars, therefore, the higher the power, the smaller the field of view. So, as you can see, power must be balanced against other desirable characteristics when choosing your binoculars.
Try out several types of binoculars for each of your particular uses. If your viewing is mostly in the bright day light, then you do not need binoculars with an objective lens of 50, 60 or higher. Those larger objective lenses do well at dusk or darker applications. Perhaps you will need both types of binoculars for all your viewing needs. It really depends on your particular usage. In either case enjoy your out-door activates with enhanced views and closer views through your favorite binoculars.
Field of View
The field of view is the area seen through your binoculars. It is properly measured in degrees. The larger the field of view the more area you can see. Field of view is particularly important when observing moving subjects, like animals or birds, at sporting events, and observing a beautiful scenic view. If you wear glasses, look for binoculars with a higher rating for field of view. You'll be much happier.