How to Select Binocular Power

The first step in selecting a pair of binoculars is to figure out the power you want. Power is described by two numbers, as in these examples: 6x30, 7x50, 8x26, 10x42, and 12x36. The two numbers are not directly related; the first represents the magnification and the second number represents the diameter of the objective lens.

Magnification
The first number, always the smaller of the two, refers to the amount of magnification that the binoculars provide. This is a measure of how much closer the viewed image will appear when looked at through the binoculars. While bigger sounds better, that's not always the case in binoculars. Selecting the correct magnification will depend on your intended usage.

Lower magnification binoculars, 7x and lower, provide for a larger field of view which makes spotting things at a distance—like birds, animals, or ships—easier. A larger field of view also makes it easier to track movement of the subject being observed. Since they take in more of the scene, lower magnification binoculars gather more light. As a result, the image will be brighter and easier to see even though it is not as highly magnified. Due to the lower magnification, they also tend to be less sensitive to vibration or movement. If you are going to be in the field using your binoculars without a tripod, this can be an important consideration. Finally, lower magnification binoculars can usually focus on objects that are closer than comparable higher magnification binoculars.

But of course, lower magnification binoculars ultimately provide less magnification, and therefore less detail than higher power models. If you really need high magnification binoculars, 11x and higher, for your application, make sure you also purchase a tripod or get a model with image stabilization. The higher the power, the more sensitive the binoculars are to vibration. Once you get up to 10x magnification binoculars, they will be nearly unusable without a tripod or an image stabilization system. Higher power binoculars also tend to be heavy. So if you plan on using them for a long period of time, a tripod may be a better choice than a model with image stabilization.

As for cost, it's probably not surprising that lower power binoculars tend to be cheaper than equivalent higher power models. Unless you have special needs or uses intended for your binoculars, a good middle-of-the-road magnification level is 8x. Binoculars at this magnification level offer a nice trade-off of magnification while providing good brightness, low weight, and lower cost.

Objective Lens Size
The second number refers to the diameter of the objective lens in millimeters (mm). The object lens is the lens in the front of the binoculars, away from your eye. Smaller objective lens usually indicate binoculars that are smaller in overall size.

Binoculars with objective lens sizes of 30 mm and under are most often found on compact and pocket models. While pocket and compact binoculars are lighter and easier to carry, the downside of them is that the smaller lenses let in less light. This can make images darker and harder to see. If you plan on using your binoculars at night, models with smaller objective lenses will not be a good choice.

If brightness, or nighttime use, is important, object lenses that are 50 mm or larger will work best for you. However, these binoculars will most often be large and heavy. If you are going to use binoculars of this size, you should definitely have a tripod available, regardless of the magnification.

The most common objective lens sizes are between 35 and 50 mm. Since magnification also affects image brightness (higher magnification is darker), the objective lens sizes tend to go up as magnification increases to compensate. As a general rule, look for binoculars that have an objective lens diameter that is at least 5 times bigger than the magnification. For example, 7x35, 8x40, 10x50, etc. While some binocular manufacturers have exceptional optics that will provide good brightness at a ratio under 5, the safest bet is to stay at or above this ratio for good brightness.