 Choosing Binoculars
There's a lot more to binoculars than magnification. As simple as binoculars are to use, features such as objective lens, angle-of-view, prism design and eye-relief are combined in different ways to serve different viewing purposes. Your understanding of these features and of your specific needs will help you find the binocular that's right for you.
What do the figures mean?
8x21. Those numbers in front of binocular models, what do they mean? The first number represents the magnification power - in this case, 8 times larger than viewed with the naked eye. The second number represents the diameter (in millimeters) of the objective lens - in this case, 21mm. The objective lens is the lens closest to the object you're looking at (not the lens that's closest to your eye) and is a good indicator of the binoculars' light-gathering power
What is angle-of-view?
The angle-of-view represents the portion of a circle you can view through a binocular. Since direct comparison of the angle-of-view between binoculars with different magnifications can be misleading, you can calculate the apparent angle-of-view by simply multiplying the binoculars' real angle-of-view by the magnification. An apparent angle-of-view of 65 degrees or more is considered wide-angle, which makes it easy to track moving objects and is therefore ideal for viewing wildlife and sporting events.
Prisms Designs.
Prism design is mostly a matter of preference. Roof prism binoculars have a slim, straight profile and are more compact. They are often more expensive, because their smaller prisms require more precise lens grinding and polishing to achieve excellent image quality. Porro prism binoculars are recognized by their offset lens barrel design. As a result of the design, the wider spacing of their objective lenses enhances perspective to produce a more realistic, three-dimensional image.
What other things are important?
If you will be doing a lot of twilight or nighttime viewing, look for multi-coated lenses. Coated lenses reduce glare and reflection and enhance light-gathering capabilities.
If you wear glasses, look for binoculars with 13mm or more of eye-relief. Measured in mm, eye-relief refers to the maximum distance from the eyepiece lens at which the entire image can be seen. A long eye-relief, such as 13 mm, ensures you will be able to see the entire image through the binoculars with your glasses on. | 
