There are many different types of binoculars designed for different purposes - astronomical binoculars, birding binoculars, image-stabilizers, opera glasses, waterproof, and so much more. How do you figure out which pair of binoculars is right for you? The best thing to do is start by learning some binocular "basics"...
What are binoculars and how do binoculars work? Binoculars are both technical and simple at the same time. They consist of an objective lens (the large lens at the far end of the binocular), the ocular lens (the eyepiece) and a prism (a light reflecting, triangular sectioned block of glass with polished edges). The prism folds the light path and allows the instrument body to be far shorter than a telescope and correctly orients the image. The traditional Z-shaped porro prism design is well suited to astronomy and consists of two joined right-angled prisms which reflects the light path 3 times. The sleeker, straight barrelled roof prism models are more compact and far more technical. The light path is longer, folding 4 times and requires stringent manufacturing quality to equal the performance. These models are better suited to terrestrial subjects and can include digital imaging systems and stabilizing features. Many manufacturers offer different models of both the porro-prism and roof-prism designs.
All binoculars are are expressed in equations - the magnifying power X the objective lens size. For astronomical applications, these two numbers play an important role in determining the exit pupil - the amount of light the human eye can accept (5-7mm depending on age). By dividing the objective lens (or aperture) size by the magnifying power you can determine a pair of binoculars exit pupil. Like a telescope, the larger the aperture, the more light gathering power - increasing proportionately in bulk and weight. Stereoscopic views of the night sky through big binoculars is an incredible, dimensional experience and one quite worthy of a mount and tripod!
Just like a telescope eyepiece, binoculars have a field of view stated in degrees. This is the amount of sky or terrain you will see while looking through them. To follow a moving target, such as studying wildlife, you may wish to consider a large field of view to decrease panning around to find the subject. No matter whether you are choosing binoculars for astronomy or binoculars for bird watching, you must also consider eye relief. Eye relief (how far away your eyes need to be to focus) is as important as magnifying power. Anything less than 9mm eye relief will make for some very uncomfortable viewing. If you wear eyeglasses to correct astigmatism, you may wish to leave your glasses on while using binoculars, so look for models which carry about 15mm eye relief. Quality binoculars should have a two-part focusing system called a right eye diopter which allows individual eye focus. Also look for multiple lens coatings, which improve color and image contrast.
For beginner astronomy binoculars, 7X50 models are an excellent choice. Larger astronomy binoculars give incredible views, but will require a mount to help steady them. For birdwatchers, a good choice of binoculars are those light enough to hold steady and carry easily. While it is tempting to go for high magnification binoculars, these models can be uncomfortable and difficult to target on a moving subject. Image stabilizing binoculars and binoculars with built-in digital cameras make excellent additions to those who like to photograph wildlife and sporting events. For boaters, water-proof binoculars are essential, just as you may wish to consider models that are "fog proof" for humid climates. While so much information may seem a little confusing at first, just a little study will take you on your way to discovering binoculars that are perfect for you.