There is a lot to know when choosing binoculars whether you're using them for hunting, birdwatching or sports viewing. Below we'll try to explain what everything means and how the numbers work in helping you to make the right choice for your needs.
When selecting a pair, you'll need to understand what the numbers mean. There are two numbers separated with an "x".
The first number refers to the power or magnification. As an example let's say it is 7 X 35. The 7 means that the object appears seven times closer than you would see it without any help.
The second number is the size of the objective lens measured in millimeters going across the lens.
The size of the objective lens will determine how much light it can obtain for effective viewing. The higher the number, the larger the lens; in effect allowing more light to pass through thereby projecting a brighter image and viewing experience.
The side-to-side measurement of the circular viewing field or subject area, is defined by the width in feet or meters of the area visible at 1000 yards or meters.
Generally, the higher the magnification, the narrower the field of view.
There are two basic types: Porro prism and roof prism. You can tell them apart by the shape. Porro-prism: These have an angled design. When standing on their barrels they appear to form an M shape.
Porros focus using an external focus wheel which, when turned, moves the eyepieces forward or backward along an external barrel.
Roof-prism: This design features two straight barrels, giving it an H shape. With this design the focusing mechanism is enclosed inside the binoculars and is adjusted with an external focusing knob or wheel.
Roof-prisms cost more to manufacture and tend to be more expensive than porros, but roof prism types are not necessarily better than porro prism types.
Quite often when comparing inexpensive, similarly priced, the porro prism is the better product.
The advantages of roof prisms are: the narrower body shape is easier to handle; there are fewer external moving parts (this increases durability; they are generally water proof; and they produce an apparently larger image.
One additional point - most are built with the bird watcher in mind, making your choice much easier.