Binoculars are typically described by two numbers, such as 7x35, referred to as, “seven by thirty-five”. The first number given is the power or magnification. A 7x (seven power) binocular will make an object look seven times closer or seven times larger than you would see with the unaided eye. The second number, 35, refers to the diameter or width (in millimeters) of the front, or objective lens. The size of the objective lens determines how bright an object will appear to your eyes. A third number commonly printed on binoculars is the angle or field of view. This number tells you how wide an area you will see.
POWER OR MAGNIFICATION
The magnification of most handheld binoculars range from 6x to 10x. Seven and eight power models are considered to be the most versatile, multi-use binoculars. Although it seems sensible that a high power model would help you see things in more detail than a low power model, this is rarely the case. The largest drawback of higher power is that along with magnifying the object, it also magnifies the movement of your hands and body, which causes the image you see to shake or appear jumpy. Keep in mind that the shake will be noticeably worse during and for a short time after any physical exertion. A second drawback is that higher power models generally have a smaller field of view, causing difficulty in finding or following objects. We do not mean that 9x and 10x binoculars should be avoided. Many experienced birders prefer 10x models, especially for birds that are difficult to approach, such as raptors and shorebirds. We suggest you try looking through a high power binocular to determine whether or not you are able to hold them steady. Handheld use of magnifications above 12x is extremely difficult, and we suggest using a tripod or window mount for the best results. When powers of 15x or higher are required to see detail at a distance, a tripod-mounted spotting scope should be considered.
The size of the objective lens directly affects the brightness of the image and the physical size and weight of the binoculars. It does not affect the field of view or area that you will see. A larger objective lens will gather more light and usually deliver a sharper, brighter image. We stress the word usually because once the light is collected, there are many factors that affect how well it is delivered to your eyes. These factors will be covered later in, “Optical Quality.” The downside of a larger objective lens is that as the size of the lens increases, so does both the weight and the size of the binoculars. Binoculars that are too large or heavy to carry comfortably tend to get left behind.
FOUR MAJOR BINOCULAR STYLES
• Full Size Porro Prism
Porro prisms have objective lenses spaced wider than roof prisms, and so can produce a slightly better stereoscopic image than the roof prism design.
• Full Size Roof Prism
Compact Design, less internal parts than porro prism design, so less to go wrong and easier to make dust and waterproof.
• Compact Reversed Porro Prism
• Compact Roof Prism