Binocular parts
Roof prism vs. Porro
What does 7x42 mean?
Exit pupil
Eye relief
Coated glass
Anti-phase shifting coatings
Field of View

Binocular parts
Birding binoculars have three basic elements: the front lenses, the rear lenses, and the prisms. The front lenses are called the objective lenses. They gather the light and focus an image of the object inside the binoculars. The rear lenses are called the eyepieces. They enlarge this image and present it to the eye. The third element are the prisms, located between the objective lenses and the eyepieces. The prisms function as mirrors, reversing the image and turning it right side up. Without the prisms, the birds would appear upside down and backwards. The prisms also fold the light path so that the overall length of the binoculars is shortened.

Roof prism vs. Porro prism binoculars
There are two main styles of binocular design depending of the kind of prism system used, roof prism and Porro prism. It's very easy to tell them apart. If the objective lenses and the eyepieces are in line with each other, they are the roof prism design. If they are offset from each other, they are the Porro prism design. Roof prisms binoculars are more compact. However, to achieve the same optical quality as Porro prism models, usually cost more to manufacture. The top binoculars of each design are now generally considered to be equal in optical quality. However many people will express a strong preference for one over the other. For example, the Porro prism design has wider spaced objectives and can show a slightly better stereoscopic image.

What does 7 x 42 mean?
You will find a formula like this engraved on every binocular. It's pronounced "seven by forty-two." The first number is the power of the binoculars. It tells you how many times as big the image will appear. It's intresting that the magnifying power of a binocular is not related to its size, but to it's eyepiece design.

The second number is the diameter of the objective lenses (the front lenses), in millimeters. It tells you the light-gathering ability of the binoculars. The bigger this second number, the larger and heavier the binoculars, and the better they will work in dim light. Binoculars are usually identified by their brand name, model name, and this formula, as in "Zeiss Classic 7 x 42" or "Bushnel Birder 8 x 40." Often the model name is dropped and they are called "Zeiss seven by forty-two's."

Exit pupil
The exit pupil relates to how well a binocular will perform in dim light. If you hold binoculars away from your eyes and up to the light, you will see a bright circle in the center of the eyepiece. The diameter of this circle in millimeters is called the exit pupil. You can calculate it by dividing the diameter of the objective lens by the magnifing power of the binoculars. For example, a 7 x 42 binocular would have an exit pupil of 6 millimeters. How useful a large exit pupil will be depends on the eyes and the age of the individual birder. With age, the eye loses its ability to adapt to low light. While a young birder's pupils may dilate to 7 mm., a 50-year-old birder's may open to only 5 mm. The older birder's eye may not be able to use all the light available and might be just as well off with a smaller, lighter-weight binocular.

Eye relief
Eye relief is how far back your eye can be from the eyepiece and still see the whole picture. It's measured in millimeters. Eyeglass wearers need to test to make sure that the eye relief is long enough to accomodate their eyeglasses. Long eye relief will be from 14 to 20 mm

Coated and Mult-coated glass
Ultra-thin coatings are put on lens surfaces to reduce reflections. Without these coatings, up to 50% of the light entering the binoculars could be lost to reflections from the many glass surfaces inside the binoculars. With uncoated optics, the images will seem dim and hazy, and have low contrast. With todays best multi-layered coatings, 95% of the light gets transmitted to the eye and the images are bright, clear, and high contrast.

Here are some terms and symbols that are used to describe binocular coatings:

(C) = coated optics: one or more surfaces coated.

(FC) = fully coated: all air-to-glass surfaces coated.

(MC) = multi-coated: one or more surfaces are multi-layer coated.

(FMC) = fully multi-coated: all air-to-glass surfaces are multi-layer coated.

Anti-phase shifting coatings
The best roof prim binoculars have anti-phase shifting coatings applied to the roof prism. This prevents an interference problem, unique to roof prism optics, that reduced image contrast. Before this technology was perfected, the image contrast of roof prism binoculars suffered by comparison with the best Porro prism binoculars.

Field-of-view
Field-of-view refers to the horizontal width of the image. In the chart it is expressed as an angle, typically between 5 and 8 degrees. It is also expressed in linear form, such as the width in feet that you can see at 1000 yards. To convert from the angle to the linear form expressed in feet, multiply the angle by 52.5. A wide field-of-view eyepice design ususally means reduced eye relief.

--Michael Porter