MAGNIFICATION POWER - What do the numbers mean? (Example 7 x 35 mm)

The first number is the magnification power, or how many times the image is enlarged. The three most common magnification powers are 7X (7 times), 8X (8 times) and 10X (10 times.) A 7X power binocular will make objects appear 7 times closer or 7 times larger than seen with the naked eye.

Don't be fooled into thinking bigger is better. More power is not always better. Although a higher power binocular will increase the size of the bird, higher magnification also magnifies the effects of your hand shaking and heart beat. The image may be bigger, but your perception of the image may remain essentially the same. Stick to a 7X, 8X or 10X power binocular for best results.

The second number indicates the diameter (in millimeters) or aperture of the larger front lens known as the objective lens. The bigger the objective lens, the more light can enter, and the greater the potential resolution of the image, especially at dusk and dawn (the two best times to view birds!) Most popular birder binoculars have objective lens ranging from 35 to 44 mm.

PORRO PRISM vs. ROOF PRISM

Binoculars are divided into two basic design classes: Porro Prism or Roof Prism. Porro prism binoculars are what you would consider traditional binoculars. They are wide-bodied with a big lens in front (the objective lens) and a smaller one in the back (the ocular lens, or the lens you place your eyes next to). In other words, the lenses are not aligned along a vertical axis.

Roof prisms are longer and sleeker in design with the objective and ocular lenses falling in alignment along the same tube.

Both designs have advantages and disadvantages. Porro prism binoculars have fewer internal elements, are generally brighter, and are easier to manufacture and repair. Roof prisms, are more expensive but also tend to be more rugged with elements firmly anchored within the barrels.

FIELD OF VIEW (FV)

The field of view is the size of the area you can see through a binocular. It refers to how much you can see left to right and top to bottom. Field of view is measured in feet at a distance of 1,000 yards from where you would be standing. (An example spec on a binocular would be: Field of View: 356 feet @ 1000 yards.) Select a field of view that offers a minimum of 300 feet at 1,000 yards. A wider field of view makes it easier to pick up and identify fast flying birds. This makes it possible to scan a sky, an ocean or an open marsh quickly. It's also easier to locate birds at close quarters in a maze of branches. Generally, lower magnification binoculars offer a wider field of view than binoculars with higher magnification.

EYE RELIEF (ER)

Closely related to field of view, eye relief refers to the distance between the ocular lens and your eye. This is the most important consideration for eye glass wearers, because glasses hold the eyes back from the eyepieces, and you may not get to experience the same wide fields enjoyed by non-eyeglass wearers. Normal eye relief for binoculars is measured in millimeters and should be from 9 to 13 mm. Even though the eyecups of most binoculars fold down to let eyeglass wearers get closer, in many cases it's not close enough. If you wear glasses, you need binoculars with a longer eye relief of 14 - 20 mm.

CLOSE FOCUS

The closest distance to the observed object that a binocular can focus down to.

EXIT PUPIL

Hold a binocular up to the light and look in the eyepiece. Exit pupils are the small , bright circles. The exit pupil is the column of light that comes through the binocular to your eye. The larger the exit pupil, the brighter the image. The exit pupil is calculated by dividing the diameter of the objective lens by the power: (Example 35 divided by 7 = 5). A 7 x 35mm binocular has a 5 Exit Pupil.

SPOTTING SCOPES