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.


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.



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.