Tom McIntyre’s Glossary of Shooting Optics Terms

EXIT PUPIL: The diameter of the light emitted from an optical device’s eyepiece. The pupil of the human eye expands, on average, from 3mm to 7mm in lowest light. The exit pupil of an optical device is calculated by dividing the diameter of the objective, in millimeters, by the power of magnification. So as an example, an 8x40mm binocular would have an exit pupil of 5mm, which would be about a minimum for use in dawn and dusk. There is, though, an exception to this rule, when it comes to low-power (1-2x) devices. Calculating these would show exit pupils well beyond the maximum 7mm the eye can absorb. So there is an explanation for why a 1x24mm riflescope, for example, does not have an exit pupil of 24mm. Dean Capuano of Swarovski offers this rationale:

“A 24mm exit pupil is not desirable or possible optically. Basically just pushing a wider beam of light through a scope is not the answer to getting a brighter image. At the lowest magnifications (1-2X) we have a light stop controlling the flow of light through the scope to give us the optimum optical performance. This is in direct relation to what is called Effective Objective Diameter. An example of this can be seen with some of our scopes at their lowest magnification. The exit pupil is way off our calculation when we use the known objective diameter.

“The light stop has effectively controlled our objective diameter. This reduction is only necessary at the lowest magnifications, as we increase the magnification we increase the amount of effective objective diameter that we are using. The higher magnifications are where the larger objectives are needed to provide us with sufficient exit pupil.”

EYE RELIEF: This is, lamentably, not another way of saying “Scarlett Johansson.” It is the distance that the eyepiece may be held away from the eye and still provide the viewer with the complete field of view, instead of a kind of tunnel vision.

This distance can fall into a range that is sometimes called the “eye box.” Some binoculars may have a very small eye box of 9 to 14 millimeters, which might be comfortable for most people who do not wear eyeglasses. For glasses wearers, though, it is estimated that an eye relief of at least 16 millimeters is required to see the whole picture.

FIELD OF VIEW: Field of view (“FOV”) can be described as “true” or “apparent.” True field of view is the angle subtended by the objective lens. An 8.5×43-millimeter binocular may have a true (or at least published) field of view of 6.1 degrees. This is the wedge, or cone, of light entering the objective. Thought of simply as width, this is how wide an area from which the objective is drawing light and accordingly is the physical width, or diameter, of the area that may be observed.

The formula for determining this in feet is: 52.5 X FOV in degrees = FOV in feet @ 1,000 yards. So for the binocular described abpve, its FOV in feet at 1,000 yards would be: 52.5 X 6.1 = 320.25 feet @ 1,000 yards.

APPARENT FIELD OF VIEW: As the light passes through the eyepiece, the angle is widened. This is the apparent field of view (called “AFOV” for convenience). Somewhat tougher to comprehend, AFOV is calculated by multiplying the degrees of true FOV by the magnification power. Thus, for the binocular mentioned above: 6.1 X 8.5 power = AFOV of 51.85

TWILIGHT FACTOR: Though it sounds as if it might be something from the Smith Corona of Rod Serling, twilight factor is actually another way of comparing the relative usefulness of different configurations of optical devices in low light. Like exit pupil, twilight factor can be calculated mathematically. The twilight factor of an optical device is found by multiplying the diameter, in millimeters again, of the objective by the power of magnification and determining the squaree root of this number. Worked out for a 60×80-millimeter spottig scope, for example, the equation would be: Twilight Factor = √80 X 60 = √4800 = 69.3

Now compare that with a 40×60-millimeter scope with an exit pupil over 25 percent larger in area (though still only 1.5 millimeters in diameter). Its twilight factor would be: √60 X 40 = √2400 = 48.9

Twilight factor was a much more significant consideration before the advent of low-dispersion glasses and multicoatings, but it remains useful for comparison purposes–think of it as the raw horsepower of an engine. With a variable-power eyepiece of say 20 – 60X on a scope with an 80-millimeter objective, the exit pupil and the twilight factor at the lowest power would be 4 millimeters and 40, respectively. Now making the assumption-and it is a major one-that glass and coatings are of exactly equal quality, then it is worthwhile noting that a 60-millimeter scope with a 15-45X eyepiece, a typical configuration, with the power setting turned all the way down, will have an exit pupil of 4 millimeters (the same as the 80-millimeter scope described earlier), but the twilight factor will be only 30. It’s a V-8 versus a straight-6, yet each has its individual usefulness.

RESOLUTION: This is an optical device’s ability to bring fine detail into sharp focus. Most of us are familiar with the big “E” Snellen Chart on the ophthalmologist’s wall. This chart is a way of gauging the resolution of our eyes. For optical devices there is the 1951 USAF Resolution Test Chart. This is a small target mounted on a square of glass and is comprised of groups of three bars, instead of letters of the alphabet, that descend in size to the nearly imperceptible. Checking a binocular, for example, against a resolution chart is not something a hunter can do in a store-and probably not something he or she will do at home, either. As a quick alternative when examing a binocular in a store, try to read the finest print you can on a product label at 30 feet by indoor light. By comparing different binoculars, you should be able to find the one with the best resoltion.

For further information about hunting optics, pick up a copy of Tom McIntyre’s new book, Field & Stream Hunting Optics Handbook, to be published by the Lyons Press (lyonspress.com) this fall.