FOV, Exit Pupil, Twilight Factor — Optics Lingo Explained

When shopping for a new riflescope or spotting scope it’s easy to get confused by all the technical terminology. Do you wish you had a better way to compare scopes — beyond just size, weight, and price? Well Swarovski Optik can help. The Swarovski Hunting Blog offers a helpful guide to technical terms used when comparing scope specifications. Here are some important definitions, expressed in layman’s language:

Objective Lens Diameter
The objective lens diameter determines the size of the optical system’s entrance pupil. The bigger the objective lens diameter, the more light the system can capture. However, the size of the objective lens does not determine the size of the field of view.

Exit Pupil
The size of the Exit Pupil is determined by the objective lens diameter and the magnification. If you look at the eyepiece from a distance of around 30 cm (11.8 in), the Exit Pupil appears as a bright disc.

For calculating the Exit Pupil the formula is:

Exit Pupil = objective lens diameter ÷ magnification (expressed in power number).

The larger the Exit Pupil, the more light will reach the eye.

Field of View
The Field of View is the size of the circular section of the area which can be observed when you look through a long-range optical device. In the case of rifle scopes, it is specified at a distance of 100 meters or 100 yards. For example, 42.5 m at 100 m or 127.5″ at 100 yards. As an alternative, the Field of View can also be stated in degrees (e.g. 6.6°).

NOTE: The technically-feasible size for the Field of View is essentially determined by the magnification. The higher the magnification the smaller the Field of View.

Twilight Factor
The Twilight Factor defines the optical system’s performance in poor light. The statement “the greater the twilight factor, the better the suitability for twilight” only applies if the exit pupil is larger than or at least as big as the eye’s pupil. The pupil in the human eye can only open to around 8 mm. As we get older, our eyes become less flexible, which limits our ability to see things in twilight or at night. Therefore [an optic’s] exit pupil cannot always be fully utilized.

For calculating the Twilight Factor the formula is:

Twilight Factor = root of ( magnification x objective lens diameter ).

NOTE: Spotting scopes have extremely high twilight factors because of their high magnification and large objective lens diameter. But [when used at high magnification] their small exit pupil can make them [somewhat difficult] to use in twilight.

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