Glossary > Binoculars > Field of view > Twilight factor

Glossary | Binoculars | Field of view | Twilight factor

Used to compare binoculars’ performance in low light conditions. The higher the number, the more suitable it is for twilight. However, the value should always be considered in connection with the exit pupil.

The twilight factor of binoculars is a function of the objective lens diameter and the magnification, and is between around 5 and 20. The higher it is, the better the resolution (performance) in twilight.

A distinction is made between the performance factors for twilight, night and day. The effective performance of binoculars is determined by the quality of the optics.

The twilight factor T is defined as:

T = square root of (O x M)
O: objective lens diameter in millimetres
M: magnification

The twilight performance P_T is often estimated by means of [1]:

P_T = T * 0.3

The daytime performance P_D depends only on the magnification:

P_D = 0.6 * M

The nighttime performance P_N is primarily determined by the lens diameter:

P_N = 0.1 * O

The entrance pupil of the eye, EP, limits the beam of rays. The product of the pupil aperture and magnification defines the usable aperture of binoculars:

O_effective = EP * M

Night glasses are usually designed for a pupil aperture of 5 mm. Used in the above equation leads to the approximation:

T = M * 2

For this reason, it makes little sense to compare the twilight factor of night glasses with normal binoculars. As can be seen from the lack of a unit specified, the twilight factor is an empirical measure that says little about the performance of an optical device.

A 25x30 scope and a 9x63 binoculars have almost the same twilight factor of 25. Nevertheless, the scope is unsuitable for the night-time, as it only uses a diameter of 1.2 of the eye pupil. Night glasses are usually optimised for a pupil aperture of 7 mm.