Twilight factor

A short tour of the twilight performance of binoculars. This article shows you how to determine it.

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