During daylight hours, when your eye pupil size will be only about 2 to 3mm, magnification will be the principal factor in image resolution. At night, with the eye pupil dilated to 6 to 8mm, aperture size is the controlling factor. In twilight conditions both of these factors control resolution effectiveness and the twilight factor is the term that compares binocular performance under these conditions.

The twilight factor is calculated by taking the square root of the product of the magnification and the aperture.

The higher the twilight factor, the better the resolution of the binocular when observing under dim light conditions. For example, a 10 x 40 (twilight factor 20) would effectively resolve better under these conditions than a 7 x 35 (twilight factor 15.4) even though the 10 X 40 has a smaller exit pupil.

Remember, however, that the twilight factor does not take into account the transmittance or quality of the optical system.