Why are light transmission and twilight factor different?
Light transmission describes how much of the light entering an optic comes out again. Some is lost inside through reflections or absorption by the materials. High transmission is therefore fundamentally different from a large objective lens. The measurement method matters too: a peak value for one colour of light cannot simply be compared with an average across several colours.
Light transmission describes how much of the light entering an optic comes out again. Some is lost inside through reflections or absorption by the materials. High transmission is therefore fundamentally different from a large objective lens. The measurement method matters too: a peak value for one colour of light cannot simply be compared with an average across several colours.
Twilight factor, by contrast, is calculated from magnification and objective diameter. It measures neither coating quality nor contrast or stray light inside the optic. Two optics can have the same twilight factor and still produce different images in low light.
Several characteristics work together to shape what you see: transmission, contrast, colour rendition, exit pupil and your own eye. A bright light source beside the image can reduce contrast without changing the stated transmission value. Judging quality solely by twilight factor or an advertised percentage therefore falls short. Observations should always be described together with the conditions under which they were made.