What is a Twilight Factor?
The Twilight Factor is supposed to be a measurement of how much you can detail can be seen in low light conditions (or twilight hours) when the human eye has not had time to adjust. This measurement tends to favor magnification which many binocular makers use to sell their binoculars. It is for this reason that it is not suggested that a binocular be purchased based on its twilight factor. It is much better to have a higher Relative Brightness Index than it is a high Twilight factor. For those how find the twilight factor useful it is calculated by multiplying the power by the objective and then finding the square root.
