Light: exit pupil, twilight factor, relative brightness · Eye relief, eye box and field of view

23.3 Light: exit pupil, twilight factor, relative brightness

How much light reaches the eye? The answer is a division: the exit pupil, that is, the diameter of the beam leaving the eyepiece, is the diameter of the objective divided by the magnification. A 56 mm at 8× produces a 7 mm beam; the same objective at 25× cuts it to 2.2 mm. And the beam has to be compared with the pupil of the eye. In daylight the pupil is contracted to 2–4 mm, and a 4 mm exit pupil already saturates it: the extra light simply does not get in, and very different scopes perform alike. At twilight and at night the eye opens to 7 mm, and that is where a wide exit pupil — and with it a large objective or moderate magnification — becomes real performance, along with the comfort of an eye that is less tired and quicker to acquire. Figure 23.2 puts the diameters side by side, at the same scale.