The actual twilight utility of, say, a particular 10x42 with a 20.5 twilight factor and 4.2 exit pupil
depends entirely on its internal optical quality.
When the seeing was steady, I used 408 x (a 2.0-mm exit pupil
) to see that some of the H II regions had definite shapes and were interesting objects all their own.
The 8x56 version boasts an incredible 7mm exit pupil
for extreme low-light performance.
Conventional wisdom tells us that the limited field-of-view and smaller exit pupil
we get at the higher power settings are not suited to close ranges and poor light.
The nitrogen-filled, watertight scope comes with an illuminated reticle, a 34 mm tube for greater strength and an exit pupil
that provides an unwavering field of view even when you're not perfectly lined up with the eyepiece.
When you arrive at a list of finalists, it's time to start calculating important data like each binocular's exit pupils
(a 7X35mm bino has an objective, or forward lens of 35mm and an exit pupil
of 5mm-the diameter of the objective divided by a magnification of 7; peak dawn and dusk performance requires a binocular with an exit pupil
anywhere from 4.7mm to 7mm, which corresponds with the widest aperture the human pupil can physically achieve) and relative brightness (a mathematical shortcut allows us to subtract magnification from the diameter of the objective.
The figure obtained by dividing the power into the objective diameter is called the exit pupil
It reduces both weight and size, yet still provides a large 6mm exit pupil
for low light use when set at 7x.
They not only give you a broader field and a larger exit pupil
for brighter images in shadow, they also offer more depth of field, helping you identify detail just short of and beyond the focus distance.
And with an 8x magnification you have but a 2.75mm exit pupil
to deliver that light to the eye.
The size of your telescope's exit pupil
is given by the diameter of the telescope's objective divided by the magnification.