The first is the magnification and the second is the aperture of each front lens in millimetres. Dividing the second by the first gives the exit pupil, which is what decides how bright the view is for your own eyes.
The naming convention is one of the few genuinely informative labels in optics, and it answers most of the buying question on its own.
The first number
Magnification. Above roughly ten, a hand-held image trembles enough to hide detail the aperture collected, which is why a tripod thread stops being a nicety at that point.
The second number
The clear aperture of each objective in millimetres. It decides how much light arrives, exactly as it does on a telescope.
The number they hide
The exit pupil, found by dividing the second by the first. If it is wider than your own dark-adapted pupil, the surplus lands on the iris and is lost — which is why the same binoculars deliver differently to two people, and why pupil width narrowing with age is a real consideration.
What is not in the name
Prism glass, coatings, eye relief and how well the two barrels are aligned with each other. Those decide how comfortable a pair is to use for an hour, and none of them is in the two numbers.
The questions that keep arriving
Is a bigger second number always better?
It collects more light and weighs more, and past a point it needs a tripod. The useful question is whether the exit pupil it produces is wider than your own pupil, because the surplus beyond that is discarded.
What about the field of view printed on the barrel?
That is usually the true field, either in degrees or as a width at a stated distance. It is what actually fits, and it is the more useful of the two field figures.
Last reviewed 17 September 2026