Focus on What Matters – Part 2: Exit Pupil

In Part 1, we talked about the misconception that a bigger objective lens automatically means a brighter scope. Now let’s talk about something that actually matters: exit pupil.

Exit pupil is simply the diameter of the beam of light leaving the scope and reaching your eye. The math is easy: divide the objective diameter by the magnification.

A 50mm objective at 10x gives you a 5mm exit pupil. A 40mm objective at 8x also gives you a 5mm exit pupil.

Here’s where it matters. Your eye can only accept a certain diameter of light. In bright conditions your pupil gets smaller, and in darkness it gets larger. A young, fully dark-adapted eye may reach around 7mm, but that maximum generally decreases as we age.So let’s take that same 50mm scope. At 5x it produces a 10mm exit pupil. Your eye can’t open to 10mm, so you can’t take advantage of all of it. Turn that scope up to 20x and the exit pupil drops to only 2.5mm.

Same scope. Same objective lens. Completely different result.

So what does this all mean? As I’ve aged, I can certainly tell my eyes don’t dilate like they used to. Years ago, I never fully understood the “tight eye box” effect people talked about. I see it now, especially when using scopes at higher magnification. It finally clicked for me.

Exit pupil and eye box aren’t exactly the same thing, but they work together in what the shooter experiences. As magnification increases and exit pupil gets smaller, your eye position becomes much less forgiving. Move your head slightly out of position and the image starts disappearing.From a hunting standpoint, I’ll take higher-quality glass and lower magnification over lesser-quality glass, more magnification, and a giant objective lens any day of the week.Think about what we’re actually doing. Many of the animals we hunt are crepuscular, meaning they’re most active around dawn and dusk. We’re trying to see clearly when available light is already limited.That’s why I wouldn’t automatically go down the giant-objective-lens path either. There are absolutely times when a larger objective is beneficial, but bigger isn’t automatically better, and depending on the magnification and the shooter’s own eyes, some of that potential may not even be usable.

Buy the optic for the job and the person looking through it. Focus on what matters.