Know Binoculars Back-to-Front

Zoom binoculars magnification tutorialBinoculars make a great gadget gift, for someone else, or for yourself—heck, you deserve it. But with all these technical-sounding numbers in their descriptions, how do you know which binoculars to get?  Does the “X” in a 10×50 pair stand for “eXceptional quality,” or “eXactly the wrong pair to take to the stadium for the big game”?

First things first: taken with the number in front of it, the “X” here is your standard multiplier, representing the binoculars’ power of magnification. So if you looked at a bear through a pair of 10×50 binoculars, you’ll be looking at a bear ten times larger than how it appears to the naked eye (if that sight doesn’t make you drop your binocs, anyway). There are also zoom binoculars, which allow you to adjust your level of magnification.Is more always better? With binocular magnification, no. Typically, the more your view is magnified, the harder it is to hold still. Anything above 12x is a challenge to keep steady with the human hand, so you’ll need a tripod, which limits the kind of places you’ll want to take it. And besides, since in most cases more magnification means a smaller view, you’ll have trouble finding and tracking anything that moves really fast—like a bird, or, on a good day, your wide receiver.

The second number is sometimes listed after the “x,” as in “10x50,” or sometimes it’s referred to separately as the “objective diameter,” or “aperture.” This number refers to the width, in millimeters, of the lenses farthest from your eye (the objective lenses), which control how much light gets into your binoculars. So in a 10×50 pair, the objective lenses are 50mm across.

Unlike with magnification, more is better when it comes to the objective diameter, up to a point. Assuming your lenses are of good quality, the more light that reaches your eye, the sharper and brighter your image will be. If you’re using your binoculars in low-light conditions, or for celestial observation, you need to take in more light to get a useful image. But while a full-size, 10×50 pair would give you enough light whether it’s day or night, it would also be a lot heavier than a mid-size 7×35. For most daytime applications—like watching your teenager mow the far end of the lawn from your hammock on the porch—an aperture in the range of 25–35mm should do the trick. With one caveat: watch your exit pupil.

No, an “exit pupil” isn’t a student who just walked out class—and don’t you give up now either, you’re too close to being a binoc master. And it’s actually a pretty simple concept. Binoculars take in a view, and then concentrate that view into a beam. Ideally that beam will be approximately the width of a dilated human pupil (7mm in an adult, though it decreases as you age), to maximize the information your eyes can receive. To find the exit pupil of a pair of binoculars, just divide the objective diameter by the power of magnification. In our case, 50/10=5mm, which is 2mm narrower than the 7mm we’re looking for.

So what does this mean? Have you ever used a pair of binoculars and noticed a black rim around your field of vision, or had trouble get your eyes lined up so you could see at all? That’s what happens with a narrow exit pupil. Because the image beam is smaller, your eyes have to stay in exactly the right place in order to see anything at all, which is tiring. And even then, you’re getting less information that your eyes are capable of taking in. You might think that an exit pupil of 7mm would be ideal then, since it matches a dilated pupil’s width—with a beam any wider, some of the light is falling outside the pupil, so you aren’t seeing it. This is true. But the wider the beam, the easier it is to line your pupils up with it. If you want to be able to pick your binoculars up at a pivotal moment and get a lock on the magnified view right away, going for a slightly more generous exit pupil is a good idea. And if you have a big lawn, you’re going to be in the hammock watching your teenager’s mowing work around the edges for a while. A larger exit pupil makes it easier to use your binoculars for longer. The rest of you is so relaxed—you wouldn’t want to strain your eyes.

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