Binoculars

NOTES: Whether you’re gazing up at the stars through a telescope or sighting a bird with a trusty pair of binocs, whoever made your glass put a lot of thought into how to gather light and hold on tight to it. This is why telescopes, primarily used when it’s dark out, have such big objective lenses. Inside a pair of binoculars – really, a joined set of telescopes, allowing for the sense of depth that comes from two-eyed vision – light passes through multiple lenses and prisms to be magnified, oriented, and shaped. Every time light strikes a surface, some of it is lost. According to Maven’s Brendon Weaver, the cheap binoculars you grew up with might transmit 75 per cent of the light that enters all the way to your eye. For these specific binoculars, that number is upward of 90 per cent. In fact, the view is so sharp that particularly on field-flattening models, which have extra optical elements that keep the view sharp, even at the very edges of the lens, people can get motion sickness while panning around: The light is so perfectly refined, it’s overwhelming for a brain used to dealing with two measly human eyes.

THE WOODS

You’re out with a friend, walking in the woods, the rubber armour (11) on the magnesium barrels (9) of your binoculars tapping lightly against your chest with each step. Then you hear it: a repeated, scratchy, beggaring call – almost like a witch’s cackle – somewhere above you. You freeze, take the lens caps (2, 13) off the binoculars, and raise them to your eyes. You scan the trees, and when you find what you’re looking for, you quietly hand the binoculars over to your chum.

THE ADJUSTMENTS

Unlike you, your friend doesn’t wear glasses, so she twists the eyecups (3) to their deepest depth. That way, when the binoculars rest against her face, the ocular lenses (4) are about as far from her eyes as they were from yours when the binoculars were resting against your glasses. Then she pivots the hinge (8) so the barrels align with her eyes, and twists the focus wheel (5) to

clear up the image. She starts to sight the binoculars where you were looking, but something is still a little off, so she makes an adjustment for the difference between her eyes: First, she covers the barrel with the dioptre ring (1) and brings one eye into focus with the focus wheel. Then she covers the second barrel and uses the dioptre to focus the other eye – it adjusts lenses inside only one barrel, so the two barrels are focused slightly differently to match the difference in acuity between her eyes. Now, when she uses the focus wheel again – it’s connected through a series of gears (7) and pins (10) to the optical elements in both barrels – both eyes will get a sharp image. She retrains her sights on the source of the sound.

THE LIGHT

Light bounces off the trees and enters the binoculars’ objective lenses (12). (The objective and ocular lenses are actually sets of lenses, with each element performing a function such

as magnification, colour correction, or sharpening.) After the objectives, the image of the trees is magnified, but also inverted, so it continues to the binoculars’ roof prisms, named for the 90-degree angle formed by two of their faces (like a gable roof). When passing through this pair’s Abbe-Koenig prism assemblies (6), the image is righted with a minimal number of ‘bounces’, meaning minimal light loss. (An Abbe-Koenig prism also ejects light on the same axis it came in on, so the binoculars can have a better form factor than older, wider pairs that have the ocular lens offset from the objective.) The light from the trees then passes through the ocular lens set, out the eyepieces, and, finally, enters your partner’s eye, and she gasps at what she sees: two pileated woodpecker chicks sitting in a hollow in the tree, yawping at their mom, who sounds a higher and calmer chirp, more laugh than cackle, and gives her babies some food. – Kevin Dupzyk