Sierra Trading Post Explores: How Do Binoculars and Scopes Work?

Whether you're an avid birdwatcher or limit your bird watching to scoping out pheasants with a shotgun by your side, you probably depend on binoculars to get a good look at the birds in question. And if you're shopping for a good pair of binoculars, you're pretty much guaranteed to end up with a pair you love if you're knowledgeable about how they work. So, keep reading to educate yourself on the inner workings of binoculars and how they make clear, focused and up-close viewing possible at a distance.

How do binoculars work?

Binoculars use two pairs of convex lenses to collect and magnify an image: the objective lens, which is farthest from the eye, and the eyepiece lens, which is closest to the eye. The objective lens faces forward to catch the light and transfers the image to the eyepiece lens. The convex eyepiece lens magnifies that image. Binoculars also contain prisms, which are housed between the objective lens and the eyepiece lens, to flip the image right side up.


Scopes and telescopes work in the same way, only with one pair of lenses instead of two.

But this explanation only brings up more questions for me. Like, how exactly do binocular and scope lenses magnify images? And why do binoculars need prisms?

So, let's get to it and figure all of this out.

How do convex lenses work?

A convex lens is curved like a dome and is thicker in the middle than at its edges. Convex lenses focus distant light rays by bending and converging light toward the thicker middle. The focused light makes distant images appear bigger and closer, which is why convex lenses are used in magnifying glasses, telescopes and binoculars.

Convex lens in Binocular

Why do binoculars have prisms?

Now we know that binoculars utilize lenses to collect the light from a distant object and focus that light into a larger image. But what about the prisms between the lenses? What's the purpose of those? Well, convex lenses are really good at enlarging distant images, but those images look upside-down because the light crosses over and flips around after passing through the convex lens. Two prisms are required to turn that image 180° so that it looks right-side-up to the viewer (each prism turns the image 90°).  Binoculars tend to have one of two different types of prisms: Porro prisms or roof prisms. To get the best pair of binoculars for your needs, you need to know the difference between these two.

What's the difference between roof prisms and Porro prisms?

Porro prisms have two offset, Z-shaped prisms that work together to flip the image 180° for right-side-up viewing. Binoculars with Porro prisms tend to be larger than binoculars with roof prisms, but they also offer a clearer image. Binoculars with roof prisms have back-to-back prisms in a straight tube, so they are smaller and lighter, but the image isn't quite as crisp or bright. It's a trade-off, so when shopping for binoculars, ask yourself if size or image quality are more important.

roof and Porro prisms in Binoculars

I covered a lot of ground here, but there's still a ton more to learn about binoculars. Check out our Optics Buying Guide to learn more about binocular components, power and care.

That wraps up this installment of Sierra Trading Post Explores. I hope you learned a thing or two. Join me next month for another look into the lesser known aspects of your outdoor adventures and gear!
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