Parts of a Binocular – Lenses, Eyepieces and Focus Wheel

All 9 parts on the diagram

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Start here: 5 main parts 5

Objective lenses The objective lenses: the big front lenses that gather the light. Their width in millimetres is the second number, as in 8×42.
Barrels The barrels: the two tubes, one telescope for each eye.
Prism housings The wider parts holding the prisms. The prisms flip the image upright. Porro prisms set the front lenses wider apart than the eyepieces.
Focus wheel The focus wheel: turned with a finger to focus both sides together.
Eyepieces The eyepieces: the small tubes at the back, one for each eye.

Other 4

Hinge It folds the two halves closer or wider apart. That matches the distance between your eyes.
Diopter adjustment A ring on one eyepiece that sets that side for your eye. Both eyes then see sharply.
Eyecups The eyecups: the cups your eyes rest against. Twist-up cups can be lowered for glasses wearers.
Strap Hangs the binoculars round your neck. It is tied to a lug on each side.

How this list was compiled: Read 2026-09-27. Wikipedia, Binoculars, https://en.wikipedia.org/wiki/Binoculars : binoculars are two telescopes side by side, one for each eye; the objective lens gathers light, and its diameter in millimetres is the second number in a description such as 7×35; prisms erect the image, and Porro prism designs set the objective lenses wider apart than, and offset from, the eyepieces; central focusing turns a central focusing wheel to adjust both tubes together, and one eyepiece can be adjusted for differences between the eyes, the dioptre; a hinged construction adjusts the distance between the two halves to the user’s interpupillary distance; twist-up eye cups can be retracted for eyeglass wearers; straps. Licence: own work

The drawing: Drawn for this site on 2026-10-05 by script, from plain geometry: one pair of Porro prism binoculars held as a small model (tapered barrels, prism housings, hinge, focus wheel, bridge, eyepieces, diopter ring, eyecups, strap) and shown in a single projection from above and a little from the front. Nothing is traced or copied. The forms and proportions were checked by eye against photographs and diagrams of Porro prism binoculars on Wikimedia Commons. Licence: own work

How it works

These are the parts of binoculars on a Porro prism pair seen from above: the big front lenses at the top, the eyepieces at the bottom. Start with the 5 main parts.

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Front to back

Binoculars are two telescopes side by side, one for each eye. Light comes in through the objective lenses at the front of the barrels. Prisms in the housings turn the image the right way up, and you look through the eyepieces.

Setting them up

Fold the halves on the hinge until the eyepieces match your eyes. Focus with the focus wheel, then turn the diopter ring on one eyepiece until that eye sees sharply too.

Read the two numbers on the body: in 8×42, 8 is the magnification and 42 is the width of each objective lens in millimetres.

Questions and answers

What is the diopter on binoculars?

A ring on one eyepiece that focuses that side on its own, to make up for a difference between your two eyes. After it is set, the focus wheel focuses both sides together.

Why are the front lenses wider apart than the eyepieces?

That is the Porro prism design: the prisms fold the light sideways, which moves the objective lenses out. Roof prism binoculars keep them almost in line with the eyepieces.

What does the hinge on binoculars do?

It folds the two halves closer together or further apart, so the eyepieces match the distance between your eyes.