Galileo was the first to discover that distant objects could be magnified by looking through a series of aligned lenses. This discovery led him to invent the telescope, and eventually, binoculars.
Binoculars are essentially two small telescopes that are joined together and mounted on a metal or plastic frame, making them portable and easy to use.
Because both eyes are used for viewing, binoculars offer a multi-dimensional and complete image of faraway objects that telescopes can't.
In binoculars, small curved lenses in the eyepieces are aligned with larger curved lenses (called objective lenses) at the opposite end. The lenses both collect light and magnify images.
Mirror-like pieces of glass called prisms are set inside most binocular models. Prisms bend and refract incoming light, supplementing light that has been lost as it passed through the lenses, and sharpening and brightening viewed images.
When an image is first captured by a set of binocular lenses, it is backwards and upside-down. Prisms invert the image so that it can be seen correctly.
Porro prism binoculars have a side-by-side prism placement, and are bulkier but relatively inexpensive. Roof prism models have a streamlined and lightweight outward design, but are internally complex and typically more expensive.
Binocular lenses are coated to reduce glare and keep images sharp. The number of coatings on specific binocular models can range from a single coating on a single lens to multiple coatings on every lens.
All binoculars are used for long distance viewing, but not all binoculars are suited to the exact same purpose. Birdwatchers will need a very different style of binoculars than outdoor sporting event enthusiasts.
For hunters, birdwatchers, and hikers, binoculars with rugged, water and fogproof construction and large objective lenses are best. For sporting events, boating, and casual daytime use, lightweight binoculars with smaller objective lenses and auto or center focus are best.