Binoculars

Binoculars, also called field glasses, are tools that help people see things far away more clearly. They are made from two small telescopes placed side by side, so you can look through both eyes at once. This is called binocular vision, and it helps you see things in three dimensions, just like you do when you look around normally.

8×42 roof prism binoculars with rainguard and opened tethered lens caps

Most binoculars are small enough to hold with both hands, but they come in many different sizes. Some are tiny enough to carry in a pocket, like opera glasses, while others are very large and used for special purposes, such as on ships.

Unlike a single telescope, called a monocular, binoculars give you a three-dimensional image. This makes binoculars very useful for activities like birdwatching, sports, concerts, and exploring nature.

Optical design evolution

Binoculars have changed a lot since telescopes were invented. Early binoculars used Galilean optics. This design has a convex objective and a concave eyepiece lens. It shows an upright image but has a narrow view and limited magnification. These are still used in simple models and special tools.

WW1 era Galilean type binoculars

Later, Keplerian optics improved the image and magnification. These use a positive eyepiece lens but create an upside-down image. Extra lenses or prisms are needed to correct it. Prisms, like the Porro and roof prisms, help make the image right-side up while keeping the binoculars compact. These designs allow for better depth perception and are used in many modern binoculars.

Main article: Galilean optics

Main articles: Porro prism, Roof prism

Optical systems and their practical effect on binoculars housing shapes

Binoculars have three main parts that help you see far away. The front part, called the objective lens, collects light and creates an image. The middle part, often using special glass pieces called prisms, makes sure the image is right-side up and not flipped. The back part, called the eyepiece, makes the image bigger so you can see it clearly.

High-quality binoculars today work very well, no matter which design they use, because of improvements in materials and technology. In the past, some designs worked better than others, but now they all perform similarly.

Optical parameters

Binoculars are made for different uses, and each type has special features that help them work better for that use. These features are called optical parameters and are often shown on the binoculars.

Magnification

Magnification makes things look bigger. For example, 7×35 binoculars make things look seven times larger. Hand-held binoculars usually have magnifications between 7× and 10×. Bigger magnifications can make the image shake more and need a steady hand or a tripod.

Parameters listed on the prism cover plate describing 7 power magnification binoculars with a 50 mm objective diameter and a 372 foot (113.39 m) field of view at 1,000 yards (914.4 m)

Objective diameter

The objective diameter is the size of the front lens. A bigger diameter lets in more light, making the image brighter and sharper. For example, 8×40 binoculars will show a brighter and sharper image than 8×25 binoculars, even though both make things look eight times larger.

Field of view

The field of view is how much you can see through the binoculars. It is usually smaller with higher magnifications.

Exit pupil

The exit pupil is the beam of light that comes out of the binoculars. It should match the size of your eye’s pupil for the best view. Bigger exit pupils make it easier to see, especially when your eyes are moving.

The small exit pupil of a 25×30 telescope and large exit pupils of 9×63 binoculars, the latter suitable for use in low light

Twilight factor and relative brightness

These are older ways to measure how well binoculars work in dim light. They help tell how bright and clear the image will be.

Eye relief

Eye relief is the distance from the eyepiece to where you should place your eye to see clearly. It is important for people who wear glasses, as they need more eye relief to see well without their glasses blocking the view.

Close focus distance

This is how close the binoculars can focus on an object. It can range from very close to quite far, depending on the design.

Eyepieces

Eyepieces are the lenses closest to your eye. Common types include Kellner and Erfle configurations, which help provide a clear and comfortable view. Some high-end binoculars also have a field flattener lens to improve sharpness across the whole view.

Mechanical design

Binoculars help you see clearly at different distances in two main ways:

Independent focusing lets you adjust each side separately. This is useful when you are outside or on a boat.
Central focusing uses one wheel to adjust both sides at once. Some central focusing systems also let you adjust one side a little more to match your own eyes.

When you look through binoculars, things far away might look blurry. This happens more the closer you look (higher magnification). Some binoculars can only focus at one set distance — these are simpler but not as flexible.

Porro type, external eyepiece bridge central-focusing binoculars with a rotating diopter on the right eyepiece allowing to adjust refractive differences between the viewer's left and right eyes

Most binoculars can be adjusted to fit different people by changing the distance between the two sides. This matches how far apart your eyes are. If this doesn't fit well, it can be hard to see clearly.

Some binoculars have special parts that help steady the picture when your hands shake. This can make it easier to use them without a tripod, but these binoculars are often bigger and cost more.

Binoculars can be made from different materials like metal or plastic. They might also have a rubber cover to make them easier to hold and protect them from bumps.

Optical coatings

Main article: Optical coating

Binoculars have glass pieces that need special coatings to work well. These coatings let more light through, reduce glare, and keep the lenses clean. They are made from very thin layers of different materials.

One important type is called anti-reflective coating. It helps reduce light loss when light bounces off the glass. This makes the picture clearer and brighter. These coatings can also help colors look more natural.

Another type is called phase-correction coating. It fixes problems that happen when light passes through certain parts of the design. This helps make the picture sharper.

There are also mirror coatings that help reflect light properly inside some binoculars. And special dielectric coatings can make reflections even better in certain designs.

Binoculars may be described as "coated optics" or "fully multi-coated" to tell you what kinds of coatings they have. These coatings help make the binoculars give you the best possible view.

Binoculars with red-colored multicoatings. Sometimes they are fraudulently advertised as "infrared" binoculars.

Accessories

Binoculars are easier to use with accessories. Straps for your neck or shoulder help you carry them. Special harnesses spread the weight if you carry them for a long time.

Other helpful items include cases to store and carry them safely, covers to keep rain off the lenses, caps to protect the ends of the lenses, and kits for cleaning the lenses and surfaces. Some adapters let you attach binoculars to a tripod for steady viewing.

Applications

Hand-held binoculars come in many sizes. Small ones, like 3×10 Galilean opera glasses, are used in theaters. Larger ones, with 7 to 12 times magnification, are used outside. Compact binoculars are small and light, good for daytime use. They usually have 7× to 10× magnification and are easy to carry and store.

Binoculars are popular for birdwatching, hunting, and other activities. They help people see faraway things more clearly. For example, birdwatchers often use binoculars with 8× to 10× magnification to see birds. Hunters might use similar binoculars to spot animals from far away. Some binoculars even have special scales to help guess how far away something is.

Main article: Birdwatching