Our glossary is an excellent quick reference you may want to have open as you browse our site or others. Whether you're just learning about binoculars or out to buy your first pair you don't want to be stumbling over terminology. We go over all the important definitions to help keep you on-track as you review binoculars, monoculars, scopes, and more. From aperture to thermal imaging, we have it all spelled out, so you don't end up cross-eyed from keeping terms straight!
When an image is distorted as it is viewed through optical devices, it is said to be caused by an aberration. It could be caused by poor glass quality, a mirror or prism out of alignment, or fogging of the objective lens. Higher quality devices may be more expensive, but will also be less likely to have aberrations.
When using an optical tool with two barrels such as binoculars, alignment refers to how clearly and well the two barrels work together. Alignment may be compromised if eyepieces are not focused in tandem or if some mechanism within the binoculars has broken or shifted.Zooming binoculars are sometimes criticized because their delicate zooming mechanisms can easily fall out of alignment, or become useless if the binoculars are subjected to jostling or dropping.
An aperture is simply the opening through which light can enter. Aperture is increased as objective lenses get bigger, and your objective lens size is listed in millimetres on binoculars as the second number in 7x50, 8x42, 10x25, and so forth. Bigger apertures and lens sizes are better in low light such as at dusk and dawn.
Armor refers to the shell or casing on binoculars and other devices, and ranges from thin and sleek through to rugged with tread hand grips. Armor quality is subjective to each individual, although in general you will want stronger, thicker armor when your binoculars will be used on the move, or if they might be dropped or bumped. It is most important for hiking, hunting, and marine use. However, it may not be worth the added expense if you plan to mount your telescope on a tripod or want a compact tool such as a pen-sized monocular.
Binoculars are optical tools that create a magnified image blended from an image to each eye. They are available in a multitude of different powers and magnifications.
Various coatings are painted on lenses to help improve optical quality. Each coating alters how the lens or glass reflects and transmits light, and many eliminate reflections. These optical coatings often result in clearer, brighter images. Roof prism devices in particular must use coatings.
Close focus simply refers to focus that is effective very close to the viewer. It is important when using a microscope or monocular to view insects, plants, and other details very closely.
Contrast is the difference of light in the foreground versus that in the background. Higher contrast means sharper, more defined edges in an image. Better contrast in binoculars means you will see objects more clearly with true, crisp colors.
DEPTH OF FIELD
Depth of field is the portion of an image that appears sharp, particularly while zooming and focusing. You'll want a wider depth of field if you are surveying a meadow during your hunting trip, but a smaller depth of field when viewing butterflies up close.
DIGITAL CAMERA BINOCULARS
Digital camera binoculars offer a digital camera and binocular combined, so you get the magnification and clear images available through binoculars and can take pictures of the images as well.
Distortion and aberration are often used interchangeably and simply suggest a poor or unclear image seen through optical devices. Good quality glass, coatings, and optimum focusing and light use help minimize distortion. Very low end binoculars may, on the other hand, contain lower end parts that result in distortion.
The exit pupil is a virtual opening at the center of each lens through which light can move. The exit pupil should be the same size as your own pupil for optimum viewing. Night viewing devices will have bigger exit pupils to match your dilating human pupils, but during day use, extra large exit pupils would simply waste light.
Eye cups are found on the edge of the binoculars you rest against your eyes, and can be molded with gentle materials and soft design to increase your comfort. If using your binoculars for a long period, you will find comfortable eye cups to be worth looking for.
Eye pieces are also on the edge of the binoculars you place against your eye, however this refers to the lenses themselves. Some devices such as telescopes and telescopic binoculars have different eyepieces you can switch in and out for different magnifications. These allow you to focus at different distances without zooming mechanisms built in to your device.
Eye relief is the distance at which you must hold the binoculars away from your eyes to use them. You will want larger eye relief if you wear glasses.
FIELD OF VIEW
Field of view (FOV) is just as it sounds; the area you can see when you use an optical device. This is given in an area of feet or meters you can see 1000 yards away through your binoculars. You will want to be able to see a bigger area if watching sports or car races, but a smaller area is fine for viewing butterflies and other insects. Higher magnifications cause smaller FOV's, although wide-angle objective lenses can increase them.
Using focus, you control light rays to make part of an image sharp and clear. Binoculars often offer a center focus which controls both barrels simultaneously, or individual eye focus so you can alter either barrel separately as you like. For example, you might want individual eye focus if one of your eyes is stronger than the other.
Giant binoculars are typically used for astronomy, and feature huge objective lenses of 70mm, 80mm, 10mm or even greater. Typically these will need to be mounted as they are quite large and heavy, but the extra large objective lenses make them usable at night while star-gazing.
Image intensification is a technology used in night vision devices to maximize the use of visible light in an environment. Some night vision devices also use infrared illumination.
Some night vision devices use infrared to illuminate an area, which is then measured and interpreted by the device so it becomes visible to humans. This allows people to view images when image intensification (maximization of visible light) is insufficient. Infrared illumination can often be switched on and off in devices that use it.
LIQUID CRYSTAL DISPLAY (LCD)
A display technology common in digital cameras to increase visibility and reduce eyestrain.
Magnification is the act of making an object appear larger. In optical devices typically magnification ranges from about 7x and 10x up, depending on the device. Astronomers will want very high magnification to see distant stars, whereas casual birdwatchers may be happy with 10x magnification. In some instances even 2x or 5x magnifications are useful; for example, with monoculars used to glimpse the chalkboard in a lecture hall, or to zero in on the tiniest features of plants.
Monoculars are single-barrelled and very compact devices, often similar in size to a pen or human finger, for magnifying images.
Night vision devices make viewing images possible in low-light and completely dark surroundings. These are commonly used by the military, aviators, and law-enforcement for surveillance. Night vision is also particularly useful for nocturnal hunters, hikers, and birders.
Objective lenses are those on the far end of binoculars as you hold them to your face, and control how much light enters. Wider lenses bring in more light, making it possible to see objects when it's dark outside. Lens sizes vary considerably however, as larger ones can get quite heavy. For example, telescopes often have giant lenses, but need to be mounted on a tripod as a result.
Opera glasses are often pocket-sized and very decorative. They offer fairly low magnification and large lenses for theater-goers.
Phosphor coating emits light when excited by energy. The screen on night vision equipment is green phosphor.
Porro prism binoculars are designed by placing prisms in a double Z-shape within the device. As a result, Porro prism devices can be bigger then other types, but may need to be re-calibrated. Porro prism sets are often less expensive and more popular.
Rangefinders are optical measuring instruments to estimate distance. They are useful for golfers, mariners, and hunters, particularly during target practice.
Sights for hunting that mount onto a rifle and make targets appear closer than they actually are.
Roof prism binoculars use small, shallow prisms, and are more compact. Binoculars that use Roof prisms, as a result, are also smaller and lighter. However, special coatings are required on Roof prisms, making them the more expensive option.
Spotting scopes are telescopes intended only for land viewing rather than used for astronomy. As a result these are often lighter weight with smaller lenses.
Thermal imaging is used in night-vision devices and takes advantage of the infrared light emitted by all objects. Every object gives off this infrared light depending on its temperature, so thermal imaging can be used in complete darkness.
Twilight factor is used particularly in astronomy, and is a calculation derived by multiplying objective lens size by magnification, then taking the square root of that number. Generally you want to work with a twilight factor of 17 or more in astronomy. The larger the twilight factor, the more you can see when light is lower.
WATERPROOF, WEATHER-PROOF, WATER-RESISTANT
Waterproof binoculars prevent leakage and damage from fogging, and are guaranteed by the manufacturer to do so. Binoculars may also be labeled weather-proof or water-resistant, which suggests they can stand up to some moisture, but are not durable enough to handle marine use.