What are binoculars?
Simply put, binoculars are optical instruments used to view distant objects - we can also say, they bring things nearer/ closer, or that they make objects appear larger than they appear with naked eyes. They are made of 2 small telescopes/ tubes set side-by-side that allow you to use both eyes for seeing ("binocular vision") and giving a natural 3-dimensional viewing experience because of the overlapping images seen by both the eyes. For convenience of understanding, we can call the arms of the binoculars as tubes or barrels.
They come in several sizes and configurations - from small pocket sized models to large ones that require a tripod, from low magnification ones that are meant to be used in a theatre to watch a play more closely, medium powered ones used for birdwatching or sports events like horse-racing, to high powered ones that you can use to see planets or use in the field for military usage or observe wildlife.
In order to know more about binoculars, it is important to get familiarized with some terms - related to the parts of the binoculars as well as usage/ configurations, so we are explaining them here. we also talk about different categories or classifications based on the operating mechanism, usage purpose, focus-ability, size and price range. Although the right way to refer to this equipment is "a pair of binoculars", we use the terms binoculars/ binocular/ binocs/ binoc/ bino interchangeably on this website.
Parts of a binocular
Regardless of the type of binocular, some common parts of binoculars are explained below. Keep in mind that not all parts are available on all models.
Objective Lens - the lens at the end of the barrel that faces away from the eyes and that directly points to the target is the objective lens. This lens collects light and passes it on towards the eyes. The larger the objective lens, the more is the light gathering capacity of the binocular. Some models have coatings on the lenses, and these can serve a variety of purposes - UV protection, anti-reflection, anti-fogging, scratch resistant, and so on.
Eye piece - the lens closest to the eye is the eyepiece. it picks up the light passed to it by objective lens and prisms, and converges it so we may view the image.
Central Focus - A large wheel or knob between both barrels that changes the distance between the objective lens and eye piece. As different targets are at different distances, this mechanism allows you to change focus, making the image sharper.
Dioptre adjustment - Often, one of the eye pieces (usually the right side) has a dioptre-adjustment feature - meaning the eye piece can also be rotated and its focus changed. This is to account for difference in power between the 2 eyes, so that the image can appear sharp to both eyes.
Eye cups - these are the parts holding the eye-pieces. They cut off stray light around your eyes so you can view comfortably through the binocs. Some models of binoculars offer "foldable eye cups" for greater convenience. If you use spectacles, you may find it better to use the binocs with the eye cups folded down. Fun fact: You can use binoculars even without wearing your spectacles, as they have their own focusing system! However, keep in mind that while the image through the binocs will not be impacted due to presence or absence of your eye-glasses, you may find it difficult to see around you without them! :)
Tripod compatible mounting screw - Most binoculars larger than palm-sized ones, typically have a 1/4 inch female screw adaptor that you can use to mount your binoculars on a tripod (using an additional accessory called L clamp or the binocular-mounting adaptor)
Prisms - Except the very basic and toy binoculars, most real binoculars have prisms between the objective lens and the eye piece. Due to the way light passes through the various lenses before reaching your eye, the image formed is inverted. The prisms perform correction of the image orientation so that you can get an erect view. Some models have porro prisms while some have roof prisms - more on this in a subsequent section.
Zoom lever - If your binocular came with variable zoom, it has a lever on one of the barrels just below the eyepiece . This lever helps you to zoom in/ out while viewing.
Optical Terminology / Specifications
Magnification (or mag.) -
Mathematical definition: the focal length of obj. lens divided by the focal length of eye piece
Practical significance: The magnifying power of the binocular can be described as how closer it brings the image, or, how much larger it makes the image appear, compared to the original object viewed from the same position. For example, a 10x magnification binocular magnifies 10 times/ brings the object 10 times closer/ makes the object appear 10 times larger than it appears without binoculars.
Another easy way to understand magnification is - it is the factor by which the distance of the viewed image reduces. Example: If you are viewing an object at a distance of 1 Km through a 10x binoc, then the object will appear as though it is just (1 Km/10) = 0.1 Km or 100 m away.
Aperture/ Objective Diameter - the diameter of the objective lens, specified in millimeters, is called the aperture/ objective diameter. The larger the objective lens, the more is the light gathering capacity of the binocular, meaning it will give you brighter and crisper images. interestingly, larger objective lenses also give more stable/ steady images, though this is not the only factor that determines stability.
Configuration - the key specifications magnifying power and objective diameter of the binoculars, mentioned together in below format is known as the configuration of the binoculars.
[Magnification] X [Obj. Diameter]
8x40 - A binocular with 8x magnification and 40mm objective lenses
10x50 - A binocular with 10x magnification and 50mm objective lenses
7x35 - A binocular with 7x magnification and 35mm objective lenses
Real Field of View (FOV) - this indicates the extent or range of objects you can observe through the binocs, as compared to the actual range of space available. FOV may be specified in 2 ways:
Linear - how many meters / yards can be seen at a distance of 1000 meters / yards (example: 110/1000 indicates an observable range of 110 m at a distance of 1000 m)
Angular - The angle of space from centre of the obj.lens, that will be visible through the binocs.
The wider the FOV, the more is the "coverage" area you can get at a given magnification (i.e., the amount of viewable space you can fit in the frame)
Apparent FOV - the angle of coverage within which the object would appear if you were to view it without binocs, from the distance at which it appears through the binocs. Conventionally Apparent FOV was calculated as Real FOV multiplied by magnification, but recently better techniques have emerged for more accurate calculations.
Focusing distance - the minimum distance at which the object or target can be for the binocs to be able to focus on them (the closest point on which the binocs can focus)
Exit Pupil - the width of the light beam created at the exit of the eyepiece. The larger the value (usually specified in mm) the brighter the image. This value can be calculated by dividing obj.diameter by magnification. Example: a 10x50 binocs will have an exit pupil of (50/10) = 5mm.
Eye Relief - the distance from the eyepiece, at which your eye can comfortably view the image being formed. This value is also specified in mm (millimeters)
Relative brightness - The square of the exit pupil (i.e., exit pupil multiplied by itself). Example: a 10x50 binocular having exit pupil 5mm will have a relative brightness value of (5*5) = 25. The higher this value, the brighter the image.
Features of binoculars
Your binocs may offer some or all of the below features. The combination of various features is different based on suitability to usage purpose.
Interpupillary Distance (IPD) adjustment - the 2 barrels/ arms of the binoc can be adjusted around the central housing/ hinge so as to change the distance between the 2 barrels. This helps in aligning the eyepieces to the distance between the 2 eyes, as the IPD varies from individual to individual.
Central Focus - explained in above section
Dioptre adjustment - explained in above section
Independant Focus - in some models, both eyepieces are individually/ separately focus-able. This feature has some disadvantages as it is not easy to adjust both eyepieces while viewing a moving object or while viewing different objects that are at different distances.
Wide angle - Some binoculars come with a higher FOV as compared to others - this feature helps in viewing fast moving subjects like birdwatching or wildlife, or are also particularly useful in astronomy and stargazing
Coated optics - Binoculars may have various coatings. Some of the advantages include UV protection, anti-fogging properties, anti-reflective qualities, and so on.
Long eye relief - A binoc offering long eye relief provides stress-free viewing. In other words, your eyes can view through the binoc comfortably without undergoing strain.
Waterproof - some specific models meant for marine use come with waterproof housing. In regular binoculars this is not a common feature, though they may be water resistant instead. (waterproof indicates can be used under water whereas water resistant means that while it is not intended to be used in water, occasional accidental contact with water may not damage the equipment)
Fixed focus / focus-free - Some binoculars come with fixed focus (they lack a focusing mechanism such as central focus wheel) and rely on your eyes' ability to adjust for focus. These have several disadvantages as their usage range is very limited and produce strain on your eyes.