Glossary of Binocular Terms

If you don't know your Diopter Adjuster from your Ocular Lens or don't know the difference between Porro Prism and Roof Prism binoculars, this article will help. Here are the most commonly used terms when talking about binocualrs and what they mean. This will in tern help you make an informed decision when purchasing the correct binoculars for your specific needs.

Amici Prisms
Also called "roof prisms" or "right angle roof prisms," an Amici roof prism is named after it's inventor, the Italian astronomer Giovanni Amici and is similar to the Schmid design. They revert and invert the image as well as bend the line of sight through a 90° angle (They deviate a beam of light by 90° while simultaneously inverting the image). They are excellent as prism diagonals in optical systems, because they erect the inverted image. Also ideal for use in spotting scopes, and any optical instrument where it is desirable to take an inverted image from an objective, turn it right side up, and bend it through a 90° angle, to maintain the correct visual orientation.

Aperture
In optics, an aperture is a hole or an opening through which light is admitted. However when we speak of aperture and binoculars, aperture refers to the diameter of a binoculars' objective lenses and is measured in millimeters. The aperture is the second number represented when describing a set of binoculars. Example: In the Steiner 8.5x26 Wildlife Pro Binoculars, 26 would represent the aperture. It is an important number when considering how well a pair of binoculars will work in low light conditions. See Exit Pupil for more information.

Aspheric Lenses
An aspheric lens is simply a lens with a surface which is not perfectly spherical or not perfectly convex or concave or, to put it another way, you can find different areas on the lens with different degrees of curvature. By using different degrees of curvature, a single aspheric lens can often do the job of two or more spherical lenses, resulting in a lighter instrument and/or a cheaper one. On the other hand, there is nothing automatic about the performance and quality of a lens just because it has the aspheric label. For instance, many low end optics use aspheric lenses that are poured into a mold, making them cheaper to produce than a conventional ground lens. At the other end of the quality spectrum are aspheric lenses which are ground just like a spherical lens but only in much more sophisticated shapes. These are very difficult and expensive to produce, but the results can be incredible.

BAK-4 prisms
BAK-4 prisms are made of superior optical glass that produces clearer images. These are what you want in your binoculars.

BK-7 prisms are usually used in lower priced binoculars. These are satisfactory, but they are inferior to the BAK-4 prisms. Some manufacturers will not tell you what kind of prisms they use, usually because they are of inferior quality.

Optical glass quality varies widely between models, this is one of the many reasons for the wide range in price that you will find in the stores ($30 - $2000+). The less expensive binoculars generally are manufactured with BK-7 prisms. If you turn your binoculars around and look down the front lens towards the inside, you can see the difference between BK-7 and BAK-4 (much better prism quality). If the binoculars have BK-7 prisms, you can see a squared off side to the general roundness of the image. BAK-4 prisms show a truer round, which translates to better light transmission and edge-to-edge sharpness.

SK15 prisms: Prisms of very high quality SK15 glass that enable minimisation of undesirable internal reflections and thus provide a crystal clear image with the best contrast.

Blacking Out
See vignetting.

Chromatic Aberration
Chromatic aberration or "color fringing" is caused by the lens not focusing different wavelengths of light onto the exact same focal plane (the focal length for different wavelengths is different) and/or by the lens magnifying different wavelengths differently. These types of chromatic aberration are referred to as "Longitudinal Chromatic Aberration" and "Lateral Chromatic Aberration" respectively and can occur concurrently. The amount of chromatic aberration depends on the dispersion of the glass. To correct this some high end binoculars use extra low dispersion glass.

Longitudinal or Axial Chromatic Aberration Focal length varies with color wavelength	Lateral or Transverse Chromatic Aberration Magnification varies with color wavelength

Collimation
Collimation refers to the optical and mechanical alignment of the binoculars. If a pair of binoculars is out of collimation, after prolonged use it kind of feels like they are trying to suck your eyes out of your head. Cheap binoculars are often shipped from the factory out of collimation. Good binoculars are carefully collimated, often with laser instruments. This requires time and expense at the manufacturing level, and raises the price at the retail level.

Depth of Field
Depth of Field refers to the distance from "near to far" that is in focus at a certain setting of the focus adjustment or at a certain distance. In a given system, as the magnification increases, depth of field decreases. This fact is one of the disadvantages of binoculars with high magnifications and why depth of field is usually more important in comparing spotting scopes or telescopes than binoculars. At very high magnifications, the depth of field can be so shallow that precise focusing is critical and so the location, size, action and feel of the focusing adjustment is an important consideration. Depth of field also changes with the distance observed, usually decreasing in depth as the distance decreases.

Dielectric Coatings
These are coatings found on roof prisms and are there to increase light reflectivity - As the light is incident at a glass–air boundary with an angle less than the critical angle, total internal reflection does not occur at that surface. To get around this problem, a mirror coating is used on those surfaces. Typically an aluminum mirror coating is used that has a reflectivity of 87% to 93%) or a silver mirror coating (reflectivity of 95% to 98%) is used. This light transmission of the prism can be improved by using a dielectric coating rather than a metallic mirror coating. This causes the prism surfaces to act as a dielectric mirror. The dielectric multilayer coating increases reflectivity from the prism surfaces by acting as a distributed Bragg reflector. A well-designed dielectric coating can provide a reflectivity of more than 99% across the visible light spectrum. This reflectivity is much improved compared to either an aluminum or silver mirror coating and the performance of the Schmidt-Pechan prism is similar to the Porro prism or the Abbe-Koenig prism.

Diopter Adjuster
A separate eyepiece-focusing tool, usually on the right lens of your binoculars, that allows you to adjust the lenses separately to allow for differences each of your eyes. It plays an important part in correctly focusing your binoculars.

Dioptric Correction
The ability to make an adjustment of the optical instrument like a pair of binoculars to the varying visual acuity of a person's eyes. It is the adjustment of one lens to provide compatible focus when the viewer's eyes have differing visual capabilities. One result is less strain on the eyes that allow for optimal viewing of depth and contrast.

Eye-cups
Eye-cups are related to the eye relief as they keep the distance from the oculars to our eyes, but also help keep stray light away from your eyes while using your binoculars. Many eye-cups are made from rubber and can roll up or down depending on whether you use glasses or not. The problem with these is that the constant rolling causes the eye-cups to break. Another type are eye-cups are ones that slide rather than roll, but these can be hard to keep in place. The third type are eye-cups that twist up and down and so they can be left at any position from all the way up to all the way down, some even have click stops at regular intervals with the eye relief distance for each stop marked on the cup so you can get the perfect eye relief for your vision.

Eye Relief
Eye relief is the distance between the ocular lens and the exit pupil. The most ideal scenario would be when the binocular is raised to the eye the exit pupil should be focussed onto the front of the eye. This allows the observer to see the whole field of view correctly. If the eye relief is too short vignetting is often found to occur around the periphery of the vision. Eye relief can be particularly important for eyeglass wearers and shooters. The eye of an eyeglass wearer is typically further from the eye piece which necessitates a longer eye relief in order to still see the entire field of view.

Exit Pupil
This is the amount of light rays that enter the objective lens and exit the ocular lens. It is an important measure if you want to know how well a binocular will perform in dim light. The measurement is achieved by dividing the lens aperture by the magnification. Example: In the Steiner 10.5x28 Wildlife Pro Binoculars, the exit pupil would be found by dividing the aperture (28) by the magnification (10.5), equaling 2.67. A higher exit pupil means the binoculars will work efficiently in dim light. For well-lit surroundings, an exit pupil of 2.5 to 4 is sufficient.

If you hold a pair of binoculars at arm's length, you'll be able to clearly see the circle of light in the eyepieces, representing the exit pupil.

It is important to note that how useful a large exit pupil will be depends on the eyes and often the age of the individual user. Let me explain: With age, the eye loses its ability to adapt to low light. While a young persons pupils may dilate to 7 mm, an older person may only open to only 5 mm. The older persons eye may therefor not be able to use all the light available and might be just as well off with smaller, lighter binoculars.

Human pupils are about 2-3 mm wide at most when bright, and the binoculars exit pupils should be about 3 mm. At night, our pupils dilate to about, so it is desirable to have binoculars with large exit pupils if they will be used at night. However, the disadvantage is that such binoculars tend to be big and heavy.

Extra Low Dispersion Glass
Extra low dispersion glass is used to make lenses on highe end cameras, telescopes, microscopes and binoculars. The Extra low dispersion glass prevents chromatic aberration because it concentrates and directs the wavelength of light more effectively onto the camera’s film or to your eyes in the case of binoculars. Lenses made from extra low dispersion glass have less air bubbles and glass deformities that are more likely to cause image distortion.

Consequently, most professionals and some serious amateurs are more likely to buy higher end optics that come equipped with extra low dispersion glass lenses. Camera's with the glass tend to take picture that are clearer and sharper with little or no chromatic aberration and binoculars and telescopes transmit clearer and sharper images to your eyes. More on Glass Quality.

Field Flattener Lenses
Improves edge sharpness and lowers the distortion by minimizing curvature of the field—aberrations that occur when focusing on the center of the field of view causing the edges to go out of focus or the centre to go out of focus when focusing on the edges. This produces sharper, clearer images all the way to the lens periphery and are used in most high end binoculars these days.

Field of View
This is the horizontal width of the image you can see while looking through the binoculars at a certain distance. The optical structure of each model of binoculars is different, so even if the magnification rating is the same, how much view the pair of binoculars can pull into your eyes will be different. The width of the view you can see through the binoculars is called the field of view. The field of view is represented as a number of feet per thousand yards of distance. It is sometimes expressed as an angle. To convert from the angle to the linear form expressed in feet, multiply the angle by 52.5. A wide field-of-view eyepiece design often means reduced eye relief and a higher field of view often means a less powerful magnification. For bird watching in a large wooded area, using a wider field of view will be more useful.

• Real field of view
  This is the view through the binoculars and it is measured from the center of the objective lens and expressed in degrees (angle). The lower the magnification the binoculars have, the wider the real field of view and the higher the magnification, the narrower the field of view. Because of this, it is hard to compare the real field of view of binoculars with that of binoculars of different magnification rating.
• Apparent field of view
  Apparent field of view = Magnification x Real field of view
  This is the value of the real field of view multiplied by the magnification. For example, if 10x magnification binoculars have a 5° real field of view, the apparent field of view will be 50°. This value represents the field of view which you will see looking through the binoculars. It is comparable even among binoculars of different magnifications. In general, if the apparent field of view is more than 62°, it is considered a wide field of view. For more take a look at this article on Wide Angle Binoculars.

• Apparent field of view ISO 14132-1:2002 standard
  With the conventional method, the apparent field of view was calculated by multiplying the real field of view by the binocular magnification. The apparent field of view based on the ISO 14132-1:2002 standard are obtained by the formula below:

Fluoride glass
Fluoride glass is a class of non-oxide optical glasses composed of fluorides of various metals. Some fluoride glasses are difficult to produce on Earth due to their fast crystallization. Optical elements made of calcium fluoride, namely of fluorite crystals, are used in some telephoto lenses, to correct color aberration. They are however being replaced with various low dispersion glasses, which have higher refraction index, better dimensional stability, and lower fragility. More on Glass Quality.

Galilean Optics
Galilean Optics have a convex objective lens but a concave eyepiece and the advantage is that it produces an upright image and so prisms are not needed to correct the images. On the down side they only produce a fairly narrow field of view and they are not capable of producing high magnifications. Galilean optics are still used in some opera binoculars & theatre glasses.

Galileo Binoculars
Named as such because the same concept was used in the telescopes made by Galileo Galilei in the 17th Century. Because concave lenses are used for the eyepiece lenses, prisms are not needed to correct the images. Also known as opera glasses, this type is used for looking at objects not too far away.

Giant Binoculars
Binoculars with objectives of 60mm or more are called giant binoculars and are common for uses such as astronomical binoculars.

Hermetically Sealed
A Hermetic seal is a seal which, for practical purposes, is considered airtight. It is often used in optics, including binoculars, to make them water and fog proof and to keep out dust.

IPD -Interpupillary Distance
This is the distance between the pupils of your eyes. As this distance is different for each person, the binocular can be adjusted to fit by opening or closing the hinge. Many binoculars include an IPD scale in millimetres, on the hinge mechanism. IPD is set correctly by first opening the binoculars right out, then looking at a distant object through the binoculars and slowly folding them shut until you see a perfect circle through the binoculars.

Lens coatings
Most binoculars have antireflection coatings on their air to glass surfaces. These coatings assist light transmission. They are what produce the blue, red, or green reflections you see when you look into the front (objective) lens of a pair of binoculars. But note how the manufacturer describes his coatings. "Coated" means a single layer antireflection coating on some lens elements, usually the first and last elements (the only ones you can see). "Fully Coated" means that all air to glass surfaces are coated. This is good. "Multi-Coated" means that at least some surfaces (again, usually the first and the last) have multiple layers of antireflection coatings. (The others presumably have single layer coatings.) Multiple layers are about an order of magnitude more effective than a single layer. "Fully Multi-Coated" means that all air to glass surfaces have received multiple layers of antireflection coatings, and this is what you want in your binoculars. The latest fad in coatings is ruby or red multi-coatings. These are intended to reduce glare in bright light.

Objective Lens
In Binoculars, this is the large lens at the end of the binocular opposite the eyepiece. This lens gathers light into the eye.

Ocular Lens
The Ocular lens is the small lens in the eyepiece of the binoculars. In some cases (as in some roof-prism binoculars), this lens is the same size as the objective lens.

Phase Correction
Phase correction is a set of coatings on the prism glass that keeps light in correct color phases. These coatings are only needed on roof prism binoculars to enhance resolution, contrast, and color fidelity. More on Phase Correction in Binoculars.

Prisms
Prisms are what let you see a correctly oriented image when you look through a pair of binoculars. There are two types of prisms in common use, Porro prisms and roof prisms. Roof prisms are essentially in line inside the optical tubes, and make for a more compact set of binoculars.

Porro Prism
Binoculars where the objective lenses and the eyepieces are not in line with each other. They have internal off-set prisms (as opposed to the aligned roof prisms) that bend the light rays inside the tubes to produce the image. These binoculars have eyepieces that are inset comparative to the objective lenses. Porro prisms have objective lenses spaced wider than roof prisms, and so can produce a slightly better stereoscopic image.

Porro prism binoculars can easily be identified by their offset tubes meaning the objective lens is not in line with the ocular lens. The front lenses are usually closer together than the rear lenses, but the reverse can also be true, particularly in compact models. Especially in medium priced class binoculars, the Porro prism design is usually optically superior to the roof prism design. Porro prism binoculars have a single pivot between the two halves of the binocular, and are therefore easy to adjust for the distance between your eyes. Like roof prisms, not all Porro prisms are created equal, BAK-4 prisms are the best.

Roof Prism (also called the Abbe-Koenig prism)
Binoculars that have the objective lenses and the eyepieces that are in line with each other, the internal prisms that are aligned (as opposed to the off-set in the Porro prisms) Roof prism binoculars tend to be sleeker, more compact binoculars. The image quality of roof-prism binoculars can suffer slightly because of the aligned prisms, although the top models of the roof-prism and porro-prism binoculars are now generally considered to have equal optical quality. Because Roof prism binoculars have straight tubes (the front/objective lens is in line with the rear/ocular lens), they are therefore more compact, which is an important consideration for the sportsman. They usually have two pivot points between the tubes, and are more difficult to adjust to the spacing of your eyes. Roof prisms can give an optical image equal to the best Porro prisms, but for technical reasons they usually do not. To be really good, roof prism binoculars have to be in the high price range. Do not attempt to economize on roof prism binoculars.

Roof prism vs Porro Prism Binoculars
Binoculars come in two main styles of design, this depends on the kind of prism system used, either roof prism or Porro prism. When you look at the binocualrs, it is very easy to tell them apart. If the objective lenses and the eyepieces are in line with each other, they are the roof prism design. If they are offset from each other, they are the Porro prism design. Roof prisms binoculars tend to be more compact, but to achieve the same optical quality as Porro prism models they often cost more to manufacture. The top binoculars of each design are now generally considered to be equal in optical quality and it will be down to your personal preference. Because the Porro prism design has wider spaced objective lenses they can have a slightly better stereoscopic image.

Schmidt Prism
Schmidt prisms are used to invert and revert an image while deviating it through an angle of 45°. Similar in function to Amici prisms, however the 45° deviation makes Schmidt prisms especially useful in eyepiece assemblies and imaging systems requiring a path bend. Aluminized roof surfaces are often used to enhance the overall light transmission efficiency.

Schmidt-Pechan Prism
The Schmidt-Pechan prism is a merger of the Schmidt prism and the Pechan prism and is used to rotate an image by 180°. Most commonly used in binoculars as an image erecting system. They can produce binoculars that are more compact when compared to binoculars using a Porro prism design. Kowa Binoculars use this design in many of their top end binoculars including their popular Genesis XD44 and Genesis XD33 binoculars.

Transmittance
As light travels through a binocular, a certain percentage of that light is lost through absorption and reflection at each air-to-glass surface or inside the prism system itself. The amount of original light available to the observer by the time it exits the eyepiece will vary from as low as 50% to as much as 97%, depending on the quality and number of optical glass elements used in the lenses and prisms, configuration and size of the prisms, collimation of the optical system, and type and amount of anti-reflection coatings present. This is an important factor that directly effects the actual brightness of the observed image. The term used to describe this percentage of light that is not lost through the optical system is transmittance and for most quality binoculars this figure will usually be above 90%. With this factor taken into account, it's possible for a 10 X 40 binocular (exit pupil 4mm) with a high transmittance (90%) to actually deliver a brighter image than a 7 X 35 (exit pupil 5mm) with a lower transmittance (70%).

Twilight Factor & Twilight Performance
Whilst it does not take into account the quality of the lenses and prisms, the twilight factor does make it possible to compare the performance of binoculars in low light conditions. It is calculated by first multiplying the magnification by the objective lens diameter and then finding the square root of the result. For example in a 7x42 binocular, the twilight factor is therefore 17.2 - the minimum for sufficient detail recognition in twilight - and an 8x56 binocular has a twilight factor of 21.2. A 8x30 binocular has a twilight factor of 15.5 and is therefore less suitable for viewing in very low light conditions. Note: The twilight factor is only one parameter among many, it does not say anything about the image quality which is a determining factor in detail recognition in twilight (twilight performance) Twilight performance is mainly determined by as high a transmission as possible in the right spectral range, as low a straylight portion as possible, as high contrast as possible and as high a resolution as possible. Only if all these requirements are met at the same time - and only then - can the twilight factor be used a measure of the twilight performance in binocular viewing.

Vignetting
This often happens when the eye relief is too short and you get a dark area around the edges or periphery of your field of view. In technical terms, it is a reduction of an image's brightness or saturation at the periphery compared to the image center.

Zoom Binoculars
Binoculars that have continuously variable magnification are called zoom binoculars. They are designated by their magnification range and can be identified by the two numbers before the objective lens diameter: For example 8-30x70 or 12-36x80. The hyphen between the first two numbers indicates that the magnification is variable from the first number to the second. For more detailed information read this article on Zoom Binoculars Review.

Binocular Terms on Video

For more on the basic terms as well as the most common binocular uses, features and accessories, thatincludes a feature on if certain features like image stabilization are needed for your intended use, check out these Binocular Videos