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 binoculars and what they mean. This will in tern help you make an informed decision when purchasing the correct binoculars for your specific needs.

Aberrations
To ensure the best possible view through a pair of binoculars with the brightest and sharpest image, correcting lens aberration is vitally important. The best optics are designed to correct the following aberrations: Spherical Aberration, Curvature of Field, Coma, Distortion, Astigmatism and Chromatic Aberration.

Achromatic lens
An achromatic lens or achromat is a lens that is designed to limit the effects of chromatic and spherical aberration. Achromatic lenses are corrected to bring two wavelengths (typically red and blue) into focus in the same plane.

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. The asphere's more complex surface profile can reduce or eliminate spherical aberration and also reduce other optical aberrations compared to a simple lens.

By using different degrees of curvature, a single aspheric lens can also can often replace a much more complex multi-lens system using standard spherical lenses, resulting in a smaller, 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.

Aspheric vs.Spherical: (see image on the right) Spherical lenses have a constant curvature on their surface, much in the same way that a sphere has a constant curvature. However, the power of the lens at different points is variable, resulting in relative defocus of more peripheral light rays. Due to this constant curvature, these lenses are easier to make; however, their optics are worse. A perfectly aspheric lens on the other hand has a variable curvature but a constant power even at the periphery of the lens. This results in the same focus of all light rays, both central and peripheral. These lenses are more difficult to make, but their optics are better.

BAK-4 prisms
BAK-4 prisms are made of superior optical glass that produces clearer images and have a better optical quality than BK-7 prisms. 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 minimization of undesirable internal reflections and thus provide a crystal clear image with the best contrast.

Blacking Out
See vignetting.

Body Shape
Binoculars come in several different shapes, often identified by letter codes: ZCF, BCF, BWCF, DCF, MCF or UCF, each of these are variations on the main Roof Prism or Porro Prism designs.

Chromatic Aberration
Chromatic aberration which is also called colour fringing, achromatism or chromatic distortion is a type of distortion in which there is a failure of a lens to focus all colors to the same convergence point. Or to put it another way: Chromatic aberration 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. If you look at an image through a lens with chromatic aberration, color fringing may occur. 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.

Colour Fringing
See Chromatic aberration.

DCF (Roof Prism)
The D is for 'Dach' which is German for 'roof'. For More See Roof Prism.

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 - The problem with the roof prism design is that it features one surface that does not have total internal reflection. It is therefore very important for binoculars' optical performance to raise the reflectivity of this surface.

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 this technique provides almost the same brightness as that perceived by the naked eye, and clear, high-contrast images that display accurate color reproduction.

Dispersion
In optics, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency, or alternatively when the group velocity depends on the frequency. Media having such a property are termed dispersive media. Dispersion is sometimes called chromatic dispersion to emphasize its wavelength-dependent nature, or group-velocity dispersion (GVD) to emphasize the role of the group velocity. The most familiar example of dispersion is a rainbow, in which dispersion causes the spatial separation of a white light into components of different wavelengths (different colors).

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.

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: With the compact 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. Compare this to a 10x42 binocular that has a 4.2 mm wide exit cone. Doubling the magnification to 20X, results in a much more critical 2.1mm exit cone. A higher exit pupil means the binoculars will work efficiently in dim light, but 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 7mm, an older person may only open to only 5mm. The older persons eye may therefore not be able to use all the light available and in theory would be just as well off with a smaller exit pupil. However whilst an exit pupil larger than the observer's pupil wastes some light, it does allow for some fumbling in side-to side movement without vignetting or clipping of the image.

Human pupils are about 2-3 mm wide at most in bright light, and therefore binoculars exit pupils should be about 3 mm. At night and in poor light conditions our pupils dilate a little more, so it is desirable in these scenarios to have binoculars with larger exit pupils. The disadvantage is that to increase exit pupil size you tend to have to increase the size of the objective lenses, making the binoculars bigger and heavier.

Extra Low Dispersion Glass (ED Glass)
Extra low dispersion glass is used to make lenses on high 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. Extra low dispersion glass is able to do this because it gives the designer of a multi-element objective lens a wider range of options with which to control and minimise aberrations, in particular, chromatic aberration. Generally speaking, the better the aberrations are controlled the cleaner and brighter the image will appear.

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.

The illustration above is one that Vanguard Sport Optics use to demonstrate the difference between using standard and extra low dispersion glass (ED glass)

ED glass and secondary spectrum
Visible light is composed of lights of various wavelengths. Gathering up all of these lights to a point is ideal for objective lenses.

With a single lens, because light is bent in the same way as with a prism, the focal lengths of lights with different wavelengths vary. As a result, not all light rays reach the same point, which causes chromatic aberration.

An achromatic lens made with conventional glass materials can match focal lengths of two different wavelengths. For red and blue colors, for example, that contain both ends of the wavelengths of visible light, chromatic aberration can be reduced to a certain extent by conforming their focal lengths. However, with more detailed examination, because light with other wavelengths such as green has different focal lengths, residual chromatic aberration results. This residual chromatic aberration is known as secondary spectrum. Combinations of conventional glasses cannot solve this secondary spectrum problem, but particular optical materials which have a unique characteristic of dispersion are needed.

ED (Extra-low Dispersion) glass has this unique characteristic and when combined with other glasses in that it minimizes the effects of the secondary spectrum. Comparing to achromatic lenses, ED glass reduces chromatic aberration to a remarkable degree.

For more read my article on Is Extra Low Dispersion Glass worth the Cost?

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 (somethimes called helicoid eyecups) 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 or the last surface of an eyepiece at which the eye can obtain the full viewing angle or to put it another way, it is the distance that an optical instrument can be held from the eye and the full field of view can still be comfortably observed. The calculation of eye relief is complex, but generally, the higher the magnification and the larger the intended field of view, the shorter the eye relief.

If the eye relief is too short vignetting is will often occur around the periphery of the vision. Whist if a viewer's eye is outside this distance, a reduced field of view will be obtained.

Eye relief can be particularly important for eyeglass wearers because 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.

The eye relief given in many product specifications does not always give a realistic view of what you as the user can expect. Although eye-cups can usually be folded down to allow the spectacle wearer to get closer to the eyepieces, there are sometimes lens mountings that do not allow the theoretical eye relief to be obtained. A better measure for those with strict needs would be one that takes account of this available eye relief, the theoretical value less any thickness of the lens' rims. This point can account for confusion in performance and is rarely expressed clearly.

What is more, when a eye-glass wearer orders a new pair of glasses, the optician will often ask if you prefers them close to the eyes or at some distance. This distance is referred to as the Back Vertex Distance, or BVD on a prescription. Since this property affects the available eye relief of any binocular or other optics used, it should be borne in mind at the eye testing stage. The matter should be discussed with the optician, though the only realistic way of testing the comfort is to try the optical device while wearing the usual spectacles. The optician can however make sure that the BVD is no worse in your new glasses than in the old ones that were used in your evaluation.

Eye relief should not be confused with the exit pupil, that is best described as the width of the cone of light that is available to the viewer at the exact eye relief distance.

Eyepiece & Eyepiece Lens
An eyepiece is so named because it usually contains the lens that is closest to the eye when someone looks through a variety of optical devices including binoculars, telescopes and microscopes. The eyepiece usually consists of several "lens elements" (often refered to as the ocular lens or eyepiece lens) contained in a housing, with a "barrel" on one end. The barrel is shaped to fit in a special opening of the instrument to which it is attached. The image can be focused by moving the eyepiece nearer and further from the objective. Most instruments have a focusing mechanism to allow movement of the shaft in which the eyepiece is mounted, without needing to manipulate the eyepiece directly.

An objective lens or mirror collects light and brings it to focus creating an image. The eyepiece is placed near the focal point of the objective to magnify this image. The amount of magnification depends on the focal length of the eyepiece.

The eyepieces of binoculars are usually permanently mounted in the binoculars, causing them to have a pre-determined magnification and field of view. With telescopes and microscopes, however, eyepieces are most often interchangeable. By switching the eyepiece, the user can adjust what is viewed. For instance, eyepieces will often be interchanged to increase or decrease the magnification of a telescope. Eyepieces also offer varying fields of view, and differing degrees of eye relief for the person who looks through them.

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 center 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.

Helicoid Eyecups
These are usually refered to as twist-up eyecups as they twist up and down using a helicoid mechanism 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.

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.

Hyperfocal Distance
In optics this is a distance beyond which all objects can be brought into an "acceptable" focus. Or to put it another way, the hyperfocal distance is the closest distance at which a lens can be focused while keeping objects at infinity acceptably sharp. When the lens is focused at this distance, all objects at distances from half of the hyperfocal distance out to infinity will be acceptably sharp.

The hyperfocal distance is entirely dependent upon what level of sharpness is considered to be acceptable. So with a binocular once you have set the focus on infinity - all objects in the background are not actually completely in focus - just an "acceptable" focus and look sharp enough to your eyes.

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 millimeters, 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.

Keplerian Telescope
Invented by Johannes Kepler in 1611, is an improvement on Galileo's design . It uses a convex lens as the eyepiece instead of Galileo's concave one. The advantage of this arrangement is the rays of light emerging from the eyepiece are converging. This allows for a much wider field of view and greater eye relief but the image for the viewer is inverted. Considerably higher magnifications can be reached with this design but to overcome aberrations the simple objective lens needs to have a very high f-ratio.

Lens coatings (Anti-reflection)
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.

Part of the light that passes through the lens is reflected by the front (incident light) and rear (exiting light) surfaces. This reduces the amount of light passing through the lens, if it is very bad the image you see will be darker than on binoculars that transmit more light. Also, the reflected light may cause ghosting and flare, affecting image contrast. To minimize reflection on the lens surfaces and ensure clear, sharp images, anti-reflective coatings are applied.

It is important to note how the manufacturer describes their 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. (A multilayer coating effectively reduces reflected light that cannot be eliminated with a single-layer coating, and increases the transmittance of light.) 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.

The table below shows Transmittance by type of coating:

Magnification Principle
Binoculars use a combination of objective lens and eyepiece lens that magnifies distant objects and makes them appear nearer. Keplerian binoculars in general use have convex lenses for objectives as well as eyepieces.

Refer to the diagram below - In principle, a real image A formed by an objective lens is magnified by an eyepiece lens and viewed as a virtual image B. As a result, the magnified objects can be observed as if they were just in front of your eyes. In this case, the image is inverted, so prisms are incorporated to rectify it so that an erect image can be viewed.

MCF - Reverse Porro Prism
The Zeiss arrangement of Porro prisms (ZCF) allows a wider separation between the objective lenses than between the eyepieces. By reversing the prisms from left to right the objectives can be moved to between the line of the eyepieces, making the overall width of the binoculars only slightly more than the inter-ocular separation. This design of binocular is sometimes, confusingly, just called 'Porro prism' but more commonly 'MCF' type. The aperture has to be much less than the eyepiece separation so is normally from 20 to 40mm.

Nitrogen Purging
Filling an instrument (particularly an outdoor one such as a binocular or riflescope) with zero grade or almost pure nitrogen in order to exclude all moisture and oxygen. This will virtually prevent any fogging and oxidation of the optics.

Objective Lens
In Binoculars, this is the large lens at the end of the binocular opposite the eyepiece and closest to the subject. This lens gathers light into the binoculars. Generally an objective lens combines convex and concave lenses to minimize color fringing / chromatic aberration, resulting in clearer images.

Ocular Lens
The Ocular lens is the small lens in the eyepiece of the binoculars, microscopes, spotting scopes and other optical devices and is usually the small lens closest to the eye. In some cases (as in some porro prism binoculars), this lens is the same size as the objective lens.

The ocular lens (or eyepiece lens as it is sometimes also referred to) usually consists of several "lens elements" that are contained in a housing, with a "barrel" on one end. This lens magnifies the image formed by the objective lens.

Various techniques have been integrated into eyepiece to meet recently rising demand for binoculars with long eye relief and wide field of view. In general, when you focus at the center of the binoculars' field of view, the peripheral area may be out of focus. And when you focus on the periphery, sometimes the image at the center may be out of focus. This phenomenon is caused by lens aberrations. One of the ways to solve this problem is by employing field-flattener lens on the eyepiece lens. Field-flattener lens can correct aberrations comprehensively, resulting in sharp images across the entire field of view.

Phase Correction Coatings
Phase correction coatings on the prism glass help keep light in correct color phases. These coatings are only needed on roof prism binoculars to enhance resolution, contrast, and color fidelity. Because these coatings keep the light in the correct color phases they produce images that have better contrast, a higher resolution and have a better color reproduction. These coatings are only really found on high end optics. More on Phase Correction in Binoculars.

Phase Shift
Phase shift is any change that occurs in the phase of one quantity, or in the phase difference between two or more quantities of a phase (wave). Manufacturers of high end roof prism binoculars will often use phase correction coatings on the prisms to help prevent phase shift. This is done because when light waves are reflected at the opposite faces of a roof prism, phase shift occurs resulting in marginal deterioration in sharpness.

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
Invented by Ignazio Porro in mid-19th-century Italy. The advantage of the porro prism design is that all of its reflective surfaces are completely reflective, so it loses no light and such binoculars are easy to produce. However, the optical path is bent like the letter Z. Accordingly, this prism system takes up considerable space, so that binoculars with a Porro prism are larger than those with a roof prism.

It is easy to spot a binocular with a porro prism as the objective lenses and the eyepieces are not in line with each other. This is because of the internal off-set prisms (as opposed to the aligned roof prisms) that bend the light rays inside the tubes to produce the image. The front lenses are usually closer together than the rear lenses, but the reverse can also be true, particularly in compact models.

Especially in the cheap to medium price range of 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. Because the objective lenses are often spaced wider than roof prisms, they can produce a slightly better stereoscopic image. Like roof prisms, not all Porro prisms are created equal, BAK-4 prisms are the best.

Refraction and Partial Reflection
Light refracts (bends) when it travels through a border between two transparent substances with different densities. In particular, it bends when leaving air and entering glass and also when leaving glass and entering air. Under ideal conditions all light will bend, but in real life about 95% bends and 5% is reflected back into the first substance. With lenses this is a serious problem because less light reaches your eyes. For a lens with n optical groups (hence 2n refraction surfaces) and coefficient of refraction r (the percentage of refracted light divided by 100), only a fraction L reaches your eyes, and L = r ²ⁿ. For r = 0.95 and n = 10, L = 0.36. This means that only 36% of the light reaches your eyes. The rest is scattered around, and some of it results in undesirable ghost images or flare.

To combat the undesirable consequences of partial reflection, scientists developed chemical coatings to be applied to glass surfaces. These coatings increase r (coefficient of refraction) - On binoculars, these coatings are most often just refered to as anti-reflection coatings.

Roof Prism (also called a Dach or Abbe-Koenig prism or DCF)
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). Because the optical path of the roof prism binoculars is straight, Roof prism binoculars tend to be sleeker, more compact binoculars.

The first prism of the roof prism system has one surface that does not feature total internal reflection, resulting in the loss of some light. It is therefore very important for a binoculars' optical performance to raise the reflectivity of this surface. To do this a coating is sometimes used on the surface to raise it's reflectivity, this is coating is only found on the best quality binoculars and is something to look out for when choosing binoculars. (see dielectric coating)

Also, to produce the roof surface of the second prism that delivers sharp images without a double image or flare, highly advanced technology to process its edge precisely is required. This is why it is important not to attempt to economize on roof prism binoculars and always buy the best you can afford.

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 binoculars, 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.

Relative Brightness
The relative brightness index is used to compare how well binoculars with different size exit pupils will perform under dark conditions. This index reminds us that as the size of the exit pupil increases, its area and ability to transmit light grow geometrically. To find the relative brightness, square the exit pupil. A binocular with an exit pupil of 5mm for example has a relative brightness of 25 (5x5=25). Because relative brightness does not consider factors such as optical quality or coatings, it should be used only as a rough guide.

Ruby Coatings
First thing to mention about "ruby" coatings on binoculars is that the coating has nothing to do with the mineral ruby. Some manufacturers filter red to compensate for their poor-quality optics that do not properly converge the color spectrum. By eliminating red from the spectrum, the optics appear to do a better job of minimizing color aberrations, but the view through the binoculars often have an unnatural greenish cast. My feeling is to stay well clear of binoculars that have "ruby" coatings as is simply a gimmick with no redeeming qualities. Read more on Ruby Coated Binoculars.

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.

Silver Mirror Coatings
Whilst the roof prism design has many advantages over the porro prism design, it does have one major problem in that one surface of the prism does not have total internal reflection. Therefore to get the best optical performance it is very important for to raise the reflectivity of this surface and to do this, a mirror coating is used on those surfaces.

On most lower quality optics, an aluminum mirror coating is used due to its low cost and fairly high reflectivity of 87% to 93%. A silver mirror coating is used when a higher reflectivity (reflectivity of 95% to 98%) is needed and the benefits outweigh the increase in cost. The practice of forming high reflectivity optical coatings with silver also takes substantially more effort than with aluminum, in part because of specific material problems that arise from the aggregated structure of depositing thin metallic films.

Note: Some optical manufacturers go even further and use dielectric prism coatings that have a reflectivity of more than 99%

Spherical Aberration
A bundle of light rays coming from one point on the optical axis is focused at a different place than the focused point depending on the distance from the optical axis when the light incidents. This deviation is caused by variations in angles of each incident light ray, and is called spherical aberration. Suppose a screen is placed at P' in the illustration below, the image of P will not be a focused point, but a blurred circle. Making the lens diameter smaller reduces such spherical aberration.

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
The binocular twilight factor is the amount of resolution that you get when viewing in the twilight or the dim light. It is a mathematical formula that shows how both the size of the objective lens and the magnification contribute to a binocular's ability to show detail in dim light and whilst it does not take into account the quality of the lenses and prisms or their coatings. It is important to remember that it can only be used to estimate performance and should only be used as a rough guide to compare the performance of different configurations of binoculars in low light or poor light conditions.

This is because the factor that has the greatest impact on resolution or image detail, will be dependent upon the amount of light available during the time of observation. During daylight hours, when your eye pupil size will be only about 2 to 3mm, magnification will be the principal factor in image resolution. At night, with the eye pupil dilated to 6 to 8mm, aperture size is the controlling factor. In twilight conditions both of these factors control resolution effectiveness and the twilight factor is the term that compares binocular performance under these conditions.

The higher the twilight factor, the better the resolution of the binocular when observing under dim light conditions, or to put it another way: The larger the twilight factor, the more detail you can see in low light.

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. (A twilight factor of about 17 or more is usually required for reasonable low light use.)

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). Also keep in mind that it has long been known from actual use in the field, that twilight factor exaggerates the importance of magnification. Any high grade bino with a small magnification, will show more image detail in low light than a cheap version with a larger magnification.

Another point to keep in mind is that the twilight factor is not a term used to indicate observed image brightness (transmittance) so you cannot assume that a binocular with a higher twilight factor will seem brighter.

Twilight performance is mainly determined by as high a transmission as possible in the right spectral range, as low a stray light 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.

A binoculars twighlight factor was more important in the past, before modern optical coatings were invented. This is because in a modern binocular, the performance in poor light now depends more on the quality of the glass and optical coatings than on just the twilight factor formula. Good coatings can double the amount of light that gets through the binocular (transmittance), when compared to those that have none or poor quality coatings.

For more, take a look at this article on Twilight Factor and how it differs to the Transmittance and Brightness of a pair of binoculars.

Twilight Factor and Exit Pupil
A 10x40 (twilight factor 20) would effectively resolve better under poor light conditions than a 7x35 (twilight factor 15.4) even though the 10x40 has a smaller exit pupil. Remember, however, that the twilight factor does not take into account the transmittance or quality of the optical system.

UCF or Dual-Axis Porro Prism
This design was introduced by Pentax around 1990. They are similar to MCF in appearance but the objective lenses are a fixed distance apart and the two eyepieces and prism housings swing outwards to adjust the inter-ocular distance. The UCF design is compact and keeps its alignment well.

Ultra Wide Band Coating
An anti-reflection coating process that is customized for every lens element in the optical path, in order to allow the best possible light from the front glass all the way back to the eyepiece. The result? Optimum brightness and true color across the light spectrum.

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.

Waterproofing - JIS Waterproof Scale
The most widely recognized standard for water-proofness is the JIS (Japanese Industrial Standards). Unfortunately not all optics manufacturers use it. The Japanese standards are used all over the world by commercial and governmental organizations involved in equipment design and manufacturing, quality assurance, construction, testing, and maintenance. The standards cover an extremely wide range of industrial and mineral products and are classified into 17 divisions, ranging from civil and architectural, to electrical, automotive and shipbuilding. For waterproof ratings, JIS utilizes a 0 to 8 scale to rate "ingress protection."

• JIS - Class 0: Non-protected
• JIS - Class 1: Protected against vertically falling water drops
• JIS - Class 2: Protected against vertically falling water drops when enclosure tilted up to 15%
• JIS - Class 3: Protected against spraying water - Water sprayed at any angle up to 60º on either side of the vertical shall have no harmful effects
• JIS - Class 4: Protected against splashing water
• JIS - Class 5: Protected against water jets - Water projected in jets from any direction against the enclosure shall have no harmfull effects.
• JIS - Class 6: Protected against powerful water jets in any direction - Watertight - Having no ingress of water into inside by receiving direct jet of water from any direction.
• JIS - Class 7: Protected against the effects of temporary immersion in water
• JIS - Class 8: Protected against the effects of continuous immersion in water

It is important to remember that this standard is used for many types of products and not just binoculars. Most high end binoculars are JIS 6, JIS 7 or JIS 8 but some low end optics can claim water-proof and only be JIS 1.

ZCF (Zeiss Centre Focus)
Zeiss pattern binoculars have their objective lenses further apart than their eyepieces and are the 'traditional' binocular shape. There are actually two varieties, the 'German style' in which the tubes holding the objective lenses are separate and screwed into the prism housing, and the 'American style' where the objective tube and prism housing are cast in one piece.

The threaded connection for the objective tubes can be a weak point so the American style tends to be more robust. The German style is known as ZCF and the American style as BCF or BWCF. ZCF and BWCF types are usually quite large aperture binoculars, 40mm upwards, and indeed are really the only design possible for apertures greater than the separation between human eyes (the inter-ocular distance). They provide bright and clear images with an enhanced stereoscopic view, but they are bulky and heavy, and really need to be carried in a case to protect them. For other designs See Body Shapes.

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, that includes a feature on if certain features like image stabilization are needed for your intended use, check out these Binocular Videos