Glossary of Spotting Scope Terms

If you don't know your Extra Low Dispersion Glass from your Dielectric Coatings or don't know the difference between Straight-through and Angled spotting scopes, this page will help. Here are the most commonly used terms reffering to scopes and what they mean. Hopefiully this will help you make an informed decision when purchasing the correct optics for your specific needs.

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.

The most common type of achromat is the achromatic doublet, here the lens is actually composed of two individual lenses, each made from glass with a different amount of dispersion. Usually one element is a concave lens made out of flint glass such as F2, which has relatively high dispersion, while the other, convex, element is made of crown glass such as BK7, which has lower dispersion; other low dispersion glasses may be used instead. The lens elements are mounted next to each other, often cemented together, and shaped so that the chromatic aberration of one is counterbalanced by that of the other.

The diagram below shows on the left, the chromatic aberration that is produced by a single lens because it causes different wavelengths of light to have differing focal lengths. On the right an achromatic doublet lens brings red and blue light to the same focus, thus reducing the amount of chromatic aberration (color fringing).

Aperture
In optics, an aperture is a hole or an opening through which light is admitted. However when we refer to aperture and scopes, aperture refers to the diameter of a scopes' objective lenses and is measured in millimeters. The aperture is often the last number represented when describing a spotting scopes. Example: In the Nikon EDG Fieldscope 85-A, 85 would represent the aperture.

Aperture is a very important factor in choosing a scope and even more important in a telescope. The prime function of all scopes is to collect light. At any given magnification, the larger the aperture, the better the image will be. The clear aperture of a telescope is the diameter of the objective lens specified in either inches or millimeters (mm). The larger the aperture, the more light it collects and the brighter (and better) the image will be. Greater detail and image clarity will be apparent as aperture increases.

For example in telescopes: a globular star cluster such as M13 is nearly unresolved through a 4" aperture telescope at 150 power but with an 8" aperture telescope at the same power, the star cluster is 16 times more brilliant, stars are separated into distinct points and the cluster itself is resolved to the core. Considering your budget and portability requirements, you should select a telescope (or spotting scope) with as large an aperture as possible.

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 with a better optical quality than BK-7 - this is what you want in your scopes.

BK-7 prisms are usually used in lower priced optics. 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 scopes generally are manufactured with BK-7 prisms.

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 scopes 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

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 optics with high magnifications and why depth of field is usually more important in comparing spotting scopes or telescopes than say binoculars that usually have a far smaller magnification. 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).

Eye-cups
Eye-cups are related to the eye relief as they keep the distance from the oculars to your eyes. They also help keep stray light away from your eyes while using your optics. 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
In simple terms this is the distance your eye can be from the eyepiece and still see the entire field of view. 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. As a general rule, you will need at least 14mm of eye relief to see the entire field of view with eyeglasses and people with thick glass lenses in their eyeglasses will probably need more.

Exit Pupil
This is the amount of light rays that enter the objective lens and the size of the column of light that exits the ocular lens. It is an important measure if you want to know how well a scope performs in poor light conditions. The measurement is achieved by dividing the lens aperture by the magnification. Example: a 15x45 model has an exit pupil, or useable light, of 3mm (45/15 = 3mm). A higher exit pupil means the scopes will probably work more efficiently in dim light although many other factors including glass quality, quality of coatings and prisim quality also affect the brightness and quality of an image produced.

It is important to note that how useful a large exit pupil will also depend on the eyes and often the age of the individual user. 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 a scope with a smaller exit pupil.

Extra Low Dispersion Glass (ED)
Extra low dispersion glass lenses are usually only found on the highest quality spotting scopes (as well as 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 your eyes. 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.

The illustration below was created by Vanguard Sport Optics and demonstrates very well the difference between using stanadard and extra low dispersion glass (ED glass).

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.

Field of View
This is the horizontal width of the image you can see while looking through the fieldscope at a certain distance. The optical structure of each model of scope is different, so even if the magnification rating is the same, how much view the spotting scope can pull into your eyes will be different. This width of the view you can see is called the field of view. The field of view is usually represented as a number of feet per thousand yards of distance or as a number of meters per 1000m of distance in Europe. It is also 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. A wide field-of-view is better for following fast-moving action or scanning for wildlife.

• Real field of view
  This is the view through the spotting scope that is measured from the center of the objective lens and expressed in degrees (angle). The lower the magnification the scopes or eyepieces have, the wider the real field of view and the higher the magnification, the narrower the field of view.
• 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. This value represents the field of view which you will see looking through the scope. It is comparable even among scopes of different magnification.

• Apparent field of view ISO 14132-1:2002 standard
  With the conventionla method, the apparent field of view was calculated by multiplying the real field of view by the 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.

Focal Length
The focal length of an optical system is a measure of how strongly the system converges (focuses) or diverges (defocuses) light. For an optical system in air, it is the distance over which initially collimated rays are brought to a focus. A system with a shorter focal length has greater optical power than one with a long focal length; that is, it bends the rays more strongly, bringing them to a focus in a shorter distance.

In most photography and all telescopy, where the subject is essentially infinitely far away, longer focal length (lower optical power) is associated with larger magnification, and a narrower angle of view. Conversely, shorter focal length or higher optical power is associated with a wider angle of view. On the other hand, in applications such as microscopy in which magnification is achieved by bringing the object close to the lens, a shorter objective lens focal length leads to higher magnification because the subject can be brought closer to the entrance pupil.

The longer the focal length of the scope, generally the more power it has, the larger the image and the smaller the field of view. For example, a telescope with a focal length of 2000mm has twice the power and half the field of view of a 1000mm telescope. Most manufacturers specify the focal length of their various instruments; but, if it is unknown and you know the focal ratio you can use the following formula to calculate it: focal length is the aperture (in mm) times the focal ratio. For example, the focal length of an 8" (203.2mm) aperture with a focal ratio of f/10 would be 203.2 x 10 = 2032mm.

Focal Ratio
The ratio of the focal length to the diameter of an optical system. To find the focal ratio, divide the focal length of the system by the diameter of the primary light gathering element of the optical system. For example, an 8" (203.2mm) telescope with a 2032mm focal length is said to be an f/10 system - (2032/203.2). The lower the focal ratio, the "faster" (and therefore brighter) the image.

Folded Light Path
A combination optical configuration that uses lenses and mirrors to create a total scope length much shorter than the total focal length of the system. This provides a compact design that still has long focal length performance.

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

Lens coatings
Coated – A single layer on at least one lens surface.
Fully Coated – A single layer on all air-to-glass surfaces.
Multi-Coated – Multiple layers on at least one lens surface.
Fully Multi-Coated – Multiple layers on all air-to-glass surfaces.

Most scopes 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 fieldscope. But 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. (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.

Objective Lens
This is the large lens at the end of the spotting scope opposite the eyepiece. This lens gathers light into the eye. The number after the "x" in the formula: (15–45x60) is the diameter of the objective or front lens. The larger the objective lens, the more light that enters the spotting scope and the brighter the image.

Ocular Lens
The Ocular lens is the small lens in the eyepiece.

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 spotting scopes to enhance resolution, contrast, and color fidelity.

Prisms
Prisms are what let you see a correctly oriented image when you look through your spotting scope. There are two types of prisms in common use, Porro prisms and roof prisms.

Porro Prism
This is 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 barrel to produce the image. Porro prisms often provide greater depth perception and generally offer a wider field-of-view.

Roof Prism

Scopes 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). The prisms overlap closely, allowing the objective lenses to line up directly with the eyepiece. The result is a slim, streamlined shape in which the lenses and prisms that magnify and correct the image are in a straight line.

The image quality of roof-prism scopes can suffer slightly because of the aligned prisms, although the top models roof prisms can give an optical image equal to the best Porro prisms - Do not attempt to economize on roof prism spotting scopes.

Schmidt–Pechan Prism
A Schmidt–Pechan prism is is a merger of the designs of the Schmidt prism and the Pechan prism and are used to rotate an image by 180°. They are commonly used in scopes and binoculars as an image erecting system.

The Pechan prism is a composite of two prisms separated by an air gap. It will invert or revert (flip) the image depending up orientation of the prism but not both at the same time. By replacing the second prism in the Pechan design with a Schmidt prism the Schmidt-Pechan prism can both invert and revert the image and so act as an image rotator.

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%

Transmittance
As light travels through a fieldscope, 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.

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 spotting scopes. Most high end waterproof scopes are JIS 6, JIS 7 or JIS 8 but some low end optics can claim water-proof and only be JIS 1.