A numerical value indicating the dispersion of optical glass, using the Greek symbol v. Also called the optical constant. The Abbe number is determined by the following formula using the index of refraction for three Fraunhofer's lines: F (blue), d (yellow) and c (red).

Abbe number = d = nd • 1/nF – nc

An optical glass characteristic distribution chart is a graph using the Abbe number as the horizontal axis and the d line index of refraction as the vertical axis.

The image formed by an ideal photographic lens would have the following characteristics:

1.  A point would be formed as a point.
2.  A plane (such as a wall) perpendicular to the optical axis would be formed as a plane.
3.  The image formed by the lens would have the same shape as the subject.

Also, from the standpoint of image expression, a lens should exhibit true color reproduction. If only light rays entering the lens close to the optical axis are used and the light is monochromatic (one specific wavelength), it is possible to realize virtually ideal lens performance. With real photographic lenses, however, where a large aperture is used to obtain sufficient brightness and the lens must converge light not only from near the optical axis but from all areas of the image, it is extremely difficult to satisfy the above-mentioned ideal conditions due to the existence of the following obstructive factors:

•  Since most lenses are constructed solely of lens elements with spherical surfaces, light rays from a single subject point are not formed in the image as a perfect point. (A problem unavoidable with spherical surfaces.)
•  The focal point position differs for different types (i.e., different wavelengths) of light.
•  There are many requirements related to changes in angle of view (especially with wide-angle, zoom and telephoto lenses).

The general term used to describe the difference between an ideal image and the actual image affected by the above factors is "aberration." Thus, to design a high-performance lens, aberration must be extremely small, with the ultimate objective being to obtain an image as close as possible to the ideal image. Aberration can be broadly divided into two classifications: chromatic aberrations, which occur due to differences in wavelength, and monochromatic aberrations, which occur even for a single wavelength.

A lens which corrects chromatic aberration for two wavelengths of light. When referring to a photographic lens, the two corrected wavelengths are in the blue-violet range and yellow range.

Another feature unique to Canon's four Image Stabilized super-telephoto lenses. Four buttons appear on the outer barrel near the front of these lenses; pushing any one will temporarily lock AF if the camera is in the AI Servo AF mode. Custom Functions on many newer EOS bodies allow these buttons to assume a variety of additional functions.

Air Lens Concept Diagram

The air spaces between the glass lens elements making up a photographic lens can be thought of as lenses made of glass having the same index of refraction as air (1.0). An air space designed from the beginning with this concept in mind is called an air lens. Since the refraction of an air lens is opposite that of a glass lens, a convex shape acts as a concave lens and a concave shape acts as a convex lens. This principle was first propounded in 1898 by a man named Emil von Hoegh working for the German company Goerz.

The area of a scene, expressed as an angle, which can be reproduced by the lens as a sharp image. The nominal diagonal angle of view is defined as the angle formed by imaginary lines connecting the lens’ second principal point with both ends of the image diagonal (43.2mm). Lens data for EF lenses generally includes the horizontal (36mm) angle of view and vertical (24mm) angle of view in addition to the diagonal angle of view

The angle between the subject point on the optical axis and the diameter of the entrance pupil, or the angle between the image point on the optical axis and the diameter of the exit pupil.

The aperture of a lens is related to the diameter of the group of light rays passing through lens and determines the brightness of the subject image formed on the focal plane. The optical aperture (also called the effective aperture) differs from the real aperture of the lens in that it depends on the diameter of the group of light rays passing through the lens rather than the actual lens diameter.

A value used to express image brightness, calculated by dividing the lens’ effective aperture (D) by its focal length (f). Since the value calculated from D/f is almost always a small decimal value less than 1 and therefore difficult to use practically, it is common to express the aperture ratio on the lens barrel as the ratio of the effective aperture to the focal length, with the effective aperture set equal to 1. (For example, the EF 85mm f/1.2L lens barrel is imprinted with 1:1.2, indicating that the focal length is 1.2 times the effective aperture when the effective aperture is equal to 1.) The brightness of an image produced by a lens is proportional to the square of the aperture ratio. In general, lens brightness is expressed as an F number, which is the inverse of the aperture ratio (f/D).

A lens which corrects chromatic aberration for three wavelengths of light, with aberration reduced to a large degree particularly in the secondary spectrum. EF super-telephoto lenses are examples of apochromatic lenses.

Photographic lenses are generally constructed of several single lens elements, all of which, unless otherwise specified, have spherical surfaces. Because all surfaces are spherical, it becomes especially difficult to correct spherical aberration in large-aperture lenses and distortion in super-wide-angle lenses. A special lens element with a surface curved with the ideal shape to correct these aberrations, i.e., a lens having a free-curved surface which is not spherical, is called an aspherical lens. The theory and usefulness of aspherical lenses have been known since the early days of lens making, but due to the extreme difficulty of actually processing and accurately measuring aspherical surfaces, practical aspherical lens manufacturing methods were not realized until fairly recently. The first SLR photographic lens to incorporate an aspherical lens was Canon's FD 55mm f/1.2AL released in March 1971. (Leica offered the 50mm f/1.2 Noctilux lens with aspherical surfaces for its rangefinder cameras many years before 1971.)

Due to revolutionary advances in production technology since that time, Canon's current EF lens group makes abundant use of various aspherical lens types such as ground and polished glass aspherical lens elements, ultra-precision glass molded (GMo) aspherical lens elements, composite aspherical lens elements and replica aspherical lens elements.

When white light (light containing many colors uniformly mixed so that the eye does not sense any particular color and thus perceives the light as white) such as sunlight is passed through a prism, a rainbow spectrum can be observed. This phenomenon occurs because the prism's index of refraction (and rate of dispersion) varies depending on the wavelength (short wavelengths are more strongly refracted than long wavelengths). While most visible in a prism, this phenomenon also occurs in photographic lenses, and since it occurs at different wavelengths is called chromatic aberration. There are two types of chromatic aberration: "axial chromatic aberration," where the focal point position on the optical axis varies according to the wavelength, and "chromatic difference of magnification," where the image magnification in peripheral areas varies according to the wavelength. In actual photographs, axial chromatic aberration appears as color blur or flare, and chromatic difference of magnification appears as color fringing (where edges show color along their borders). Chromatic aberration in a photographic lens is corrected by combining different types of optical glass having different refraction and dispersion characteristics. Since the effect of chromatic aberration increases at longer focal lengths, precise chromatic aberration correction is particularly important in super-telephoto lenses for good image sharpness. Although there is a limit to the degree of correction possible with optical glass, significant performance improvements can be achieved using man-made crystal such as fluorite or UD glass. Axial chromatic aberration is also sometimes referred to as "longitudinal chromatic aberration" (since it occurs longitudinally with respect to the optical axis), and chromatic difference of magnification can be referred to as "lateral chromatic aberration" (since it occurs laterally with respect to the optical axis).
Note: While chromatic aberration is most noticeable when using color film, it affects black-and-white images as well, appearing as a reduction in sharpness.

Since all lenses contain a certain amount of spherical aberration and astigmatism, they cannot perfectly converge rays from a subject point to form a true image point (i.e., an infinitely small dot with zero area). In other words, images are formed from a composite of dots (not points) having a certain area, or size. Since the image becomes less sharp as the size of these dots increases, the dots are called “circles of confusion.” Thus, one way of indicating the quality of a lens is by the smallest dot it can form, or its “minimum circle of confusion.” The maximum allowable dot size in an image is called the “permissible circle of confusion.”

Certain Canon lenses feature a new Circular Aperture diaphragm unit, which uses curved aperture blades to provide for a more rounded opening as the lens is stopped down. It's especially effective at rendering out of focus background highlights as natural rounded shapes. In lenses such as the EF 70-200mm f/2.8L IS lens, the lens opening is virtually circular from f/2.8 to f/5.6. These lenses retain all the benefits previously available with Canon's Electromagnetic Diaphragm – smooth and consistent stop-down operation (even at up to 10fps with the EOS-1v), near-silent aperture control, and total absence of mechanical levers or switches in the lens mount.

A circular polarizing filter is functionally the same as a linear polarizing filter as it only passes light vibrating in a certain direction. However, the light passing through a circular polarizing filter differs from light passing through a linear polarizing filter in that the vibrational locus rotates in a spiral pattern as it propagates. Thus, the effect of the filter does not interfere with the effect of half-mirrors: allowing normal operation of TTL-AE and AF functions. When using a polarizing filter with an EOS camera, be sure to always use a circular polarizing filter. The effectiveness of a circular polarizing filter in eliminating reflected light is the same as that of a linear polarizing filter.

When light enters and exits a lens, approximately 5% of the light is reflected back at each lens-air boundary due to the difference in index of refraction. This not only reduces the amount of light passing through the lens but can also lead to repeating reflections which can cause unwanted flare or ghost images. To prevent this reflection, lenses are processed with a special coating. Basically this is carried out using vacuum vapor deposition to coat the lens with a thin film having a thickness l/4 the wavelength of the light to be affected, with the film made of a substance (such as magnesium fluoride) which has an index of refraction of n, where n is the index of refraction of the lens glass. Instead of a single coating affecting only a single wavelength, however, EF lenses feature a superior multi layer coating (multiple layers of vapor deposited film reducing the reflection rate to 0.2-0.3%) which effectively prevents reflections of all wavelengths in the visible light range. Lens coating is carried out not only to prevent reflections, however. By coating the various lens elements with appropriate substances having different properties, coating plays an important role in providing the overall lens system with optimum color balance characteristics.

The color reproduction fidelity of a photo taken through a lens compared to the original subject. Color balance in all EF lenses is based on ISO recommended reference values and maintained within a strict tolerance range that is tighter than ISO's CCI allowable value range.

Coma, or comatic aberration, is a phenomenon visible in the periphery of an image produced by a lens which has been corrected for spherical aberration, and causes light rays entering the edge of the lens at an angle to converge in the form of a comet instead of the desired point, hence the name. The comet shape is oriented radially with the tail pointing either toward or away from the center of the image. The resulting blur near the edges of the image is called comatic flare. Coma, which can occur even in lenses which correctly reproduce a point as a point on the optical axis, is caused by a difference in refraction between light rays from an off-axis point passing through the edge of the lens and the principal light ray from the same point passing through the lens center. Coma increases as the angle of the principal ray increases, and causes a decrease in contrast near the edges of the image. A certain degree of improvement is possible by stopping down the lens. Coma can also cause blurred areas of an image to flare, resulting in an unpleasing effect. The elimination of both spherical aberration and coma for a subject at a certain shooting distance is called aplanatism, and a lens corrected as such is called an aplanat.

The degree of distinction between areas of different brightness levels in a photograph, i.e., the difference in brightness between light and dark areas. For example, when the reproduction ratio between white and black is clear, contrast is said to be high, and when unclear, contrast is said to be low. In general, quality lenses producing high quality images have both high resolution and high contrast.

States that light fall-off in peripheral areas of the image increases as the angle of view increases, even if the lens is completely free of vignetting. The peripheral image is formed by groups of light rays entering the lens at a certain angle with respect to the optical axis, and the amount of light fall-off is proportional to the cosine of that angle raised to the fourth power. As this is a law of physics, it cannot be avoided. However, with wide-angle lenses having a large angle of view, decreases in peripheral illumination can be prevented by increasing the lens’ aperture efficiency (ratio of the area of the on-axis entrance pupil to the area of the off-axis entrance pupil).

Curvature of field is a phenomenon which causes the image formation plane to become curved like the inside of a shallow bowl, preventing the lens from producing a flat image of a flat subject. When the center of the image is in focus, the periphery is out of focus, and when the periphery is in focus, the center is out of focus. The degree of curvature of field is largely affected by the method used for correcting astigmatism. Since the image plane falls between the sagittal and meridional image surfaces, good correction of astigmatism results in small curvature of field. Since curvature of field cannot be improved very much by stopping down the lens, lens designers reduce it as much as possible using various methods such as changing the shapes of the various single lens elements making up the lens and changing the position of the aperture. In doing this, one necessary condition that must be satisfied to simultaneously correct astigmatism and curvature of field is Petzval’s Condition (1843). Petzval’s Condition states that a lens element is good if a result of zero is obtained when the inverse of the product of the index of refraction and focal length of that lens element is added to the total number of lens elements making up the lens. This sum is called Petzval’s Sum.