A phenomenon where light entering the lens is partially blocked by an obstruction such as the end of a lens hood or the frame of a filter, causing the corners of the image to darken or the overall image to lighten. Shading is the general term used for the case where the image is degraded by some type of obstacle that blocks light rays which should actually reach the image.

The distance from the film plane (focal plane) to the subject. The position of the film plane is indicated on the top of most cameras by a “ “ symbol.

This aberration exists to some degree in all lenses constructed entirely of spherical elements. Spherical aberration causes parallel light rays passing through the edge of a lens to converge at a focal point closer to the lens than light rays passing through the center of the lens. (The amount of focal point shift along the optical axis is called longitudinal spherical aberration.) The degree of spherical aberration tends to be larger in large-aperture lenses. A point image affected by spherical aberration is sharply formed by light rays near the optical axis but is affected by flare from the peripheral light rays (this flare is also called halo, and its radius is called lateral spherical aberration). As a result, spherical aberration affects the entire image area from the center to the edges, and produces a soft, low-contrast image which looks as if covered with a thin veil. Correction of spherical aberration in spherical lenses is very difficult. Although commonly carried out by coming two lenses-one convex and one concave-based on light rays with a certain height of incidence (distance from the optical axis), there is a limit to the degree of correction possible using spherical lenses, so some aberration always remains. This remaining aberration can be largely eliminated by stopping down the diaphragm to cut the amount of peripheral light. With large aperture lenses at full aperture, the only effective way to thoroughly compensate spherical aberration is to use an aspherical lens element.

The opening which adjusts the diameter of the group of light rays passing through the lens. In interchangeable lenses used with single lens reflex cameras, this mechanism is usually constructed as an iris diaphragm consisting of several blades which can be moved to continuously vary the opening diameter. With conventional SLR camera lenses, the aperture is adjusted by turning an aperture ring on the lens barrel. With modern camera lenses, however, aperture adjustment is commonly controlled by operating an electronic dial on the camera body.

The distance from the lens’ front principal point to the subject.

All EF lenses are coated in accordance with Canon’s own standards, which are even more strict than the CCI tolerances set by the ISO (International Standards Organization), and the variety of single and multilayer coatings used are selected to optimally match the refraction of the lens to which it is being applied. Named Super Spectra coating by Canon, this process features a high permeation rate, ultraviolet ray filtering, highly durable surface hardness and features and stable characteristics. The superior imaging characteristics realized by these exacting coating procedures includes sharp, vivid images with high contrast, uniform color balance throughout the EF lens lineup, and true color reproduction that does not change over years of use.

The high cost of synthetic fluorite crystal production makes fluorite lenses extremely expensive. One answer was found in the latter half of the 1970’s with the appearance of UD (ultra low dispersion) glass that could provide characteristics similar to fluorite but at a lower cost. While the indexes of refraction and dispersion of UD glass do not equal that of fluorite, they are significantly lower than those of other types of optical glass. Moreover, UD glass does display partial dispersion characteristics similar to fluorite. The selection of the proper lens element combination in consideration of the intended focal length and other factors can provide close to the same effect as fluorite, (two UD lens elements are equivalent to one fluorite element). Another breakthrough was made in 1993 when Super UD glass was introduced as a new material that achieves almost the same performance as fluorite while achieving a new balance of greater cost reduction and even higher quality.

To achieve a high level of sharpness both at the center and out to the edges of an image when shooting with a telephoto lens, it is desirable for the index of refraction of the front convex lens element to be as small as possible. Accordingly, the use of fluorite with its low index of refraction effectively improves image quality over the total image area.

In this type of lens, the lens group behind the diaphragm has nearly the same configuration and shape as the lens group in front of the diaphragm. Symmetrical lenses are further classified into various types such as the Gauss type, triplet type, Tessar type, Topogon type and orthometer type. Of these, the Gauss type and its derivations is the most typical configuration used today because • its symmetrical design allows well-balanced correction of all types of aberrations, and • a comparatively long back focus can be achieved. The Canon 50mm f/1.8 released back in 1951 succeeded in eliminating the comatic aberration which was the sole weak point of Gauss type lenses of that day, and thus became famous as a historical landmark lens due to the remarkable improvement in performance it afforded. Canon still uses a Gauss type construction in current lenses such as the EF 50mm f/1.8 II, EF 50mm f/1.4 USM and EF 85mm f/1.2L USM. The Tessar and triplet type symmetrical configurations are commonly used today in compact cameras equipped with single focal length lenses.

The ratio between the overall length of a telephoto lens and its focal length is called the telephoto ratio. Put another way, it is the value of the distance from the apex of the frontmost lens element to the focal plane divided by the focal length. For telephoto lenses, this value is less than one. For reference, the telephoto ratio of the EF 300mm f/2.8L USM is 0.91 , and that of the EF 600mm f/4L USM is 0.78.

With general photographic lenses, the overall length of a lens (the distance from the apex of the frontmost lens element to the focal plane) is longer than its focal length. This is not usually the case with lenses of particularly long focal length, however, since using a normal lens construction would result in a very large, unwieldy lens. To keep the size of such a lens manageable while still providing a long focal length, a concave (negative) lens assembly is placed behind the main convex (positive) lens assembly, resulting in a lens which is shorter than its focal length. Lenses of this type are called telephoto lenses. In a telephoto lens, the second principal point is located in front of the frontmost lens element.

When a convex fluorite lens is combined with a concave wide-dispersion optical glass lens to correct red and blue wavelengths, the partial dispersion characteristics of the fluorite also effectively compensate the green wavelength as well. This greatly reduces the presence of secondary spectrum and brings all three wavelengths ~ red, green and blue ~ together at the same focal point to realize virtually ideal chromatic aberration compensation, (apochromatic performance).

Lenses made from fluorite are extremely expensive due to the high cost of synthetic fluorite crystal production. UD (ultra low dispersion) glass made an appearance in the latter half of the 1970’s, delivering a special optical glass which could provide characteristics similar to fluorite but at a lower cost and thereby answering another desire of lens designers. While the index of refraction and dispersion of UD glass are not as low as fluorite, they are significantly lower than other types of optical glass. Moreover, UD glass does display similar partial dispersion characteristics. The selection of the proper lens element combination in consideration of the intended focal length and other factors can provide close to the same effect as fluorite, (two UD lens elements are equivalent to one fluorite element). Super UD glass was introduced in 1993 as a new material that achieves almost the same performance as fluorite while achieving a new balance of cost reduction and higher quality.

Canon became the first camera maker to apply the use of an advanced USM (Ultrasonic Motor) in 1987 when the EF 300mm f/2.8L USM amazed the world with its silent, super-fast autofocus performance. Then, in 1990, Canon developed the lower cost ring-type USM could be used in a variety of more affordable lenses. This feat was followed in 1992 by the development of a new type of micro USM that enabled the automation of production. Every day Canon comes closer to realizing the goal of equipping every EF lens with a USM. Features of the ring-type USM include its ability to easily achieve the low-speed, high-torque characteristics needed to realize direct drive. Large holding torque means the disc brake automatically holds the lens in place when the motor is stopped. Its construction is extremely simple, operation is virtually noise-free, and it demonstrates excellent start/stop response and control. High efficiency and low power consumption allow the lens to be powered by the camera’s battery. The motor’s ring shape is optimally suited to lens barrel applications and its low rotation speed is ideal for lens drive purposes. Rotation speed control covers a wide variable range from 0.2 rpm to 80 rpm to realize high-precision, high-speed lens drive control. Variable-sensitivity electronic manual focus is also available. The broad operating temperature range of -30ºC to +60ºC (-22ºF to 140ºF) ensures stable operation even in severe environments. And, all lens drive control is performed by the microprocessor housed within the lens. More information