Parallax Error - A phenomenon common to cameras that have viewing systems not coupled to the lens or focusing system (i.e. rangefinder and twin-lens reflex cameras) in which the accuracy of the frame decreases as the camera-to-subject distance decreases. Coupled finders shift the frame lines accordingly as the focus distance changes, which eliminates parallax issues.
Peripheral Illumination - Some lenses, wider-angle lenses in particular, produce images that appear darker towards the edges of the frame compared to the center of the image field. This is called vignetting. Peripheral illumination is a measurement of the degree of light falloff from the center of the image field to the corners expressed as a percentage.
Polarizing Filter (Circular Polarizing Filter, Linear Polarizing Filter) - A filter designed to eliminate or greatly reduce reflections on shiny or polished non-metallic surfaces, such as water and glass. By eliminating glare caused by stray ambient light, polarizing filters make clouds seemingly pop from equally intensified blue skies. Polarizing filters are essential tools for landscape, architecture, product photography, and any number of similar applications.
Polarizing filters have a dark gray appearance, are set in a rotating filter mount, and absorb about 2.5 stops of light at maximum polarization. They are most effective when the light source is behind and off-center to the camera. Polarizing filters should be restricted to lenses no wider than approximately 20mm, after which uneven darkening may occur.
Portrait Lens - A portrait lens has a focal length about twice as long as a normal lens. In the case of a 35mm full-frame camera, a typical portrait lens would be in the 85mm to 105mm range (26º30' to 23º diagonal AoV). Lenses in this range tend to slightly flatten perspective, which helps render facial features in a visually pleasing way. Prime Lens A fixed focal length lens.
Rangefinder - A non-reflex viewing system in which two semi-transparent images of the scene are lined up and become a single, clear image when the lens is in focus.
Rear Focusing - Unlike all-group focusing, in which all of the element groups shift when the lens is focused, rear-focusing lenses require less energy to shift the lens groups, allowing the lens to focus more quickly. Rear-focus lenses can also be designed smaller and lighter than conventional focusing lenses, and because the lens barrel doesn't rotate, they are easier to use with polarizing and graduated neutral density filters.
Reflection - When light strikes a glass surface, depending on the angle and intensity of the light, a certain percentage bounces off the surface rather than continuing through the glass. Left unchecked, this stray light can cause flare, loss of contrast, and other issues not conducive to good image quality. To reduce the reflective qualities of glass, manufacturers rely on advanced lens coatings, lens barrels with flat-black interiors, and lens shades.
Fisheye and ultra wide-angle lenses with larger curved front elements are particularly susceptible to flare due to their inherent protruding design.
Resolution - Also referred to as resolving power, resolution describes the ability of a lens to resolve the detail of the subject being photographed. The resolution of a lens is typically based on the results of photographing a resolution measurement chart to determine how many lines-per-millimeter the lens can resolve. The numerical value assigned to a lens in regard to resolving power alone does not indicate resolution clarity or contrast.
Rounded Diaphragm - The diaphragm blades in many modern lenses are slightly curved in order to create a rounder aperture opening when the lens is stopped down. Round aperture openings benefit creating the softly rounded out-of-focus highlight areas many photographers prefer (see Bokeh).
Shading - Unlike the darkened corners caused by vignetting, shading is shadowing of the image caused by a physical blockage such as an improperly seated lens hood, a wide-rimmed filter, or a matte box.
Shooting Distance (Camera Distance) - The distance from the film plane (focal plane) to the subject being photographed.
Spherical Aberration - When the light rays passing through the lens fail to come into focus at the same focal point, it is usually because the light rays passing through the edges of the lens refract at an angle different from the light rays passing through the center axis of the lens. Spherical aberrations reveal themselves in the form of halo-like flare and an overall reduction of contrast, or "flatness" in the affected areas.
Most common to faster (wider-aperture) lenses, optical engineers can greatly reduce spherical aberrations through the use of combinations of convex and concave lens elements and aspheric lens surfaces. Stopping down the lens also reduces spherical aberrations by cutting down the amount of peripheral light from the image field.
Stop (See Aperture, Diaphragm, and f/stop)
Subject Distance - The distance between the subject and the camera's focal (film) plane.
Super Spectra Coating - All Canon EF lenses are coated with Super Spectra coatings, which are optimally matched to the refractive characteristics of the lens elements to which they are applied. Super Spectra coatings feature a high permeation rate, UV filtering, and a durably hard surface to protect the glass surfaces. Super Spectra coatings are designed to better ensure sharp, high-contrast imagery with levels of true color fidelity that remain constant amongst all Canon EF lenses.
Super Telephoto - A telephoto lens with a focal length of 300mm or greater.
Super UD Glass - Lens elements made from a class of optical glass that has a very low dispersion index. Super UD (ultra low dispersion) lens elements are widely used in Canon EF lenses, super-telephoto L-series lenses, and select telephoto, zoom, and wide-angle lenses. Super UD lens elements evolved from Canon's original UD glass formulations.
Teleconverter - Sometimes referred to as a tele-extender, a teleconverter is a secondary lens accessory that mounts between the lens and the camera body that magnifies the focal length of the lens, typically by factors of 1.4x, 1.7x, 2x, or 3x. In almost all cases the light loss of teleconverters equals the magnification ratio, i.e. a 1.4x has a 1-stop loss of light, a 2x loses 2-stops of light, and a 3x loses 3-stops of light.
Teleconverters are handy in that they can greatly expand the focal range of an existing lens system without adding additional weight and bulk to one's camera bag.
Telephoto Lens - A telephoto lens is any lens with a focal length longer than a standard normal lens, or in technical terms, a lens in which the front-most lens element to the focal plane is longer than its focal length.
Telephoto lenses bring distant subjects seemingly closer, and are ideal for sporting events and wildlife photography. Telephoto lenses in the 85mm to 105mm range are considered the ideal focal range for portraiture. Currently, the longest fixed focal length lens available is Canon's 800mm f/5.6L IS USM.
Tilt-Shift (TS-E) Lenses - Special lenses in which the optical axis and film plane center and the optical axis vertical to the film plane can be tilted and shifted in order to correct keystone distortion (vertical convergence). TS-E lenses can also be used effectively to either increase or decrease the depth of field in the photograph without having to change the aperture. Canon TS-E lenses are available in 17mm, 24mm, 45mm, and 90mm focal lengths. TS-E lenses retain auto-exposure capabilities but are manual focus only.
UD Glass Lenses - UD (ultra low dispersion) glass is a special optical glass that features refraction and dispersion characteristics similar to fluorite but at a lower cost. UD lens elements have been supplanted by more advanced Super UD glass elements.
Ultra Wide-Angle Lens - A wide-angle lens with an angle of view greater than 84º, or the equivalent of a 24mm lens on a full-frame camera.
USM (Ultrasonic Motor) - Microprocessor-driven autofocus motors developed by Canon that use advanced in-lens electronics to drive the AF system quickly and silently compared to earlier AF drive systems.
Variable Maximum Aperture - Zoom lenses are available with constant or variable maximum apertures. Constant maximum aperture lenses, which maintain a consistent maximum aperture regardless of the set focal length, tend to be larger, heavier, and pricier than variable aperture lenses. Variable maximum aperture lenses, which effectively lose one to two stops of light transmission as the lens is zoomed, are lighter, smaller, and often less expensive than their constant maximum aperture counterparts.
Vignetting - Common to a degree in most lenses, and most visible among wider-angle lenses, vignetting describes the darkened corners of a photograph. Vignetting is due to the inability of the lens to transmit a consistent amount of light to the edges of the image field compared to the center portion of the photograph. Though aesthetically pleasing in some cases, vignetting is most prevalent when the lens is wide open, and can usually be reduced or eliminated by stopping down the lens.
Wide-Angle Lens - A lens wider than 50mm (about 46º AOV) on a full-frame, 35mm format camera.
Working Distance - The distance from the front edge of the lens barrel to the subject, which can be an important factor when shooting close-ups.
Zoom - The ability to quickly shift from one focal length to another using a zoom lens.
Zoom Lens - Unlike fixed focal length lenses (prime lens), zoom lenses allow the photographer to quickly recompose a photograph, i.e., get closer or further away from the subject by simply turning a ring on the lens barrel in order to change the effective focal length of the lens.
Source: Canon USA