Lens Glossary, H-O

H

Hyperfocal Distance - The hyperfocal distance of a lens is a focus setting based on the closest point at which the lens can be focused and still maintain focus at infinity based on the lens's aperture setting (f/stop). 
As an example, a 24mm lens set at f/11 and the focus distance set to 4.9' (1.5 m) will have a depth of focus ranging from infinity to about 2.3' (70 cm) from the camera position. Focusing a lens to its hyperfocal distance is a popular method of capturing spontaneous, journalistic-style photography and ensuring sharp focus throughout the frame.
The hyperfocal distance of a lens can be determined using the following formula: Hyperfocal distance = f2 / (d ∙ F); where f is the focal length, d is the minimum circle of confusion diameter, and F is the F-number.

I

Image Circle - The diameter of the circular image formed by the camera lens and focused onto the imaging sensor. The size of the image circle must be large enough to cover all four corners of the camera's rectangular image field. Full-frame cameras require image circles at least as wide as the diagonal of a 24 x 36mm frame, while APS-C format cameras require lenses that can cover the slightly smaller diagonal of the smaller sensor size.

Canon EF lenses have an image circle of approximately 43.2mm. Canon's EF tilt-shift (TS-E) lenses have an image circle of approximately 58.6mm in order provide extended image coverage for tilt and shift movements.

Image Distance - The distance between the rear principal point of the lens to the focal point (film plane), which varies according to the focus point of the lens.

Image Magnification - The image-size ratio between the physical size of the subject and the physical size of the imaging sensor or film. A macro lens that can focus down to life-size, or 1:1, will reproduce the subject at life size. A macro lens that can focus down to half life size, or 1:2, will record the subject at half of its actual size.

Image Stabilization (IS) - A technology that makes it possible to capture sharp photographs at shutter speeds up to four stops slower than normally recommended. Optical image stabilization, the type used by Canon, uses vibration-sensitive sensors to detect and measure the degree of vibration. These measurements are then used to rapidly shift a motorized set of elements inside the lens in the opposite direction of the vibration, effectively neutralizing any image blur.

Internal Focusing - Internal focusing is a lens design in which the length of the lens barrel remains constant regardless of whether the lens is focused at infinity or at its closest focusing point. This is achieved by having one or more of the lens groups shift within the confines of the lens barrel rather than having to extend outwards as the lens focuses closer.

Internal-focusing lenses tend to be more compact, and because the barrel doesn't rotate during focusing, internal-focus lenses are preferable for use with polarizing filters.

L

Lens - One or more concave, convex, or aspheric elements that can focus a light image on a focal plane (camera sensor or film).

Lens Barrel - A metal, polycarbonate, or plastic tube that holds the lens elements, and other lens components if applicable, in place.

Lens Cap - Protective cover that screws, snaps, or clips onto the front and rear of the lens to keep dust and other foreign particles off of the surfaces of the exposed front and rear lens elements.

Lens Mount - The metal or plastic coupling at the rear of the lens that connects the lens to the camera mount.

Lens Speed - A term describing the widest (fastest) aperture of a lens.

Lens Type - Categorization of a camera lens based on the number of elements it contains, its focal length, maximum aperture, and intended use.

Light (Visible Spectrum) - Light, i.e., the visual spectrum, consists approximately of the 400 to 700 nanometer portion of a greater known range of electromagnetic frequencies that include infrared and ultraviolet radiation, and wavelengths that transmit radio and TV broadcasts. Traveling at about 186,282.4 miles per second, light is the basis of photography.

Linear Polarizing Filter (Circular Polarizing Filter and Polarizing Filter) - Linear polarizers, which look and work identically to circular polarizers, are strictly for use with manual focus lenses and manual exposure metering. Due to the lack of a quarter wave plane, linear polarizers are not accurate when used with autofocus lenses, and can interfere with the accuracy and functionality of the AF and AE system.

M

Macro Lens - Unlike traditional curved-field lenses, in which the edges of the frame are not always as sharp as the center of the frame, macro lenses are flat-field lenses that maintain edge-to-edge sharpness when photographing flat subjects (documents, artwork, etc.) from an angle parallel to the subject. As the name implies, macro lenses can also focus closer to the subject than standard lenses, typically down to full (1:1) or half life size (1:2).

Canon produces macro lenses in five focal lengths-50mm, 60mm, 100mm, 180mm, and a 65mm lens with the ability to focus as close as 5x life size for extreme macro applications.

Macro Mode - Macro mode is a close-focusing mode found on many lenses, particularly zoom lenses. When engaged, macro mode enables the lens to focus closer than the lens's standard close-focusing distance.

Even though macro mode enables close focusing, the image quality of the same close-up captured with a true macro lens will usually be significantly better. Nonetheless, macro mode is a handy feature to have on any lens.

Maximum Aperture - The widest aperture of a lens.

Maximum Magnification - The maximum size ratio of the subject when the lens is at its closest focusing distance.

Medium Telephoto - A lens with a focal length between 85mm (28º30' AoV) and 135mm (18º AoV) on a full-frame camera.

Minimum Aperture - The smallest aperture of a lens.

Mechanical Distance - The measured distance from the leading edge of the lens barrel to the film plane.

Mirror Lens (Catadioptric Lens, Reflex Lens) - Essentially a short telescope, mirror lenses use curved, counter-reflective mirrors in place of glass elements that enable the design of short, lightweight telephoto lenses typically ranging in focal length from 300mm to 1000mm. Some inherent attributes of mirror lenses are that they have only one fixed aperture, usually about f/8; are capable of complete elimination of chromatic aberrations; and produce distinctive doughnut-shaped bokeh.

MTF Chart (Modulation Transfer Function)MTF charts are used to objectively measure the optical performance potential of a lens and utilize pairs of lines, organized perpendicular to one another, and at 30 lines/mm and 10 lines/mm, to measure the contrast and resolution resolving abilities of a lens against a theoretically perfect lens. Canon publishes MTF test results for each of its lenses at two aperture settings-the widest aperture and at f/8, both with the lens focused at infinity.

N

Normal Lens (Standard Lens) - A lens that has a focal length equal, or close, to the diagonal of the sensor or film. A normal lens for a full-frame sensor or 35mm film is approximately 50mm, and 35mm for APS-C format cameras. Normal lenses have a diagonal angle of view of about 46º.

O

Optical Axis - The straight line which passes through the hypothetical centerline connecting the center of curvature of each lens surface within a lens.

Optical Zoom - A zoom lens is an optical zoom. Unlike digital zoom, which is no more than a form of image cropping and interpolation, a zoom lens brings subjects variably closer by physically changing the effective focal length of the lens. Because the image quality remains virtually constant regardless of how tightly the subject is framed, optical zoom is a superior method of tightly framing a subject compared to other means of zooming.

Source: Canon USA

Lens Glossary, P-Z

P

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.

R

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).

S

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.

T

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.

U

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.

V

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.

W

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.

Z

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