Lens Glossary, A-G

A

Abbe Number - Also referred to as the "optical constant," Abbe numbers are an industry standard for categorizing the light and color dispersion qualities of optical glass. The higher the Abbe number, the higher the glass quality. Determined by the composition and optical characteristics of the glass, there are about 250 distinct Abbe classifications, and it's not unusual to find multiple Abbe classifications of glass elements used in a single lens design.

Aberration (Flare, Spherical Aberration, Coma, Field Curvature, Distortion, Vignetting) - Aberrations are visual artifacts that occur when the dynamics of the picture being taken go beyond the limitations of the lens design or the materials used to make the lens. Depending on the specifics of the aberration, aberrations most commonly appear in the form of flare, color fringing along the edges of contrast-rich subjects, and optical distortion. These are typically triggered by the angle and intensity of the light entering the lens, the shapes and configurations of the lens elements, internal reflections, and the fields-of-view of the lenses, singularly or in combination with one another.

Achromat or Achromatic Lens - Achromats are lenses that correct for chromatic aberrations in the blue-violet and yellow wavelengths of the visual spectrum.

AF Stop Feature - AF Stop is an advanced autofocus mode that temporarily locks the focus point of the lens when shooting in AI Servo AF mode. Unique to Canon IS super-telephoto lenses, AF Stop can be activated at any time by simply pressing one of four small buttons located along the circumference of the forward portion of the lens barrel. Once activated, your subject remains in focus as long as the subject-to-lens distance remains unchanged.

Air Lens - An air lens is the space formed when two concave (or one concave and one convex) lens elements are placed together. Having the same refraction index as air (1.0), air lenses are frequently incorporated into the lens design, though because air and glass have opposing refraction indexes, convex air lenses function as concave lenses and concave air lenses function as convex lenses.

Angle of View (AOV) - The area of a scene, expressed as an angle in degrees, which can be reproduced by the lens as a sharp image. Typically given as a diagonal angle of view, this measurement is defined as the angle formed by imaginary lines connecting the lens's second principal point with both ends of the image diagonal.

Aperture (f/stop)Commonly called f/stops (or f-stops), the lens aperture is the opening formed by the lens's diaphragm blades, allowing an adjustable amount of light to pass through the lens. Just as the iris in the human eye opens up to allow more light into the eye when light levels are low and gets smaller as light levels increase, the lens diaphragm blades similarly open and close, allowing more or less light through the lens. Along with the camera's ISO setting and shutter speed, the lens aperture is a key component of a camera exposure.

APS-C Format - Named after a similarly sized (25.1 x 16.7mm) and named film format, the 22.3 x 14.9mm APS-C imaging sensors found in Canon's APS-C DSLRs and mirrorless cameras have a 1.6x crop factor compared to full-frame (24 x 36mm) imaging sensors.

Aspherical Lens ElementsConventional or spherical lens elements have limitations when it comes to controlling spherical aberrations in wide-aperture lenses and distortion in wider-angle lenses.

To counter these issues, lens designers incorporate additional "peaks and valleys" into convex and concave lens element designs, called "aspheric" lens surfaces. In addition to damping aberrations and distortion, one aspheric lens element can often replace two conventional lens elements, enabling lens manufacturers to produce smaller and lighter lenses.

Autofocus - The ability of a lens to electronically lock focus on a static or moving subject without the need of manual assist. Many autofocus lenses can be set to fix the focus at a set distance or track the subject continuously as it moves across the image field.

Auxiliary Lens - A secondary lens that screws onto, or otherwise attaches to, the front of a camera lens to either increase or decrease the effective magnification of the primary lens, therefore simulating a wider or narrower field of view. Macro auxiliary lenses that enable the lens to focus closer to the subject are also available.

B

Bokeh - Bokeh is a transliteration of the Japanese term "boke," which describes the visual characteristics of the out-of-focus portions of a photographic image. "Good" or "pleasing" bokeh is typically identified as smooth, rounded out-of-focus highlights, while "bad" bokeh typically describes out-of-focus highlights that mimic the more geometric form of straight diaphragm blades.

C

Chromatic Aberration - Most common in pictures taken with longer focal length lenses, chromatic aberrations occur when the lens fails to focus all of the color frequencies at the same point of focus, with blue being the shortest frequency and red being the longest frequency.

Chromatic aberrations typically appear in the form of "purple fringing," which is recognizable by the purple halos that run along the more contrast-rich edges of objects in the frame. Axial chromatic aberrations appear as color blurring or flare.

When this occurs, there is a loss of perceived sharpness and an increase in color flare and fringing. Chromatic aberrations are less problematic among lenses containing extra-low dispersion and fluorite lens elements.

Circle of Confusion - There are limits as to how far down a lens can focus a point of light. Circles of confusion (CoC) are defined as the largest blurs of light a healthy human eye can perceive as a point of light at the image's final output size from a distance of 25cm.

Circular Apertures - Circular Aperture diaphragm units, found in select Canon lenses, use curved aperture blades to create more rounded aperture openings and, therefore, pleasing bokeh by rendering out-of-focus background highlights as naturally rounded shapes.

Circular Polarizing Filter (Linear Polarizing Filter, Polarizing Filter)Polarizing filters are used to reduce or eliminate reflections on glass, water, and other non-metallic reflective surfaces. Polarizing filters also darken blue skies and make clouds 'pop' from their blue surroundings by polarizing stray light from the scene.

Circular and linear polarizing filters look and function the same with one difference. Unlike standard linear polarizers that filter light in a linear fashion, circular polarizers utilize a quarter wave plane to filter light in a circular pattern, enabling normal functionality of TTL, auto exposure, and autofocus systems. Circular polarizing filters can be used equally well with manual and autofocus lenses, but linear polarizers should only be used with manual focus lenses.

Dark gray in appearance, polarizing filters absorb 2 to 3 stops of light.

Click Stops - Also referred to as detents, click stops are touch-sensitive positions often found on shutter speed, aperture, and other exposure adjustment dials that physically identify specific camera settings.

Coatings (Lens Coatings) - Only a fraction of a wavelength thick, lens coatings help eliminate flare and low contrast levels while increasing light transmission efficiency and overall image quality. Applied to all lens surfaces in single or, more commonly, multiple coatings (multi-coating or multi-layered coating), lens coatings play a dramatic role in a lens's ability to produce top-notch image quality.

Lens coatings are applied using a vacuum vapor deposition process that coats the surface of each lens element with a micro-layer of magnesium fluoride or similar compound. While uncoated glass reflects upwards of 7% of the light that strikes its surface, Canon's multi-coated EF-series lenses reflect as little as 0.2 to 0.3% of the light, effectively reflections of all wavelengths in the visible spectrum.

Coma (Comatic Aberration) - Coma is an optical phenomenon that causes off-axis points of light to appear as a comet-shaped blurs of light rather than sharper round points of light. The "tails" of coma shapes usually point toward or away from the center of the image, and the degree of coma usually increases proportionally to the angle of the light entering the lens. Depending on the lens design and the nature of the light source, coma can often be reduced or eliminated by stopping the lens down to a smaller aperture.

Contrast - The degree of distinction between darker and brighter areas of a photographic image. High contrast is when the ratio between white and black is clearly distinctive and crisp, while low-contrast images contain higher degrees of gray tones, making for lower-contrast photographs.

Coupled Rangefinder - An in-camera rangefinder viewing and focusing system that is mechanically coupled to the focusing mechanism of a lens. Focusing is achieved by turning the focusing ring until the dual images in the viewfinder overlap. Coupled rangefinders are usually parallax-corrected.

Crop Factor - A mathematical formula for determining the field of view of a lens used on a smaller-format camera compared to the same lens used on a full-frame camera. As an example, the APS-C sensors used in Canon's APS-C-sized DSLRs and EOS M-series cameras have a 1.6x crop factor compared to a full-frame (24 x 36mm) imaging sensor (whose crop factor is essentially 1x). This means a 50mm lens mounted on an APS-C format camera has the field of view of an 80mm lens (50 x 1.6 = 80). The lens is still a 50mm lens but the smaller APS-C sensor can only record the central 40% of the lens's actual image circle or, rather, it is cropping into the full image circle of the lens.

Curvature of Field - Unlike macro lenses, which feature flat planes, that when focused parallel to a flat subject deliver sharp edge-to-edge detail even at wide apertures, conventional lenses focus on a curved, bowl-like plane. As a result, when photographing a series of same-size objects lined up perpendicular to the lens at the lens's widest aperture, the objects in the center of the frame often appear sharper than the objects located closer to the edges of the frame.
 

D

Depth of Field - Depth of field (DoF) describes how much of the area in front of and behind the main point of focus appears sharp. The actual depth of field of a lens is determined by the aperture of the lens (how wide or stopped down it is), the focal length of the lens, the size of the imaging sensor, and the distance between the focus point and the camera. Depth of field can be characterized by the following attributes: -Depth of field is greater at short focal lengths and shallower at long focal lengths. -Depth of field is greater with small apertures and shallower with larger apertures. -Depth of field is greater at longer shooting distances and shallower at closer shooting distances. -Front depth of field is shallower than rear depth of field.

Depth of Focus - The area fore and aft of the plane of focus (the point where the image comes into and leaves focus) that retains enough definition to reproduce a sharp photograph. Unlike depth of field, depth of focus is distributed equally in front of and beyond the plane of focus.

The depth of focus of a lens, regardless of focal length, can be determined by multiplying the minimum circle of confusion by the F-number.

Detents - Also referred to as "click stops," detents are touch-sensitive positions on shutter speed and aperture dials that physically identify camera settings.

Diaphragm - The lens diaphragm is the mechanism that creates f/stops by opening and closing the diaphragm blades to preset positions.

Diaphragm Blades - The individual blades that collectively act as an iris, opening and closing, allowing more or less light to pass through the lens. The size of the openings created by the blades as they open and close is what we call aperture, incrementally quantified by the term "f/stop."

Diffractive Optics (DO) - An optical design technology that enables engineers to design longer focal length telephotos that, in addition to greatly reducing the appearance of chromatic and spherical aberrations, are shorter and lighter than conventional telephoto lens designs.

Diffractive elements are created by applying diffractive coatings to the surfaces of two or more lens elements and bonding them together into a single DO lens element that contains light dispersion characteristics that, when combined with optical glass elements, cancel the effects of chromatic aberrations across a broad range of wavelengths.

Dispersion - Also known as color dispersion, dispersion is the breakdown of white light (the visible spectrum) into its individual color components. Light passing through a prism, cerating subsequent rainbows, is an example of color dispersion.

Colors always break down in the order in which they appear in the visible spectrum: red, orange, yellow, green, blue, indigo, and violet (mnemonic: Roy G. Biv) in varying proportions.

Distortion - Even the best lenses can display signs of distortion. Most commonly noticed when photographing scenes containing straight or parallel lines, optical distortions fall into two main categories.

Barrel distortion is when straight lines curve outwards from the center of the frame (like a wooden barrel), and pincushion distortion is when the lines curve inward. A third type of distortion-mustache distortion-is when straight lines take on a handlebar mustache-like shape, especially towards the edges of the frame.

Though distortion is easier to detect in photographs taken with wider-angle lenses, it can also appear in images from normal and telephoto lenses. Zoom lenses often, but not always, display barrel distortions toward the shorter focal lengths of the lens and pincushion distortion toward the telephoto end of the zoom range.

Barrel and pincushion distortion can often be corrected, post capture, in a number of photo-editing programs.

E

Element - One of a number of glass "pieces" in a lens.

EMD (Electromagnetic Diaphragm) - All Canon EF-series lenses contain electromagnetic diaphragms, which unlike their mechanical-linkage counterparts, offer higher levels of precision, quicker response times, and aperture openings that remain accurate, exposure after exposure.

Designed as part of Canon's EF lens mount, the EMD is a diaphragm drive control actuator made up of a deformation stepping motor and diaphragm blade unit that opens and closes the lens's diaphragm blades quietly and smoothly, and-because it's electronically controlled-extremely accurately.

Exposure - Camera exposures are determined by calculating, manually or automatically, a proper balance of shutter speed, lens aperture, and the ISO sensitivity. If you change any of these variables, one or both of the other variables must be changed reciprocally in order to maintain consistent exposure.

Changes to the shutter speed, aperture, and ISO sensitivity levels are typically made in full, half, or third-stop increments, though in certain auto-exposure modes, changes to the aperture, shutter speed, and ISO can be made in much finer increments.

Extension Tubes - Tubes with male and female lens mounts on either end that enable a lens to focus closer to the subject by extending the distance between the rear element of the lens and the film or sensor plane. Extension tubes are available in several sizes, can be stacked for further extension and, depending on the brand and model, may or may not enable automatic exposure or autofocus control. When using extension tubes, the lens will not be able to focus to infinity.
 

F

F/stop (Aperture, F-Number, or Focal Ratio) - The quantitative measurement of the iris opening created by the lens's diaphragm blades. The formula for determining the actual f/stop is N = f/d (The f-number equals the focal length of the lens over the diameter of the entrance pupil (effective aperture).

F/stops and shutter speeds are reciprocal: if you stop the lens down a stop (make it a stop smaller), you must increase the shutter speed by one stop to maintain a consistent exposure. And when you double the f-number, you decrease relative brightness by a factor of four.

Field of View (FoV) - The diagonal measurement of the image captured by the lens as recorded by the imaging sensor or film. The field of view of any given lens is greatly determined by the size of the camera sensor or film format. (see Angle of View)

Filter Thread - Filter threads, located inside the front lip of the lens barrel, are for attaching filters and other accessories to the front of the lens. They are measured by the diameter and typically expressed in millimeters.

Fisheye - An ultra wide-angle lens that captures circular or full-frame images with image fields of up to 180º. Unlike retro-focus type wide-angle lenses, which render straight lines parallel when the camera is positioned at a level angle, fisheye lenses display distortion regardless of how level the camera is positioned.

Fixed-Focus (Focus Free) - Lenses that maintain a fixed point of focus and require, nor allow, any further focus adjustments. Fixed-focus lenses are commonly found on inexpensive point-and-shoots and security cameras.

Flange Back (Flange-Focal Distance, Focal Flange Distance, or Flange Distance) - The distance between the rear of the lens mount to the surface of the camera's imaging sensor or film plane. A key attribute of mirrorless and rangefinder cameras is that they have shorter flange distances compared to SLRs and DSLRs, which is why they readily adapt to non-OEM and third-party lenses. Nearly any lens designed with a longer flange distance can be adapted to fit a camera system with a shorter flange distance, whereas lenses from short flange distance systems cannot be adapted to longer flange distance systems without the addition of corrective optics for attaining infinity focus.

Flare - A lens aberration caused by stray direct or reflected light from the lens or camera's mirror box that strikes the film or imaging sensor, causing loss of detail and contrast-reducing aberrations. Though not always desirable, flare can often be used as a creative element in a photograph.

Flare, a term used to describe the blurring and halo effects caused by spherical and comatic aberrations, can often be controlled by using a lens hood or shade.

Floating System (Floating Elements) - In order to better control aberrations when focusing at closer distances with certain lenses, select elements move independently from the other element groups in order to reduce aberrations and improve edge sharpness among wider-angle lenses. This is also referred to as a close-distance aberration compensation mechanism.

FluoriteFluorite is a rare-earth mineral known for its extremely low refraction and dispersion indexes compared to traditional optical glass. In 1968, Canon's optical engineers created a method of manufacturing artificial fluorite crystals, making it possible to design affordable, high quality, and high performance lenses for professional applications.

Focal Length - The distance, typically expressed in millimeters, from the rear principal point of a lens to the surface of the camera's imaging sensor or film along the optical axis of the lens when focused at infinity.

Depending on their respective focal lengths in regard to specific film or sensor sizes, lenses fall into three main categories – wide-angle, normal, and telephoto. Zoom lenses can "zoom" from wide-angle to telephoto, normal to telephoto, ultra wide-angle to wide-angle, or telephoto to super-telephoto.

Because there are numerous sensor sizes, manufacturers continue to use the 35mm film format (24 x 36mm) as a standard when referencing lenses designed for smaller sensor formats. This is why a lens described as a 28mm to 90mm equivalent zoom might have an actual focal range of 18mm to 55mm or 14mm to 45mm.

Focal Plane - The flat plane (film or sensor surface) onto which the lens focuses the light to form an image.

Focal Point, Focus - The focal point of a lens is the convergence point of the light rays entering the lens. A good example of a focal point is the sharp point of light that forms when you focus the sun's light onto a smooth surface with a magnifying glass.

Focus - The point at which the image projected by the camera lens onto the recording surface of the imaging sensor is sharpest.

Focus Preset - A special feature found on many Canon EF IS-enabled telephoto lenses that memorizes a preset focus point and can quickly return to that point by simply turning a "playback" ring located on the lens barrel, regardless of where the lens is focused at the time.

Fungus - Lenses stored in dark, humid environments for long periods of time can sometimes develop a spider web-like fungus that, if left unchecked, can cause permanent damage to the lens. If spotted early enough it can usually be stopped from causing further damage to the lens elements.

Full-Frame Image Sensor - An imaging sensor measuring approximately 24 x 36mm, which is the size of a 35mm film frame. Many modern digital cameras are modeled after traditional 35mm film cameras, which is why the focal-length range of many smaller-format camera lenses is invariably referred to in regard to a 35mm equivalent.

Full-Time Manual Focusing - A signature feature of USM-enabled Canon EF lenses, instant manual focus override of the lens's AF system is possible by simply rotating the manual focus ring.

Fully Electronic Interface - When all of the components of a camera system, including the camera, lenses, flash, remotes, and related electronic components in a system share a common source of real-time exposure, flash, and white-balance data to better ensure consistently exposed stills and video.

G

Ghosting - A type of flare that typically appears when a strong light source, i.e., the sun, a spotlight, etc., causes strong reflective flare on the side of the image field opposite the light source. Ghosting greatly reduces contrast and detail in the affected areas. Reframing the image, use of a lens hood, or otherwise blocking the light source can usually eliminate ghosting.

Source: Canon USA

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