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Canon - Lens Dictionary

Aspherical Lenses

Light rays entering at the edges of spherical lens elements converge at slightly different focal points to light rays entering from the centre. Known as spherical aberration, this optical phenomenon produces soft, low contrast images that look as if covered with a thin veil. Canon developed aspherical lens elements to counteract this problem. A special non-spherical surface converges central and peripheral light rays at a single focal point to ensure uniform sharpness and clarity over the whole image area. Now found in almost every EF lens, aspherical lens elements are particularly useful for large-aperture lenses, wide-angle lenses and high quality compact zoom lenses. (source)

Fluorite and UD Lenses

If you hold a prism up against sunlight, a rainbow spectrum will appear. This is due to the fact that different wavelengths of light refract - or bend - at different points within the prism. The same phenomenon occurs to a lesser degree in photographic lenses, where it is known as chromatic aberration. It's most noticeable in photographs as colour fringing at the edges of objects. Combining convex and concave lenses helps to correct the problem but does not entirely resolve it. Fluorite, which boasts a very low dispersion of light, is capable of combatting the residual aberration that standard optical glass fails to eliminate. Canon succeeded in artificially creating crystal fluorite in the 1960s, producing the first interchangeable SLR lenses with fluorite elements. In the 1970s, Canon achieved the first UD (Ultra Low Dispersion) lens elements incorporating low-dispersion optical glass. This technology was further improved to create Super UD lenses in the 1990s. A combination of fluorite, UD and Super UD elements are used in many of today's super-telephoto L series lenses, telephoto zooms and wide-angle lenses. (source)

DO (Diffractive Optics) Lenses

Diffraction is an optical phenomenon in which light waves bend as they pass around the edges of an object. In the late 1990s, a team of young Canon engineers came up with the idea of using this phenomenon to counteract chromatic aberration. A single diffractive optical (DO) element features diffractive gratings - very fine parallel grooves on the surface - which change the direction of light. However, this process generates diffracted light that is not suitable for photographic lenses and can cause flaring. Employing multiple DO elements ensures that nearly all light allowed to pass through the DO elements can be used for photography. Canon DO lenses use multiple DO elements whose diffractive gratings are bonded face-to-face. Combining this arrangement with a refractive convex lens almost completely cancels out chromatic aberration while allowing the lens elements to be placed much closer together within the lens barrel. The result is a high performance EF lens that's significantly shorter and lighter than typical refractive lenses. In fact, the EF 400mm f/4 DO IS USM is 27% smaller and 31% lighter than a conventional 400mm f/4 lens would be. Fast becoming a favourite with sports and news photographers, DO lenses are making high performance telephoto shooting easier and more manageable than ever before. (source)

Super Spectra Coating

When light enters a lens, a small percentage is reflected off the internal lens element surfaces. This reflected light, if not dealt with effectively, can result in distortion of the recorded image. There are two common types of image distortions: Ghosting, in which a faint second image appears on the photo; and flare, which occurs when light from the back of the lens barrel is reflected onto the image. Light travels in waves since each colour travels at different wavelengths, multi layers coatings are required to effectively reduce reflection. Each layer works on different wave lengths of light. Canon's multi layer Super Spectra coating allows up to 99.9% of light through to the CMOS sensor, over a range that extends from ultra violet to near-infrared light. As well as minimising ghosting and flare, Super Spectra Coating ensures a consistent colour balance across all EF lenses and plays a key part in delivering the sharp, high-contrast results that Canon lenses are renowned for. (source)

Focusing Systems

Floating System
Extending the focusing lens group at close focusing distances can sometimes cause distortions such as curvature of field - particularly noticeable with wide-angle lenses. Canon counteracts this problem by incorporating 'floating' lens elements into many EF lenses. Because the floating component is separate to the focusing lens group, aberrations caused by lens extensions are significantly reduced. This means you can be sure of high image quality at all focusing distances.

Inner and Rear Focusing System
Many photographic lenses use either all-group focusing (all lens groups are moved together along the optical axis) or front-group focusing (only the front lens group is moved). These systems require far more power from the drive mechanism, limiting auto focus speeds. The need for large front lens groups also limits the potential for compact lens sizes.

The majority of Canon's lenses use inner focusing systems (placing the focusing lens group between the front lens and diaphragm) or rear focusing systems (placing the focusing lens group behind the diaphragm). These systems contribute to more compact lens sizes, rapid auto focusing and shorter minimum shooting distances. Lenses are also easier to handle since they do not change length during focusing. And because the front frame of these lenses does not rotate, polarizing filters are easier to use. (source)

Full-Time Manual Focus

Many EF lenses are equipped with a ring-type USM which allows full-time manual focus in AF mode. This means you can manually fine-tune focusing as soon as auto focus is complete, without having to switch to manual focusing mode. Full-time manual focusing on most compatible lenses uses a mechanical system for instant responsiveness. Full-time manual focus is also possible on some lenses equipped with a Micro USM, such as the EF 50mm f/1.4 USM. (source)

Ultrasonic Motor

Canon's Ultrasonic Motor (USM) was the world's first lens-based motor to harness ultrasonic oscillation energy. Electronic vibrations created by a piezoelectric element power the mechanical action of the lens. This technology makes focusing precise, virtually noiseless and incredibly quick - some lenses focusing literally faster than the human eye. The USM starts instantaneously and stops the moment focus is achieved, with no overshooting. What's more, the USM requires minimal battery power so you can shoot for longer on a single charge. Two types of USM are used in Canon EF lenses: Ring-type USM and Micro USM. Ring-type USM is found in large-aperture and super telephoto lenses, while Micro USM is used in more compact lenses. (source)

Image Stabilizer

Camera shake is a major cause of blurred images, especially with handheld photography or in low light situations where a slow shutter speed is required. Canon's Image Stabilizer (IS) technology uses a lens shift-type system to detect and correct slight camera shakes. Pressing the shutter button halfway releases the lock on the Image Stabilizer system. Minute gyro sensors detect lens vibrations caused by hand shake and send a signal to a microcomputer, which controls an IS lens group that compensates for the pitch and yaw (movement along the horizontal and vertical axis of the camera) of the lens. To ensure that intentional panning and tilting isnt mistaken for camera shake, some telephoto and tele-zoom lenses have IS Mode 2. This mode detects if movement in a certain direction occurs for longer than a preset time and switches off the Image Stabilizer in that direction. EF lenses with IS technology enable steady shooting at up to four stops lower than is possible with conventional lenses, making them ideal for situations where the use of flash is not permitted or desired. (source)


EF (Electro Focus)
The EF lens mount is the standard lens mount on the Canon EOS family of SLR film and digital cameras. EF stands for "Electro-Focus": automatic focusing on EF lenses is handled by a dedicated electric motor built into the lens. Mechanically, it is a bayonet-style mount, and all communication between camera and lens takes place through electrical contacts; there are no mechanical levers or plungers. In 2003, Canon introduced the EF-S lens mount, a derivative of the EF mount that is strictly for digital EOS cameras with a 1.6x crop released after 2003. EF lenses can be mounted on EF-S bodies, albeit with cropped image, while EF-S lenses cannot be mounted on EF bodies. (source)

EF-S (Electro Focus - Short back focus)
The EF-S lens mount is a derivative of the EF lens mount created for a subset of Canon digital single-lens reflex cameras with APS-C sized image sensors. It was released in 2003. Cameras supporting the EF-S mount are backward-compatible with the EF lens mount. Only Canon cameras with the APS-C sized sensor released after 2003 support the mount. All other cameras do not support it. The "S" in EF-S stands for "short back focus", which means that the rear element of the lens is closer to the image sensor than on regular 35 mm SLR cameras. The proximity of the rear element to the image sensor greatly enhances the possibilities for wide angle and very wide angle lens, enabling them to be made smaller, lighter (containing less glass), faster (larger aperture) and less expensive. Most current Canon EF-S lenses are wide angle. Lenses designed specifically for APS-C sized sensors are often optically designed to provide a narrower light cone to match the sensor. However, not all such lenses require the shorter back focus, and may feature the standard EF mount. Such lenses will give noticeable vignetting if used on a 35mm film or sensor camera. (source)

L-series lenses

Top-of-the-line Canon EF lenses are designated as L-series, or "Luxury" lenses.L-series lenses have good optical performance and a solid construction. Canon ships all L lenses with a hood and pouch; photographers must purchase the hood and pouch separately for non-L lenses. Distinctive visual characteristics include a red ring around the lens and an off-white color on some lenses (to reduce heat absorption). Some L lenses have environmental and weather protection against the other elements. All L lenses include at least one fluorite or ultra-low dispersion glass element, combined with super-low dispersion glass and ground aspherical elements. Other mechanical characteristics of L lenses (but not exclusive to them) are the USM (particularly in recent years) and Image Stabilization technologies. (source)

TS lenses (Tilt-Shift)

These lenses expand photographic possibilities. Tilt movements allow you to obtain a wide depth of field even at the maximum aperture and still keep the entire subject in focus. Shift movements correct the trapezoidal effect seen in pictures taken of tall objects, so as not to distort the subject. (source)