Substantially corrects chromatic aberrations in the telephoto range
Fluorite is a type of mineral that is the crystallised form of calcium fluoride. It possesses optical properties such as a low refractive index and a low dispersion. Natural fluorites are small in size, and cannot be used in DSLR camera lenses. In 1969, Canon established a technology for creating artificial crystal fluorite and developed a special technology with a polishing process that takes four times longer than usual, the first in the world to succeed in commercialising the use of fluorite in photographic lenses. By utilising the low-dispersion characteristics of fluorite, the occurrence of secondary spectrum, which is unavoidable in conventional optical glass, can be eliminated almost entirely. Image quality is significantly enhanced as a result, enabling vivid, sharp, and yet detailed image expression.

Effect similar to fluorite lens elements
The chromatic aberration correction effect is remarkable when a fluorite element is used. However, due to the high cost of fluorite compared to other materials, it cannot be employed widely on many lenses. To resolve this issue, Canon developed the UD (Ultra Low Dispersion) lens element, which is characterised by a low refractive index and low dispersive properties. It is widely employed in many lenses due to its ability to enhance the elimination of secondary spectrum.

In-lens correction of camera shake
Canon was the first in the world to develop the IS (Image Stabilizer), an in-lens system to correct camera shake in DSLR cameras. Compared to the in-camera Image Stabilizer feature, not only is the IS capable of stabilising the viewfinder image, it also helps to improve AF accuracy.
The underlying principle is that the amount of misalignment between the image of the subject and the optical axis of the lens is detected by two vibration-detecting gyro sensors. The corrective optical system inside the lens is then moved accordingly in the vertical or horizontal direction to stabilise the image of the subject on the imaging plane.
Most blurry pictures occur as the result of camera shake, which inevitably happens when you take handheld shots. Such blurring is more obvious at slow shutter speeds, which usually occur at low light settings such as during indoor portrait or night street photography because the low amount of light requires the sensor to be exposed for longer. For that reason, photographers usually make up for that by setting a higher ISO speed or using a tripod and remote shutter release. Both have their merits, but also some demerits.

For instance, while increasing the ISO speed also increases the shutter speed, noise occurs at high ISO speeds. There is therefore a limit to how high you can set the latter without compromising on image quality. Similarly, while using a tripod and remote shutter release keeps camera shake to a minimum, they are practical only for certain shooting scenes as prior setup is required.

IS, on the other hand, is able to suppress camera shake to a certain degree even for such scenes, increasing the range of scenes for which you can take handheld shots. Depending on the lens, the image stabilization effect may allow you to shoot at 2-5 shutter speed stops slower than without IS.

With IS function

Without IS function

Enables silent and high-speed focusing

USM (Ultrasonic Motor) was first put into practical application in the world by Canon. USM drives the lens by transforming ultrasonic vibrations into rotation energy. Not only does it consume little power, it has high efficiency, torque and response, enabling high-speed focusing with almost no operating sound emitted.

There are three types of USM . The round-shaped Ring USM is simple in structure, and is best suited for use in a circular lens tube. It is especially useful for driving lenses such as large-diameter or super telephoto ones due to its high torque, and it also allows for full-time manual focusing. Meanwhile, the Micro USM can be employed on many types of lenses because it is low in cost and not subject to restrictions on the lens diameter. In 2002, Canon contributed further to the downsizing of lenses with the development of the Micro USM II, which is about half the size and weight of the Micro USM . And in 2016, Canon introduced Nano USM, which not only realizes a high speed AF that rivals that achieved by Ring USM, but also is able to operate quietly and seamlessly. It’s not an exaggeration to describe Nano USM as the most ideal motor for normal-use lenses that are meant to handle both still photography and movie shooting.

Rung USM

Micor USM (Left), Micro USM II (Right)

Nano USM—The thin and tiny USM motor that achieves fast and smooth AF

Thin and compact, this USM, the latest to be developed by Canon, made its debut in 2016 on the EF-S18-135mm f/3.5-5.6mm IS USM. Computer chip-shaped (a factor that allows a more compact lens design) with power sources in the form of transducers, it connects directly to the AF actuating mechanism. It possesses a speediness and responsiveness that rivals that of the Ring USM, combined with a control that equals that of STM (lead screw-type). Its main benefit lies in enabling quiet, smooth AF operation in compactly-sized lenses such as standard zoom lenses. Also very suitable for movie-shooting, its high speed AF is able to handle scenes with subjects that are moving at a fast speed as well as those with unpredictable movements.

The Nano USM is shaped like a computer chip, and has 2 transducers in its center.

A cross section of the EF-S18-135mm f/3.5-5.6 IS USM, released in 2016 and the first lens to carry the Nano USM. Moving along the top of the slider, the Nano USM directly drives the main AF power source. As this motor not only allows for fast and smooth AF but is also small in size, it makes it possible for the lens that utilize it to be around the same size as lenses with STM.

Enables high-speed and precise AF

The inner focusing and rear focusing systems adjust the focus by moving only some of the lens elements inside the lens. Inner focusing refers to the type where the focusing lens group is located in front of the aperture diaphragm, while this group is positioned behind the aperture diaphragm in the case of the rear focusing type. As a smaller number of lens elements need to be moved, AF speed can be increased, and the size and weight of the entire lens can also be reduced. Besides, the front of the lens does not extend out during focusing, thus making it easy to achieve an integral lens structure, which in turn contributes to the enhanced robustness of the design. Yet another advantage of this system is that it enhances the operability of polarising filters as the filter mount does not rotate during focusing.

EF24-70mm f/2.8L USM Rear System

EF400mm f/5.6L USM Rear System

EF70-200mm f/2.8L IS III USM Inner System

Enables MF immediately after AF

Many of Canon’s EF lenses are equipped with “full-time manual focusing”, which allows manual focus right after auto focus is complete, just by turning the focusing ring and without the need to switch to MF mode. All Ring USM and STM lenses support full-time manual focusing.

Dust- and drip-proof for shooting under harsh conditions

Many of Canon’s L lenses come with a dust- and drip-proof structure, with rubber sealing applied to the interface area between the joint of the mount and the camera, as well as sealing at the movable parts such as the focusing ring and the lens extension tube. Rubber materials are also employed even at the smallest parts, such as the switch panel and the insertion slot of the drop-in filter compartments. The excellent dust- and drip-resistance enhances the usability of lenses under adverse weather, and helps to address the needs of professional photographers who are required to perform shoots under harsh shooting conditions. Fluorine-coated lenses allow dirt attached to the lens to be wiped off easily.

Expresses point light sources in beautiful round bokeh

The aperture is made up of several blades, the number of which determines the shape of the bokeh effect, such as in the case of illumination, light seeping through the leaves of a tree, and light reflection from the water surface. Light normally appears round when seen through our eyes, thus expressing light in a circular shape adds a more natural and soft touch to the resulting image. Almost all EF lenses employ a circular aperture diaphragm.