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 characterized 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.

Aspherical lens elements capable of converging light rays to a point

Traditional spherical lens elements are unable to ensure that light rays entering from the peripheral areas and those from the centre converge at the same point. Generally, it is difficult for spherical lens elements alone to correct spherical aberration on large-diameter lenses as well as distortion on wide-angle lenses. In contrast, aspherical lens elements have a lens surfacing with varying curvature, designed to make the light rays converge at the same. The manufacturing of aspherical lens elements requires a high level of processing technology and precise measuring techniques in order to produce a lens curvature for obtaining the ideal imaging point. Canon embarked on developing the technology for manufacturing aspherical lens elements at an early stage, and succeeded in 1971 in commercialising the world’s first lens with built-in ground and polished glass aspherical lens elements. Today, Canon has been able to contribute to enhancing the image quality of many EF lenses through the mass production of glass-mould and replica aspherical lens elements.

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

Silent and smooth AF

The STM (Stepping Motor) is an AF drive motor that is able to control the rotational operation of the motor using the fluctuation of pulse signals. Each electrical pulse signal can rotate the stepping motor by one step, and start-stop response is superb. The simple mechanical structure makes it easy to adopt a compact lens design. Also, the STM helps to reduce operating sound, and is thus ideal for video shooting. Canon developed two different types of STM according to the lens characteristics. The “STM + lead screw” unit adopts a lead-screw-type drive that is capable of generating a large torque, and enables silent and smooth AF drive on zoom lenses. The “STM + gear” unit allows for compact lens designs, and is optimised as a drive motor for compact lenses such as the pancake lens.

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 II 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.

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.