With the motor now in the lens the whole focus operation could be made more efficient, more accurate and quieter. And in addition, the motors could be individually designed for each lens. The challenge then became how to design a motor suitable to the purpose.

The EF  (Electro Focus) lens mount made its appearance in 1987. Mechanically the mount is a bayonet style but all communications between lens and camera are electric. There are no levers or plungers on the base of the lens but rather a series of electrical contacts.

At the time it was introduced the EF mount had the largest internal diameter (54mm) of all mounts on SLR cameras. It had a Flange Distance of 44.0 millimeters as opposed to 42.0 millimeters on the FD mount. As a result the earlier FD system is not compatible with the EF mount. Adapters with focus to infinity are impossible without added lenses which inevitably reduces image quality substantially.

If you compare the listing of “FD” lenses with the EOS lenses you will see a strong movement to zoom lenses over primes. With advances in technology, new glasses, fluorite, aspherical lenses, superior coatings, and computers to do the required calculations, it has been possible to bring the quality yielded by zoom lenses up to the level of primes, or almost to that level.

Notice another thing: the “FD” zooms that do exist have almost constant aperture regardless of f/stop. The f/stop is constant throughout the range and maximum f/stop does not vary with focal length. Constant maximum aperture was required in the “FD” lenses because exposure calculations could not take into account an f/stop that varied with focal length. Remember that maximum aperture is communicated to the camera body by the height of a pin on the rear of the lens. You could set up the camera for a single lens but not for interchangeable lenses. Maximum aperture had to be fixed regardless of focal length so the camera body could do the required exposure calculations.

Because the EOS lenses are controlled electronically it was possible to accommodate varying f/stops from lens to lens and focal length to focal length. So the lens designers were relieved of worrying about this parameter. The f/stop at each focal length could wander and the on board CPU would compensate. This was undoubtedly one of the factors that led to the rising quality of zoom lens images.

Eventually the EF line of lenses contained over sixty from 8 to 1200mm. Throughout our discussion of lenses I have used the names assigned to lenses in the lens listings in the Canon Online Museum. Some definitions are in order to understand the lens designations used by Canon.

Armed with these definitions and abbreviations the lens names should make perfect sense.

Ultrasonic Motor (USM)

The USM appeared in Canon lenses with the EF 300mm f/2.8 L USM of 1987. Canon was the first camera maker to successfully make use commercially of this technology. Basically these motors take advantage of ultrasonic vibrations induced in the motors parts which result in rotational movement. They do not rely on magnetism as in conventional electric motors. One day I will write about how this actually works.

There are two types of USM’s. The first is the ring type which really does not look like a motor at all. It consists of two rings that encircle the lens barrel and work against each other to move the focus elements.

The second is the micro motor which looks more like a conventional motor but uses the same high frequency vibrations to generate rotational torque. Both types of motor allow for full-time manual (FT-M) focusing without switching out of AF mode. However, to do this with a micro motor requires extra components and so most micro-USM lenses do not offer FT-M focusing.

The micro motor is usually found on the low end less expensive lenses due to its ease and lower cost of manufacture. There is little advantage to the micro motor USM over the conventional magnetic motor but they are cheaper to make than the ring type.

StepThere are two kinds of USM micro motors. The first version, the earliest one, is larger. The second, USM II, is half the size making them easier to employ in modern AF lenses.

Stepping Motor (STM)

Canon has also used a “stepping motor” in some of the latest lenses which is a more precise and quieter form of traditional motor that can be controlled with great accuracy using pulses of current. They are very good a producing smooth precise movement in increments which suits the requirements of AF systems.

Image Stabilization (IS)

The image stabilization technology detects handheld motion and optically corrects it. It only corrects handheld motion and not motion of the subject. First used in the 75-300mm lens of 1995 it provided about 2 f-stops of stability but was much slower than modern lenses. The capability of these lenses has been slowing improving over the years and is much more effective now over the early versions.

Diffractive Optics (DO)

Lenses with a green ring on the front of the lens barrel use Diffractive Optics (DO) elements which are special lens elements. DO lenses are usually smaller and lighter and are better at handling chromatic aberration, compared to conventional lenses of similar focal length and aperture value. They are more expensive to make. Only the EF 400 mm f/4 DO IS USM, its updated Mark II version, and the EF 70–300 mm f/4.5–5.6 DO IS USM contain DO elements. DO lenses have a green ring on the barrel.

L-series Lenses

Top range Canon EF lenses are designated “L-series”, or “Luxury” lenses. L series lenses are compatible with the EF mounts and, as they are aimed at the high-end user, also include environmental or weather sealing and a constant maximum aperture. All L lenses are supplied complete with a hood and pouch, which are not included with non-L lenses. Distinctive visual cues include a red ring around the lens and an off-white color on longer-focal-length models. The latter, as well as looking distinctive, also helps to reflect light and reduce heat absorption.

Tilt Shift Lenses (TS-E)

These are specialty lenses that tilt the plane of focus and can also shift the lens to correct for perspective. These are very specialized lenses that most people will not come across.

Power Zoom Lenses (PZ)

Canon did produce at least one zoom lens whose focal length was changed by the push of a button. It was a development that apparently died a quick death.

I think we got everything!  You may also see a “II” or a “III” after these designations, which just denotes “second” or “third” generation of a lens.  With this knowledge you can now read the name of a lens and know in some detail how the lens is constructed .

All L lenses include at least one fluorite or ultra-low-dispersion glass element, and/or certain types of aspherical elements. Other mechanical characteristics of L lenses (but not exclusive to them or present in all L lenses) are the use of an ultrasonic motor (USM) for focusing (particularly in recent years) and image stabilization (IS).

The Prime Lenses

The Zoom Lenses