The key to all these lens names is that they have the character E in the first few characters and it signifies the lens being designed for EOS.

• EF – is for Electro Focus
• EF-S – is for Electro Focus with Short back focal design
• MP-E – is for Macro Photography for EOS
• TS-E – is for Tilt Shift for EOS

The F character in the lens name indicates that the lens has a focus motor fitted, it can be automatically focussed by the EOS camera. If the F character is missing then the lens has no focus motor and must be focussed manually.

After the first few letters there’s a number or combination of numbers, these give the focal length or range of focal lengths of the lens. Lenses with variable focal length ranges are zoom lenses. Changing the focal length changes the angle of view making subjects appear further away or closer than they are in reality.

Since human’s normal vision has a similar angle of view to a 43mm lens then 50mm became a popular standard focal length, and in time zoom lenses that covered the 50mm focal length were often termed “standard zoom”. With the advent of digital EOS cameras with smaller APS-C sensors the angle of view changes. The effect is similar to multiplying the actual focal length by 1.6x. So the 50mm standard lens on an APS-C sensor camera shows the angle of view of an 80mm lens on a full-frame camera.

Shorter focal lengths indicate a lens has a wider angle of view, and conversely longer focal lengths tell of narrower angle of view and the appearance of bring distant objects closer to the camera.

The set of numbers prefixed by f/ describes the maximum aperture of the lens. Smaller numbers mean a bigger diameter aperture that lets in more light. Some zoom lenses change their maximum aperture as the lens focal length is changed. Seeing a lens with an f/4-5.6 aperture described means it will have a f/4 aperture at the shortest focal length and an f/5.6 aperture at the longest. It’s often interesting to know at which focal length setting the aperture will change.

An EF-S 10-22mm f/3.5-4.5 USM for example starts at f/3.5 at the 10mm focal length then the aperture drops to f/4 at 12mm and to f/4.5 around 18mm. Professional photographers are usually keen to have the same maximum aperture throughout the zoom range.

Canon’s premium professional lenses are the L-series. L-series lenses include the L designation written next to the aperture value.

L-series lenses also have a red coloured band around the front too. L was chosen to indicate luxury, and L-series lenses generally have the best optical quality, more durable construction, and in many cases are resistant to dust and water ingress. L-series lenses also employ Canon’s fluorite glass or ultra-low dispersion glass elements and/or certain types of aspherical shaped elements.

These letters designate either key lens technologies or specialist use.

• Macro is reserved for lenses that provide the posibility of capturing a subject at 1:1 magnification. Size of the subject is the same on the image sensor.
• IS is used to designate that the lens has an optical Image Stabilizer that can counteract the effects of camera shake. Lenses with IS are often preferred  choices and IS is available in many lenses in the current Canon range.
• USM and STM designate the kind of focus motor that is used to drive the focus assembly in the lens.  UltraSonic Motor is a fast, silent motor that starts moving quickly and stops without overshoot. It is favoured for higher performance lenses. In 2012 Canon introduced the STepper Motor technology to some lenses that allow them to combine with specific AF sensors configurations to enable continuous tracking focus when shooting movies. If the USM and STM letters are not present and the lens is an autofocus lens then it uses a simple DC motor that is not quite so good.
• DO is Canon’s abbreviation for Diffractive Optics, a special type of lens element within the lens itself bends light using diffraction. The result is a lens that can be made substantially shorter than one using conventional glass elements. Currently there are two DO lenses; EF 400mm f/4 DO IS USM and EF 70-300mm f/4.5-5.6 DO IS USM. DO lenses can be easily identified due to the green ring around the front of the lens.

The Roman numerals indicate that this is the second, third or later version of the lens. The numerals are used to differentiate lenses that have been updated in performance, construction, size or weight but not so that the designation letters described above would need to be changed.