Lenses have bits of text printed on the barrel or around the front element which convey a lot of important information about their properties. Be really careful when shopping for lenses. Two lenses may have very similar-looking names, but may actually be completely different one from the other.

Here are two examples. Note that all the technical terms used here are described later on in this FAQ.

CANON LENS EF 28-80mm 1:3.5-5.6. Ø58mm.

EF means that the lens is of the Canon EF type. EF lenses fit Canon EOS cameras and virtually no others.

28-80mm refers to the focal length of the lens. In this case there are two values since the lens is a zoom lens which can go from 28mm at its widest to 80mm at its longest. These numeric values in millimetres essentially indicate the coverage area of the lens.

1:3.5-5.6 refers to the widest aperture of which the lens is capable. The 1: is there since f/stops are, technically speaking, ratios. Since the lens is a zoom there are two aperture values - f/3.5 and f/5.6. This particular lens is an inexpensive amateur lens which can be opened up to f/3.5 at the widest end (28mm) but only f/5.6 at the longest end (80mm). This means it’s a fairly slow lens - it can’t let in much light, even when its aperture diaphragm is fully open.

Note that the autofocus motor type is not indicated, which means that the lens uses either an AFD (arc-form drive) or micromotor drive. Both types are slow and noisy compared to ultrasonic (USM) motors.

Ø58mm refers to the filter ring diameter. In other words, screw-on filters 58mm in diameter will fit this lens.

CANON LENS EF 200mm 1:2.8L II USM. Ø72mm.

EF means that the lens is a Canon EF lens for EOS cameras.

200mm refers to the focal length. Since only one value is indicated, this lens is a prime lens (ie: a lens with a fixed focal length).

1:2.8 refers to the widest aperture of which the lens is capable. f/2.8 is reasonably wide, especially for a 200mm telephoto lens, and so this is considered a fast lens.

L indicates that the lens is an L or “luxury” series Canon lens. Such lenses are generally the best that Canon manufacture. They are marked with a characteristic red stripe around the end of the lens barrel.

II indicates that this is the second version of this lens with these particular numerical specifications that Canon have made.

USM refers to the autofocus motor type used by the lens. In this case it uses an ultrasonic motor - see below. Non-L USM lenses are marked with a characteristic gold stripe around the end of the lens barrel. L lenses usually have USM motors, but have a red stripe instead (ie: the red stripe takes priority and you don’t see lenses with two stripes).

Ø72mm refers to the filter ring diameter. In other words, screw-on filters 72mm in diameter will fit this lens, making it a fairly large lens.