Canon Lens Buying Tips

You're taking great shots with your new Canon DSLR. Landscapes, buildings, portraits of your family and friends: The camera rarely leaves your eye. But, you're feeling a little limited, hemmed in with the kit lens that came with the camera body. After a quick look at the Canon lenses online you find yourself lost in bizarre nomenclature.

Canon EF 70-300 f/4-5.6 IS USM
Canon EF-S 17-55 f/2.8 IS USM
Canon EF 50 f/1.4 USM
Canon EF 24-105 f/4L IS USM

What? What does that mean? I'm not a professional, only a studious shutterbug. What's EF and EF-S? What is IS and USM? Fear not, Canon lens nomenclature is not as hard as it seems. Let's break it down.

EF
The SLRs in Canon's EOS line can accept EF or EF-S lens. The letters identify the type of mount on the lens. EF stands for Electro-Focus: An autofocus system that uses only electronic contacts between the camera and lens, not gears or levers. The EF lens and its bayonet-style mount were introduced by Canon in 1987, and replaced the long serving FD mount.

Simple, just a name. If you have a Canon EOS camera, you use an EF lens.

But, what about EF-S?
Not all Canon DSLRs are created equal. There are two main categories of DSLRs: Full frame and APS sensors. Great, more terms. There's not a lot here you have to know, unless you really want to. Only two of Canon's DSLRs have the larger full frame sensor: The 1D and 5D models. These top-of-the-line camera bodies will only accept EF lenses.

EF-S lenses are made for Canon's other bodies that lack a full frame sensor. The S stands for short back focus. The back of an EF-S lens projects further into the camera body, is closer to the sensor, and provides for better wide-angle shots.

You can use an EF or EF-S lens on APS sensor Canon bodies, but you cannot use an EF-S lens on a full frame body (EOS 1D or 5D). Canon has made it impossible to mount the wrong lens on the wrong body. Because an EF-S lens projects further into the body, using one on a full-frame sensor camera would result in damage to both the lens and camera.

70-300? Is that speed? Like a race car?
No. Terms like 17-55, 24-105 and 70-200 represent the zoom factor of a lens. The smaller the number, the wider the angle. A 17-40, for example, is a lens that zooms from 17 mm to 40 mm. A 70-200 zooms from 70 mm to 200 mm, the higher numbers meaning it's a telephoto zoom capable of getting in tight.

But there's a single number, like 50 or 85...
A single number means this lens is a prime lens, or fixed. A number of 50 would mean the lens is a 50 mm prime lens. These lenses don't zoom and the optics are generally higher quality than zoom lens.

f/2.8? f/4-5.6? Those sound dirty.
The f in this section of the lens description is not vulgarity. The f stands for f-stop, or the maximum diameter of the pupil that admits light into the camera. This f number is also a measure of how fast a lens is. Lower is always faster. A lens with f/2.8 can let in a lot of light, and allow you to use very fast shutter speeds.

F/4-5.6 is a notation used with zoom lens. Let's look again at our example above.

Canon EF 70-300 f/4-5.6 IS USM

The f numbers follow the zoom. Yes, there is a reason. In this example at 70 mm the maximum f-stop would be 4. At the other end, 200 mm, it would be 5.6. Expensive zoom lens will have only one number, f/4 or f/2.8. This means the value does not change when the zoom is used.

Alphabet soup
After the zoom and f-stop numbers in a lens description, you will see a veritable alphabet soup of acronyms. They aren't hard to understand.

L
Sometimes you will see an L rating after an f-stop: f/2.8L. The letter L denotes Canon's professional L-series line of lenses. In camera-speak, L has taken on the unofficial moniker of "luxury," because L-series lenses are expensive. These lenses are built with weather and dust seals, and razor-sharp optics. L-series lenses can be visually identified by the red ring around the front of the lens.

USM
Simple: Ultrasonic motor. This means the lens uses a special motor for autofocus. USM is fast and quiet. Just to make things difficult, all Ultrasonic motors are not created equal. More expensive lenses use ring-type USM. The focus ring on the front of these lenses can be used to manual focus an image without switching out of autofocus mode. Micromotor USM is used on less expensive lenses and doesn't offer this capability. Canon lenses with USM can be identified by a gold ring around the barrel, or have the word Ultrasonic printed in red on the lens.

IS
Welcome to the cool realm of Image Stabilizers. An IS lens is special: It's designed to help minimize camera shake and vibrations from your hands when using slow shutter speeds. It offers shutterbugs the capabilities of using slower shutter speeds without having to resort of a tripod. IS lens will have Image Stabilizer written on the lens in silver.

DO
Diffractive Optics lenses are not something seen by the average photographer. They are expensive lenses with superior optics, built to be smaller and lighter. Very few are made. They can be identified by the green band around the barrel.

Ready?
Welcome to the club! You can now read the specs of a Canon lens and understand what the different terms mean.