Focal Length
The first number used to describe a lens is its focal length; in combination with the camera’s sensor size, this defines the angle of view covered by the lens, with smaller numbers indicating a wider angle. Zoom lenses are named using two numbers which indicate the extremes of the range, for example 18-55mm for a typical kit zoom lens. Fixed focal length lenses which don’t zoom (also widely known as ‘primes’) just have a single number (e.g. 50mm).
Image Stabilization
Image stabilization has become widespread across camera systems over the past few years, but the various manufacturers implement it in different ways. Pentax and Olympus incorporate it into the camera body, whereas Canon, Fujifilm, Nikon, Panasonic and Samsung use systems built into the lens. Sony (slightly confusingly) uses in-body ‘Super Steady Shot’ for its Alpha SLT cameras, but in-lens ‘Optical Steady Shot’ for its Nex system cameras. Image stabilization is especially useful with telephoto lenses, so is worth bearing in mind when comparing the available options.
Aperture
The aperture of a lens is the second major parameter used in its specification, and describes how much light it is capable of gathering. Apertures can be expressed in several different ways, with F4, f/4, 1:4 all meaning the same thing. A smaller number means the lens has a larger maximum aperture and therefore gathers more light; an F2.8 lens collects twice as much light as an F4, for example.
A lens with a larger maximum aperture allows you to shoot in lower light, and (for example) take pictures indoors without using flash. Larger apertures also give decreased depth of field (i.e. how much of the picture in front of and behind the focus point appears sharp), which is an important aspect of creative photography.
Format Coverage
Most affordable SLRs and mirrorless cameras use APS-C sensors, which are approximately 24mm x 16mm in size, or less than half the size of the old 35mm film negative (Nikon calls these cameras ‘DX format’). However high end Canon, Nikon and Sony cameras sport so-called ‘full frame’ sensors, which means purely that they are about the same size as a 35mm negative, i.e. 24mm x 36mm (Canon’s older high speed professional cameras used an intermediate sensor size, called APS-H, which provided a 1.3X crop). Panasonic and Olympus, meanwhile, use the slightly smaller Four Thirds sensor format in their interchangeable lens cameras.
All of the major manufacturers (aside of course from Olympus and Panasonic) now make a range of lenses specifically optimized for APS-C cameras, and these generally provide the best choice for general-purpose and wide angle zooms in particular. Lenses designed for full frame will also work just fine on APS-C cameras. However APS-C lenses won’t work properly on full-frame cameras, and in the case of Canon, it’s physically impossible to attach an APS-C-optimised EF-S lens to a full-frame camera. This is something worth bearing in mind if you are thinking of upgrading to a full frame system in the near future.
Lens Mounts
Each camera maker uses its own proprietary lens mount, meaning that lenses can’t be swapped across brands; a Canon lens won’t fit on a Nikon body, for example, and you’ll cause damage to lens and camera if you try. There are a couple of exceptions – Olympus and Panasonic both use the Four Thirds mount for DSLRs, and the Micro Four Thirds mount for their mirrorless interchangeable lens compacts (ILCs). Samsung’s now-obsolescent SLRs were essentially re-badged Pentax KAF-mount models, however the company is now concentrating on its NX ILC series.
A number of third party manufacturers, most notably Sigma, Tamron and Tokina, also make lenses in a number of different mounts to fit the multiple camera brands. The table below lists the currently available lens mounts.
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