Camera Labs lens buying guide
Focusing is obviously a crucial aspect of any lens, and some models do it better than others. While the auto-focusing decisions are normally made within the DSLR, the actual focusing itself is performed by the lens, and there’s several things to look out for.
Some lenses feature built-in focusing motors, whereas others rely on a motor that’s built into the DSLR. In most situations you won’t need to worry too much about this, but there are a couple of important exceptions.
To save weight and money, Nikon removed the internal focusing motor from its budget D40 and D40x DSLRs, so they will only auto-focus with newer lenses that have their own focusing motors. In Nikon’s own range, you’ll need a lens with AF-S to auto-focus with the D40 or D40x. If it doesn’t have AF-S then you’ll be manually focusing with a D40 or D40x. If you’re buying a Sigma or Tamron lens, look out for models specifically compatible with the D40 and D40x.
All of Canon’s current lenses have built-in focusing motors, but again some are better than others. Canon lenses with USM in their title have special ultrasonic motors which are quicker and much quieter than non-USM models. So if you’re buying a Canon lens and want quick and quiet focusing, buy a model with USM.
Nikon’s equivalent technology is called Silent Wave Motor, or SWM for short. This is the S in AF-S, so for quick and quiet focusing on a Nikkor lens, choose an AF-S model. The equivalent technology from Sony, Pentax, Olympus and Sigma is called SSM, SDM, SWD and HSM respectively; note to fully exploit SDM (DA*) lenses from Pentax though, you’ll need a compatible body like the K10D.
It’s also worth mentioning internal focusing which as its name suggests takes place within the lens. This means the end section of the lens barrel doesn’t rotate while focusing, which is important for users of polarising filters.
Auto-focusing has become so dominant that good manual focusing facilities are frequently neglected on cheaper lenses. Some budget models feature little more than a tiny ring on the end for manual focusing. If manual focusing is important to you, look for a model with a decent manual focusing ring.
While on the subject of focusing, so-called macro lenses are optimised for close-up photography of subjects like flowers and insects. Typical macro lenses have focal lengths between 50 and 100mm and can also be used for other subjects, although they are optimised to perform best at close range. So if you’re into close-up photography, a dedicated macro lens is the way forward.
Some lenses feature anti-shake facilities which allow you to typically handhold at shutter speeds three to four times slower than normal. This won’t stop a moving subject from blurring, but it can greatly reduce the effect of camera shake.
Lens-based anti-shake systems all work in the same way by detecting wobbles and adjusting a special optical element inside the lens to counteract them in real-time. The benefit of fitting it inside the lens is you’ll see the stabilising effect through the viewfinder, which can be very reassuring, especially at longer focal lengths.
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While stabilisation is most commonly employed on telephoto lenses, it can be equally useful on standard or even wide angle focal lengths. Regardless of the focal length, stabilisation will still let you handhold at shutter speeds three to four times slower than normal, so for wide angle, that gives you the chance to handhold some seriously slow exposures. Ideal if you want to blur waterfalls and rivers – see our DSLR Tips workshop.
Stabilisation systems can however get confused in certain circumstances. If you’re panning the camera to follow something (see our DSLR Tips blurring action workshop), the stabilisation could mistake it for a wobble and try and counteract the motion.
Some anti-shake lenses offer a panning mode which ignores horizontal motion and only stabilises vertically. Some of the latest models can even detect this motion and switch their mode accordingly. Older, or more basic anti-shake lenses won’t work with panning though and the feature should be temporarily switched off. Likewise if you’re using a tripod, you should switch the stabilisation off or the system could actually introduce wobbling.
Each manufacturer has a different name for anti-shake. Canon calls it Image Stabilisation or IS for short. Nikon calls it Vibration Reduction, or VR for short. Sigma calls it Optical Stabilisation, or OS for short. So if you want a lens with anti-shake, these are the letters you should be looking for in its name.
One final note: each DSLR manufacturer uses a different lens mount, so if you have a Canon body, you’ll need a lens with a Canon mount and so on. The major manufacturers of DSLR bodies also only produce lenses for their own systems, so Canon lenses will only work on Canon bodies and Nikkor lenses on Nikon bodies. Some adapters may be available, but to support all the features, you should stick by this rule.
The exception are third party manufacturers like Sigma and Tamron which produce different versions of their lenses for different mounts. So a Sigma lens for example may be available in Canon, Nikon and Pentax lens mounts.
Now you’re equipped with the background knowledge, you’re ready to start shopping for a new lens.
To see full reviews of a number of popular models, please see our lens reviews.