Choosing a Lens: Page 2
Each camera manufacturer has their own lens mount and they aren’t compatible with one another. If you own a Canon, for example, you can’t use Nikon lenses, though you can use independent brands such as Sigma and Tamron – if you purchase them in the right mount.
If you’re migrating from a 35mm SLR, your lenses won’t provide the same field of view on a DSLR unless you have a ‘full-frame’ model such as Canon’s EOS 5D. For Nikon, Pentax, Fuji and Sony DSLRs, magnify the focal length by 1.5x to get the 35mm equivalent (eg a 100mm lens becomes 150mm); for Canon it’s 1.6x; Sigma is 1.7x; and the Four-Thirds mount, 2.0x.
You pay a premium for lenses with wider maximum apertures, but for many users they’re worth it. Wider apertures mean you can use faster, motion-stopping shutter speeds, and shoot handheld in available light with less camera shake. Zooms whose maximum apertures are fixed throughout the range (instead of getting smaller as you zoom) also cost more.
Most digital sensors are smaller than 35mm, which is why lenses designed for digital can be smaller than those designed for 35mm. Although pre-digital lenses can be used on digital SLRs they may not perform as well, due to their optical characteristics. Lenses designed solely for digital sensors, however, can’t be used on 35mm cameras (or digital SLRs with 35mm-sized sensors) because the image they produce isn’t big enough in diameter to cover the sensor/film, causing severe vignetting.
It’s impossible to hold a camera dead steady without support, but at the faster shutter speeds any shaking isn’t noticeable in pictures. As the shutter speed drops, though, our pictures become progressively less ‘crisp’ until they’re downright blurry. Optical Image Stabilisation (used by Canon) and Vibration Reduction (Nikon) technologies use a floating element inside the lens and a gyroscope to compensate for our body movement. It’s effective enough to give us two or three extra stops of extra shutter speeds to play with before shake becomes visible. Some cameras (e.g. Sony, Pentax, Samsung and Olympus) use a moving sensor instead, so IS/VR lenses aren’t necessary.
Buiilt-in Focus Motor
Some lenses incorporate a motor within the lens to drive the auto focusing, while others are powered by motors within the camera. Lenses with their own motors will generally focus more quickly than those without. Some cameras, such as the Nikon D40, don’t have a built-in focus motor so in these cases you can only use lenses that have their own. Canon brands its lens motors USM (Ultrasonic Motor), Sigma calls its HSM (Hypersonic Motor) and Nikon’s are called Sonic Wave.
Most lenses have letters as part of their names. These usually denote what technology they feature and how sophisticated they are. For example, manufacturers who incorporate image stabilisation or low-dispersion elements in their lenses usually state this as part of the lens’s name. Other information gathered from these letters may be the type of mount the lens uses, whether any aspherical elements feature and whether the lens sits in the company’s ‘professional’ range, which can include weatherproofing and sturdier constructions.