The most popular lens on a consumer digital SLR is 18mm – 55mm. This is often referred to as a kit lens.
The sensor on the consumer digital SLR cameras is an APS -size sensor. These are smaller than a 35mm film frame. The above lens on this type of camera is has a crop factor, focal-length magnification, or focal-length multiplier. So what do all these terms mean?
A lens has an angle of view, e.g. 24mm lens has a vertical angle of view of 53 degrees and a horizontal angle of view of 73 degrees. These figures apply to camera held in the landscape or horizontal format. A 50mm lens has a 38 degree vertical view and a 40 degree horizontal view. A 210mm lens has a 6 degree vertical view and a 10 degree horizontal view. As the focal length of a lens increases the angle of view decreases.
On an digital SLR with an APS-size sensor 50mm lens captures the same angle of view as a 75mm lens. That is were the multiplier or crop factor comes in. APS-size sensor digital SLR cameras have a crop factor of 1.5.
Wide angle lenses have barrel distortion. This means vertical lines tend to curve outward near the edge of the image frame. On an APS-size sensor digital camera, while a 33mm lens is the equivalent of a 50mm lens still has the barrel distortion of a 33mm wide angle lens.
The crop factor has advantages and disadvantages. Sports or wildlife enthusiasts will love the extra pull of a “longer” lens. Landscape and architecture photographers may not find the 33mm lens wide enough. They will have to move to an 18mm or super wide angle 10mm lens.
If you are from 35mm film background and find all this confusing, then you may want to move up to a full-frame digital SLR camera body, e.g. Canon 5D MKII. All your lenses will do exactly what you expect them to do.
Some camera makers make digital only lenses. These lenses are made to cover the smaller sized APS sensor. Since all lenses create a circular image, the image must cover completely cover the rectangular sensor. The digital only lenses tend to be lighter, smaller and less expensive, but only work on an APS-size sensor camera. They will not work on a full-frame sensor camera.
Some camera makers put the Image Stabilization (IS) in the lens, e.g. Canon; while some put it in the camera body, e.g. Nikon. They also refer to this feature by different terms. Canon – IS; Nikon – VR; Sigma and Panasonic – OS. IS in the lens makes the lens more expensive.
Auto focus uses virtually silent small electric motors to focus the subject. Each maker has their own term. Canon – USM; Nikon – SWM; Sigma – SWD; Sony – SSM.
Apochromatic glass is an expensive special glass used in one or more lens elements to reduce chromatic aberration. This is an optical defect that reduces an image’s sharpness and causes colour fringing at the edge of high-contrast pixels in an picture. This is especially true with telephoto lenses. Therefore, when considering the purchase of a long zoom lens, e.g. 70mm – 200mm, 100mm – 300mm, if may be to your advantage to invest in a top of the line lens made with APO glass. While fringing can be reduced during image processing, why have to bother with it in the first place?
