Canon single lens reflex or SLR lenses are classified into four types based on their focal length: ultra wide angle, wide angle, standard and telephoto. The focal length defines the lens’ viewing angle, and this determines what kind of photograph is produced.
Focal length is the distance between the lens’ optical center and the camera’s sensor. It is measured in millimeters, with a smaller number indicating a wider viewing angle. When shorter focal length SLR lenses such as 15 millimeters, 20 millimeters or 35 millimeters are used, the subject fills the frame, but background objects seem farther away. These wide-angle or standard lenses are best suited for landscape and portrait photography, respectively. However, wide-angle lenses cause some distortion at the edge of the frame.
Photographers use telephoto lenses with focal lengths between 100 and 300 millimeters when they have to stand far away from the subject but want to get clear close-up photos. These lenses are ideal for photographing sports events, concerts and wildlife.
The focal length of a lens is either variable or fixed. Variable focal length or zoom lenses are useful for recomposing images without changing the lens. Fixed focal length or prime lenses don’t zoom, but they are fast and give sharp images even in low-light conditions.