Lenses
The fundamental difference between telephoto lenses and wide-angle lenses is use and perspective. Each lens is used for vastly different reasons and subjects, and the technical measurements help you tell the difference. Professional level telephoto and wide-angle lenses are measured in millimeters, which relates to how close the glass in the lens is to the digital sensor in your camera that makes the picture.
Telephoto lenses
A telephoto lens
A telephoto lens is designed to "zoom in" on your subject, make their face fill the frame and get a nice close up. This magnification of the image is the key characteristic of a telephoto lens. A lens is considered a telephoto lens when it measures 85 millimeters or more and can go all the way to 400 millimeters and beyond.
Wide-Angle Lenses
A wide angle lens
A lens is considered wide angle when the measurements are 35 millimeters or lower. At 28 millimeters and below, you will be able to see a lot more through the eye of the camera, and some lines may begin to distort. A lens that measures 20 millimeters and below is considered a fisheye. Parts of the image will begin to curve and distort, most noticeable in straight lines like cracks in a sidewalk or the sides of buildings.
Quality and optics
Good telephoto and wide-angle lenses come at a high price because of durability, quality of glass and general use. The glass must be of high quality to avoid unnecessary distortion or lens flares, especially in telephoto lenses, whose characteristics are clean images. The distortion of straight lines is expected in wide-angle lenses, but the optics are crafted carefully to avoid lens flares and other lens anomalies.
Uses
Telephoto lenses are typically used when the photographer needs to be close to a subject but can't be for various reasons. Photojournalism, nature photography and wedding photography are good examples of when a telephoto lens is required to get the shot. Wide-angle lenses are used when the photographer needs to capture as much of the scene as possible, such as in photojournalism and sports photography.
Overall
Many photographers have a variety of telephoto and wide-angle lenses to be prepared to get the shot in any situation. It all comes down to what kind of photography you wish to do, how prepared you wish to be and how comfortable and confident you feel with each lens.
Tags: telephoto lens, telephoto wide-angle, telephoto wide-angle lenses, wide-angle lenses, lens considered, lens flares, lens lens