Focal length
Measured in millimeters, this represents the distance between the optical center of a lens and its focal plane. The lower the focal length, the wider your angle of view; the higher the focal length, the narrower the angle of view.
The focal length for the three lens types generally breaks down as follows:
Standard lens: Standard camera lenses typically have a 50mm to 55mm focal length
Wide-angle lens: Wide-angle scamera lense have a 18mm to 35mm focal length. Fish-eye lenses range from 6mm to 16mm.
Telephoto lens: Telephoto camera lenses have a 80mm to 500mm focal length.
Zoom lens
Lenses have either fixed focal length lenses (35mm, 50mm) or zoom lenses, where focal length changes. For example, the Canon EF zoom lens offers a zoom from 75mm–300mm. Zoom allows you to get closer to your subject by magnifying the image; you move a ring to alter the focal length and zoom in or out.
Auto focus and manual focus
Auto focus (AF) lenses do the focusing for you automatically. Manual Focus (MF), on the other hand, allows you to manually focus the camera lens. The former obviously provides ease and simplicity, while the latter is good for focusing in low light and when you need to focus in very precise increments.
Some camera lenses offer both types of focus functionality. Auto focus can be turned off or on, letting you manually focus whenever you want.
Aperture
Latin for “opening,” aperture indicates how much light a camera lens lets through to the focal plane. The smaller the aperture number (f/2.8, for example), the more light a lens will let in, thus better controlling the brightness of the exposure. And the more light a lens allows through, the faster the shutter speed.
Most important in low-light, outdoor picture-taking situations, a lens’s aperture will usually increase along with the focal length of a zoom lens. Both aperture and focal length will usually appear on the barrel of the lens.
