Common Terms/Numbers Found on Camera Lenses
Focal Length. For a prime lens, this will be a number mentioned in millimetres. A zoom lens contains two numbers indicating the lowest and highest possible focal lengths for the lens.
With zoom lenses, you can also see a series of numbers next to a slider or a rotating grip, with a pointer pointing to one of the numbers. This indicates the current focal length of the lens.
Examples: 50mm, 100mm, 17-40mm, 70-300mm. The first two numbers are for prime lenses with focal length 50mm and 100mm. The third one indicates a zoom lens whose focal length can vary from 17mm to 40mm. Similarly, the last one indicates a lens with focal length varying from 70mm to 300mm.
Focusing distance. This is a series of numbers mentioned in feet, usually starting from a few feet and going to infinity. This indicates the distance at which the lens is currently focused.
Minimum Focusing Distance. Most lenses indicate the minimum distance at which the lens can focus properly. This is usually mentioned in feet. This can be a single number for a prime lens and a range for zoom lenses. If the lens has macro capability, it is indicated by the picture of a flower.
Aperture. This is a set of numbers mentioned as a ratio that indicate the widest possible aperture for a lens. In case of a zoom lens, you can see one or two values, while a prime lens will have a single value.
Example. 1:2.8, 1:4-5.6. In the first example, the numbers indicate that the widest aperture possible for the lens is f/2.8, and this value remains constant at all focal lengths. In the second example, the value varies between f/4 and f/5.6 depending on the focal length.
Filter Diameter. This is a number at the far end of the lens mentioned in millimetres, usually preceded with the symbol of a line across a circle. This indicates the diameter of the lens thread where the filters are fitted.
AF/MF. This is a switch that can toggle between the two values. Setting the switch to AF turns auto-focus on. MF indicates manual focus.