Magnification is usually expressed as a number with the letter X after it, which represents the magnification multiplier of a particular lens. For instance, a 3x magnification lens magnifies images by three times more than what the human eye can see under normal circumstances. This value is typically not reported with camera lenses, as convention is to report camera lenses by focal length. Focal length, measured in millimetres, refers to the distance between the front of a lens's front glass element and the point at which parallel lines will converge when shone through the camera lens.
Write down the maximum focal length of the lens for which you want to find the magnification. For instance, a 70-250 mm lens will have a maximum focal length of 250mm. A prime lens will have only one value listed, and this value should be considered the maximum focal length.
Multiply this value by 1.6 if you are attaching the lens to a digital camera. Most digital cameras have a 1.6x "crop factor", which means that the outside edges of a scene will be cropped because they do not physically fit on the sensor, making the effective focal length 1.6 times greater than it would be on a film camera. For a 250mm lens, the effective focal length would be 400 (250 times 1.6).
Divide the effective focal length by 100. An easier way to do this is to move the decimal point two spaces to the left. The effective focal length of a 250mm lens on a digital camera is 400mm, so divide 400 by 100 to arrive at four.
Multiply this value by two to find the magnification of the lens in terms of viewfinder magnification used in binoculars or telescopes. So a 250mm lens would have an 8x magnification (400 divided by 100 multiplied by two) on a digital camera.
Camera lenses are not typically expressed by their magnification factor, but this value is useful in determining how much magnification a lens has if you wish to compare it to binoculars, telescopes or magnifying glasses that use this value.
If your lens is advertised with a number followed by an X, this value probably refers to the zoom range, not the magnification factor. These two values refer to different things: zoom range is the distance between a lens's maximum focal length and minimum focal length, while magnification factor is the magnification between the lens's maximum focal length and the human eye.