Lenses are characterized by two fundamental specifications, f-stop and focal length. The f-stop is a number which specifies the aperture versus focal length ratio of the lens. It is also known as the lens ‘speed’ as it indicates how much light can pass through the lens. The lower the number, the greater the light throughput (and the smaller the depth of field/focus).
The focal length can be used to calculate the ratio between image size and throw distance. To do this, the actual size of the image-producing element (e.g. the LCD panel) has to be known and the relationship is as follows.
Most projectors come standard with a zoom lens, i.e. a lens with a variable focal length. Specification can be the zoom range in the focal length, e.g. 60-105mm, or expressed as a zoom ratio, e.g. 1.3x, which indicates the ratio between the longest and shortest focal length.
There is no clear standard for specifying lenses on projectors. Some manufacturers specify throw ratio, some focal length, some zoom ratio, and some use a chart showing minimum and maximum throw distances in relation to image sizes. It can be confusing – so read the data with care!