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The numbers giving the aperture size, sometimes called stops, are smaller as the aperture size increases. This may seem strange, but there is a reason. To understand this we first need to understand focal length, which is the length from where light is first ‘bent’, i.e. the front of the lens, to where the light converges, i.e. the film plane or sensor plane. The aperture number is a ratio of the aperture diameter to the focal length. Thus with the aperture wide open, the focal length may be only 2.8 times the aperture diameter. With the aperture at its smallest size, the focal length may be 22 times the aperture diameter. These limits vary from lens to lens.
A lens is designated by its focal length and maximum aperture. Thus a lens designated as 55mm f2.8 is one with a 55 millimeter focal length which is 2.8 times longer than the maximum aperture diameter. A zoom lens has two focal length designations – the minimum and maximum. Thus a zoom lens may be designated 30-120mm which would be a 4 to 1 zoom. Note that the actual length of the lens does not increase from 30 to 120 millimeters. The front of the lens moves a minimal distance and elements inside the lens are also adjusted to give the desired focal length. When the focal length is increased and the actual size of the aperture is unchanged, the ratio of the focal length to the aperture is changed. Thus a lens with a maximum aperture of f2.8 at 30mm may have a maximum of f 5.6 at 120mm. In that case a zoom lens would be designated 30-120mm f2.8-5.6.
