nominal focal length

nominal focal length

An approximate value of the focal length, rounded off to some standard figure, used for the classification of lenses, mirrors, or cameras.
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3 Di II ([pounds sterling]430) features a nominal focal length of 16mm at the widest setting (equivalent to 24mm when mounted on a cropped sensor DSLR), and extends out to 300mm.
For example, a 32 wavelength wide lens aperture would have a nominal focal length of 20 [Lambda], so a normalized path length error of 0.
This has tremendous advantages for longer lenses: a telephoto lens with a nominal focal length of 200mm in the 35mm format has a working focal length of 320mm when attached to a digital SLR.
By measuring the difference between these foci, expressed as a percentage of the nominal focal length, we can determine the amount of longitudinal spherical aberration.