Manufacturers indicate focal length for a lens focused at infinity. Even so, it may not match the indicated focal length: manufacturers are fond of rounding the focal length to familiar values: no one markets a 27mm or 29mm wide angle! A 28mm lens may actually be 27.5mm, 28.1mm, 28.5mm or something else. Schneider states in their online specifications that the effective focal length of the PC Super Angulon is 29.2mm.
Lenses may also change focal length as they are focused closer depending on the type of focusing the lens optics employ. diglloyd has three different lenses with 200mm markings, yet the actual focal length differs by more than 5% at some focusing distances, an unusually wide variance. The Schneider 28mm/f2.8 PC Super Angulon incorporates a floating element to enhance performance at close-focusing distances; this different approach may help account for its focal-length shift (see table further on).
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