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Focal Length

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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Since 2006, Diglloyd Advanced Photography is the authoritative review and reference for DSLRs and their lenses: Canon and Nikon primarily, but also Pentax and some medium format. Reviews of key DSLRs are included, but the primary focus is on lens performance. Also included is a wealth of technique and workflow approaches.

DAP includes thousands of pages of exclusive coverage and high resolution Retina-grade examples detailing Canon and Nikon DSLR bodies and their lenses, with in-depth analysis of sharpness and contrast, color, bokeh, vignetting, distortion, MTF, and flare, behavioral and practical usage.

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