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p.1 #3 · Contax 645 80mm equivalent lenght for 35 mm? 50mm?  
When comparing formats with different aspect ratios, (645 = 1.35, 135 = 1.5), I prefer to use the horizontal angle of view (AOV.h). The alternative approach (and most common) is to use the diagonal AOV.d.
Based on AoV.h, the 645 80mm lens is equivalent to a 51mm focal length lens on 135 (35mm) format. Based on AoV.d, it's 50mm. Either way, they're all 'normal'.
The equations are simple trigonometry. You can grab an "equivalent focal length" calculator from my web site, http://www.jcolwell.ca/photography/calcinfo/index.htm
Of course, if you put an 80mm 645 lens on a 135 (35mm) or FF DSLR body, it produces the same AoV as does an 80mm 135 (35mm) lens on the same body.
P.S. the typical 645 frame size is 56mm x 41.5mm.
