In the olden days of 35mm film it was correct to accept that an 85mm (+-) focal length was the optimum for portraiture. For most digital SLR's that we discuss here (with the exception of a precious few costing many thousands of dollars) this benchmark 85mm focal length has been dropped to 57mm (85 / 1.5 = 57). This 1.5 FLM would be applied to my digital Nikon SLR, others, such as the Canon, would have a slightly different multiplier.
It is the size of the sensors (which vary slightly between manufactures), as they relate to the size of 35mm film, that brings about the need for this calculation.
The 85mm benchmark is now a relic of the days of film. All the optical attributes of the 85mm focal length have been transported to the new number: 57mm (+-). This is true whether you are using the new digital lenses or the lenses you used on your old 35mm film camera.
For a more in depth discussion of this subject you can refer to the following link:
http://www.dpreview.com/learn/?/key=...gth+multiplier
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