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For 35 mm cameras, the normal lens is not 50 mm, as many people think, but 43.27 mm, which is the diagonal of the sensor size, 24 x 36 mm. This explains the 85 mm portrait lens (roughly 2 x normal) and the 135 mm lens (roughly 3 x normal), and why the first Nikkor zoom ever made (introduced in 1963) was the 43 – 86 mm (great to use, but not very good optically). Rounding down, the normal lens for 35 mm cameras should therefore be 40 mm, not 50 mm.
For the GF lenses, normal would be 54.78 mm, the diagonal of the sensor size, 32.9 x 43.8 mm. The 50 mm is therefore the closest to normal among all the GF lenses, and this explains the lens’ polyvalence, good for all sorts of subjects.

Source: Jonas Rask