50mm "Normal" Lens... Ok, but "Normal" on FF or APS-C Sensor?

Started Nov 10, 2012 | Discussions thread
OP REShultz Senior Member • Posts: 1,108
Re: Erwin Puts on the notion of the 50mm normal lens


Charles2 wrote:

Erwin Puts, Leica Lens Compendium, old 2005 edition, p. 93:
The focal length of 50mm has been designated as the "standard" for the 35mm format. There are however, no hard or fast rules here. The statement is derived from the notion that the standard lens should have a focal length equal to the diagonal of the negative area. For a 24x36mm negative the diagonal is exactly 43.27mm. In reality most standard lenses of 50mm focal length are closer to 52mm. That is a difference of almost 10mm and too large to be inconsequential.
A second, related explanation, has it that the angle of view of the standard lens (about 47 degrees) corresponds with a natural viewing angle of the human eye. That again is a myth and cannot be supported by research. The angle of view of the eye where good discrimination of details is maintained, is about 20 degrees. And the total angle is 140 degrees. The angle of 45 to 50 degrees has no special significance for the human eye.
There is a psychological and a technical argument that can explain the preference for the 50mm length. If we look at a print with dimensions 15 x 20cm (diagonal 25cm) at the closest normal viewing distance (25cm) the eye is located at the so-called centre of perspective, corresponding to the optical centre of the taking lens. From that location of the eye, we look at the picture as if we were standing in the centre of the negative at the sharpness plane. At this distance the eye can capture the whole print area without eye movement, providing for easy viewing. Technically the focal length of 50mm is a very good compromise between high speed, small dimensions and excellent optical correction.

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