[Leica] 50mm lenses

[lrzeitlin at aol.com]

I would like to remind the LUG that Leica made a number of 40 mm lenses and equipped the Leica CL with an excellent 40 mm F 2.0 Summicron. This lens was reputed to be one of the best available but Leica's weasel worded explanation was that the focusing mount was not compatible with the M cameras. This was a naked ploy to avoid competition with the much more expensive lenses sold for the M camera.

Here is the explanation from Erwin Puts' Leica lens compendium:

The true focal length of many ‘standard’ lenses of
 50mm (indicated) is 52mm! Why a designer would choose 90mm or 85mm is not clear. Presumably the calculations dictate a certain physical volume or a certain front 
lens diameter, which is convenient or necessary. A second consideration when
 discussing lenses is the angle of view or angular coverage. I would like to
 draw attention to the fact that the negative format is 24x36mm, which gives 
three different angles of view. As any lens produces an image circle, in which
the rectangular format of the negative has to be fitted, we have a diagonal, a
 horizontal and a vertical angle of view. For a 50mm lens the diagonal angle of 
view is 45, but the horizontal angle is 41 and the vertical only 28. For the
 35mm lens the numbers are, 64, 56 and 37. It is evident that the horizontal
 angle of view is more important than the diagonal. When taking photographs, we habitually look at the 
horizontal line to see what part of the scene is covered by the lens. This
 intuitive gaze, corresponds to the horizontal angle, which is invariably smaller
 than the quoted diagonal and can explain the disappointment sometimes noted with
the covering power of a wide-angle lens in practical situations. 

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 center of perspective, corresponding to the optical center of the taking lens. From that location of the eye we look at the picture as if we were standing in the center 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. In the world of the microscope lenses, where Barnack looked for a suitable lens, the focal lengths of 42mm to 60mm were available with good corrections. This might have inspired him to search for a solution within this range. The 50mm focal length has been the workhorse of all Leica photographers since the early thirties. 


Larry Z