[Leica] 50mm lenses

[steve.barbour at gmail.com (Steve Barbour)]

excellent Larry.


> On Sep 17, 2016, at 8:26 AM, Larry Zeitlin via LUG <lug at 
> leica-users.org> wrote:
> 
> 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
> 
> 
> 
> _______________________________________________
> Leica Users Group.
> See http://leica-users.org/mailman/listinfo/lug for more information