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 p.5 #6 · Distagon 28mm f2 C/Y or Distagon 28mm f2 ZE/ZF? | |
The following excerpt is from the preface of a Leica R-lens catalog published in 1984. This is an excellent catalog if you can find it. While it is not a Zeiss published white paper on curvature of field, it does make some important points about astigmatism, curvature of field, floating lens elements, and MTF curves. Leitz is basically justifying its lens design objectives, but the arguments they make are good to keep in mind when choosing lenses.
Basically, MTF measurements are always comparable with pictures of test charts. It is therefore not surprising that LEICA lenses usually show much better practical results than test photographs or MTF measurements would suggest. This at first glance paradoxical statement can be explained by the Leitz approach to correction:
Many lens manufacturers endeavor to keep astigmatism and curvature of field at a minimum. This must often be bought with the loss of maximum possible contrast, although it is the contrast performance of a lens that decides the visual impression of sharpness in the picture. Leitz are therefore less keen on a flat image field and minimum possible astigmatism, if this is at the expense of contrast performance. Particularly good examples of this are the 400 and 560mm TELYT-R f/6.8 follow-focus lenses. In both lenses, resolving power and colour correction are outstanding owing to the use of special glasses developed by Leitz. It is a well-known fact that achromats at full aperture suffer from slight curvature of field. As a result, sharpness in the marginal zones is shifted towards objects at a slightly shorter distance. In practice, this is usually considered a positive feature by photographers, because it normally affects the reproduction of the foreground. And a minor (apparent) gain in sharpness in the near foreground is almost always felt to be pleasant from a pictorial point of view. For plane objects, such as walls of houses, focusing should be a compromise, i.e. it should be a little outside the center of the picture, and the lens should accordingly be stopped down sufficiently if optimum sharpness is also required in the marginal portions. If this characteristic of the lenses is not allowed for during the test, the test result must be completely negative and would be in gross contradiction to the judgement of many sports photographers.
A similar discrepancy between test result and practical result can also occur in the testing of wide-angle lenses. Many of these have a floating element, which according to the claims of the publicity departments is often synonymous for outstanding optical performance in the near-focusing range. This may be, but is not necessarily the case. Here is the explanation.
As a rule, 35mm camera lenses reach their optimum optical performance when focused on infinity, which corresponds roughly to 50-100x the focal length of the lens. Within the nearer range, image quality decreases unavoidably. This falling-off of performance is usually unnoticeable in practice. But with some lenses, for example very wide-angle high-speed lenses, an appreciable worsening of image quality can occur. One of the methods of correction is changing the distance between individual elements or components within a lens. This floating element is moved backwards or forwards corresponding to its optical effect, and is coupled with the focusing mechanism of the lens. Investigations at Leitz have shown that a floating element clearly improves the image quality in the focusing plane. When test charts are photographed, lenses with a floating element will, therefore, always produce good results. Unfortunately, the good image quality is achieved only in the focusing plane. The space in front and behind, the depth of field, does not equally profit from it. When the foreground of the subject is focused on, more distant objects in the corners of the picture will appear unsharp, although according to the tables and scales, the depth of field would be deep enough to include these features in the zone of sharpness. This effect is enormously disturbing in pictorial photography, because it confines three-dimensional subjects by the exclusion of plane areas. Leitz, therefore, checks with every wide-angle lens for the LEICA whether or not a floating element offers advantages to photographic practice, whether it can be dispensed with, or if it makes sense to build a more complicated lens. Guided by these practical considerations, Leitz have not yet decided on the indiscriminate installation of floating elements in all wide-angle lenses – even if this has a negative effect on the measuring curves during testing.
We repeat: Leitz obviously endeavour to compute lenses that produce a completely flat field. Even so, the practical requirements are never neglected. And if a compromise has to be found, which is often necessary, it will only be in favour of applied, i.e. practical, photography.
It should always be borne in mind that in photographic practice, i.e. in the photography of subjects staggered in depth, there is not one, but an infinite number of subject planes. If you focus on a certain subject plane, there will others in front and behind it, which fully benefit from the higher contrast of the Leitz lenses, bought at the expense of some curvature of field. The result is an excellent picture.
For test chart pictures of MTF measurements, on the other hand, there is only one subject and, therefore, only one image plane. Here, curvature of field and astigmatism are disturbing features. Which means that when Leitz lenses are tested with the aid of test charts and MTF measurements, the effect on the result will be negative.
But the typical and, above all practical, lens characteristics are never appreciated in the published measurements, and are, therefore, neglected in the judgement of a lens. If you do not know this characteristic feature, you cannot help but get the wrong impression.
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