21 mm lenses
tained with 28mm lenses can be noticed without difficulty, but image details are slightly ‘fuzzier’. Optical progress can easily be followed in the discussion that follows, with the 21 mm f/2.8 Elmarit-M ASPH representing a very high level of progress. It is the only 21 mm lens that compares favorably with the 28mm f/ 2.8 Elmarit-M. The Elmarit-M at full aperture shows very fine details, but it renders them with slight fuzzy edge contrast and extremely fine details are lost. One needs to stop down to f/5.6 in order to render these extremely fine details with good separation. The Elmarit ASPH produces better images at full aperture. Overall contrast is higher, and extremely fine details are just discernible. The ASPH gives a noticeably crisper image at all apertures and the level of just discernible details is higher (more details that is). At f/4 performance is as good as the predecessor’s at f/5.6 to f/8.
[22] Leica M Lenses
Is a direct comparison between lenses of different focal length acceptable? Not really! The reproduction factor of the extremely fine details is higher with a 21 mm than it is with a 28 mm lens. Even if the grain and the film would allow it, the 21 mm image needs a higher magnification to show the same details at the same size and therefore it needs a higher degree of optical corrections at a higher spatial frequency. The prerequisites for such an infinity test are simple: extreme care must be taken if one wants best quality at infinity: rigid tripod, correct exposure, distance setting at infinity, low speed film. The test shows clearly that performance at infinity with the latest generations of 21mm lenses is at a very high level. When comparing results between different focal lengths one should take
into account the reproduction factor and all other image-degrading components. I also conducted a test to ascertain whether the performance level changes if one sets the focusing ring just a fraction before the infinity mark, thus using depth of field to secure good rendition of details. The drop in performance between these two settings is clearly visible. So, if you need the best image quality at infinity, focus the lens at infinity and forget about depth of field concerns in such cases. We also noted that image quality will be reduced substantially if the objects at infinity are overexposed, which is quite often the case with small objects (like trees) against the sky. This behavior is inherent in 35 mm format photography where overexposure is one of the most serious causes of image degradation.