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Leica 16-18-21mm f/4 Tri-Elmar-M ASPH
(not to scale)

MTF vs 21/3.4

The 21/3.4 SEM at f/3.4 trounces the Tri-Elmar at f/4 across the imaging field showing tightly controlled astigmatism and far superior MTF to the edges and corners. While the Tri-Elmar might be suffering from field curvature, this only makes the 21/3.4 SEM even more realiably useful in practice: a half-stop brighter helps focusing visible (10X Live View), and with higher performance conforming to a near-planar imaging field for subject matter at a uniform distance. Those considerations are compelling in practice.

Note that astigmatism can be a serious performance problem when applying a Leica M lens on a mirrorless camera because the sensor cover glass filter can not only amplify an ugly double image effect but it also amplifies chromatic errors. Hence the astigmatism of the Tri-Elmar is not a friendly behavior for mirrorless, but it might be substantially mitigated by a relatively telecentric optical design. However, the 21/3.4 SEM also has a relatively telecentric design.

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Diglloyd Guide to LEICA contains in-depth coverage of Leica M system cameras and lenses, with additional coverage of Leica M Monochrom, Leica Q.

Special emphasis is placed on Leica M lenses and certain Zeiss ZM lenses.

  • Make better images by learning how to get the best results right away.
  • Save money by choosing the right lens for your needs the first time, particularly the Summilux/Summicron/Elmarit choice and/or Zeiss ZM.
  • Make better images, a sort of “cheat sheet” saving yourself months or years of ad-hoc learning. Processing parameters are discussed and shown.
  • Jaw-dropping image quality found nowhere else utilizing Retina-grade images up to full camera resolution, plus large crops [past 2 years or so].
  • Real world examples with insights found nowhere else. Make sharper images just by understanding lens behavior you won’t read about elsewhere.
  • Aperture series from wide open through stopped down, showing the full range of lens performance and bokeh.
  • Optical quality analysis of field curvature, focus shift, sharpness, flare, distortion, and performance in the field.

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