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On Secondary Color: Collective Context-Dropping regarding Zeiss Milvus 50/1.4 Distagon

Get Zeiss Milvus at B&H Photo.

There is a thread of (non) thought running around the web that somehow the color correction of the new Zeiss Milvus 50mm f/1.4 Distagon is not good enough. The absurdities and gross context-dropping are running amok to the point of hysteria.

This is not to say that the rendering style of less well corrected classic symmetric designs is inferior per se (it may be preferable for some), but every optical design has tradeoffs. And so the discussion must be based on the core nature of the thing being discussed! In this case, the Milvus is a highly corrected lens, not a classic design with its hazy wide-open performance mush. Stepping away from lenses: criticizing a Tesla Model S or Prius because it cannot take me up Silver Canyon would be absurd. So it must be for the design and usage goals of a lens.

What is the issue about SLOCA?

The issue at hand is secondary longitudinal chromatic aberration (SLOCA). SLOCA causes out of focus areas in the background to develop blurs with a magenta core and green exterior (these often overlap to confuse the issue). The better corrected the lens for other aberrations, the more purity the colors will retain, hence SLOCA will look worse in highly corrected lenses, a point generally ignored.

In essence, these blurs are differential defocus by color; if all colors focus (and defocus) identically, then there would just be a true-to-reality blur with no color change, as with the Coastal Optics 60mm f/4 UV-VIS-IR APO Macro as shown in Coastal 60/4: Zero Longitudinal Chromatic Aberration. However, the Coastal 60/4 is an f/4 lens! Even the Otus lenses at f/4 are not as pure in color as the Coastal! Shall we then also say the Otus lineup is problematic in failing to perform well enough?

SLOCA should not be confused with primary longitudinal chromatic aberration (LOCA), which causes violet fringing on high contrast transitions, and also lays a veiling haze over the image, dropping contrast and impairing clarity (and making focusing more difficult).

The Milvus 50/1.4 and 85/1.4 have less LOCA than any other f/1.4 DSLR lens on the market (except Otus), and in my examples with the 50/1.4 and 85/1.4 Milvus, LOCA is simply not seen, even in extremely challenging lighting approaching 10,000°K (very blue) on high contrast white branches.

Example of SLOCA

Shown below is an aperture series from f/1.4 to f/6.3. It is an actual pixels crop from a 36-megapixel Nikon D810 image. The background is very strongly out of focus and the crop is from 36 megapixels, a point which must be remembered for anyone used to a 22 or 24MP camera (Canon or Leica M).

The magenta/green blurs are SLOCA and can be seen to converge with depth of field (stopping down). The amount of SLOCA seen here is comparable to the vast majority of APO lenses on the market. I deem it as least as good as Leica M APO lenses*. Also, less well corrected lenses diminish the appearance of SLOCA by diluting the color via other aberrations, such as spherical aberration and LOCA and more—a point rarely if ever considered in discussions.

Also seen on the leading white branches is the magenta halo indicating out of focus; similar color can be seen with the Zeiss 135mm f/2 APO-Sonnar in the house comparison.

The behavior here is undesirable to be sure, but even Zeiss Otus cannot avoid such effects, though the amount is lessened. And if more persuasion is needed, note that the Zeiss 135mm f/2 APO-Sonnar takes several apertures to clean up such color blurs in similar circumstances. Is the 135/2 APO also then an inferior lens?

* The term “APO” today in general is a vague marketing term indicating some improved level of color correction. It no longer has any clear technical meaning, having been corrupted in usage to the point of being almost useless. For Zeiss, APO is a far higher grade designation than with any other brand. In general, read “APO” as “we made some effort to improve color correction”.

Actual pixels from Nikon D810
f1.4 @ 2.0 sec, ISO 64; 2015-08-14 20:14:56
NIKON D810 + Zeiss Milvus 50mm f/1.4

[low-res image for bot]

Example of SLOCA: Leica 50/2 APO

Shown below is a crop from the Leica 50mm f/2 APO-Summicron-M ASPH, which I deem the best corrected of Leica’s APO M lenses. It is an f/2 lens (a full stop slower than the Milvus 50/1.4), the letters are not much out of focus, and the crop is from a 24MP sensor (not 36MP as above). Be sure to view it at actual pixels, and note the magenta/green color blurs.

Now imagine far greater defocus and at 36 or 42 or 50 megapixels. How much “color bokeh” (SLOCA) will be apparent with the 50/2 APO? Well, a lot. And it is an f/2 lens costing over $8000. Also, the SLOCA at only 18MP with the Leica 75/2 APO-Summicron-M ASPH (and with the 90/2 APO) is very strong (and those two have other problematic behaviors also). Those Leica APO lenses are well corrected, and thus their SLOCA remains pure, undiluted by most other aberrations. Shall we also say that the Leica APO lenses are poor performers?

So the Zeiss Milvus 50mm f/1.4 Distagon is on par in SLOCA with the Leica APO lenses (Otus is easily superior), and yet it is a stop faster and costs a fraction of the price. Where is the internet frenzy over the SLOCA trouble with those Leica APO lenses? :)

Click for actual pixels. This crop from the Leica 50/2 APO: Color Bokeh (Secondary Longitudinal Chromatic Aberration, Dolls) series.

Actual pixels from 24-megapixel Leica 50mm f/2 APO-Summicron-M ASPH at f/2
(not much out of focus!)

Context matters

  • No f/1.4 or f/2 lens on the market today is free of SLOCA. Achieving that would likely result in a $10,000 price tag, and perhaps far more, with a great deal of special glass types and fluorite.
  • Every fast DSLR and mirrorless and rangefinder lens on the market (including Zeiss Otus) has secondary color (SLOCA); those magenta/green blurs for out of focus areas.
  • Lenses that are not as well corrected will mask SLOCA via a veiling haze over the image, diluting the color. Highly corrected lenses like the Milvus 50/1.4 Distagon will thus retain greater color purity for everything, including the magenta/green blurs of SLOCA. If one prefers this masking of SLOCA, choose a lower performing lens!
  • A highly-corrected lens brings light rays together in a very tight point spread function. Those same light rays blur-out quickly relative to this tight focus, thus making SLOCA more apparent. A lens with some “slop” makes these rays overlap more (more “jumbled”) thus reducing apparent SLOCA. One need only shoot a lens high in spherical aberration to see how much this jumbled imaging overlays and dilutes everything.
  • Rational analysis shows that the SLOCA of the Zeiss Milvus 50mm f/1.4 Distagon as good or better (and perhaps better) than most (perhaps all) Leica APO lenses, even those that are a slower f/2. And the Milvus 50/1.4 has no focus shift and less field curvature and superior flare control.
  • Actual pixels at 22 or 24 megapixels (Canon or Leica) is a much lower magnification than at 36 or 42 or 50 megapixels (Nikon, Sony, Canon). This ought to be numerically obvious, but it is a core fact which seems to have been forgotten entirely in the discussion.
  • A lens that is f/1.4 should not be compared at f/1.4 to an image produced by an f/2 lens at f/2! Both lenses must be compared at the same aperture.
  • It is more difficult to correct an f/1.4 lens than an f/2 lens in every way, and far more difficult to correct some aberrations.
  • Balanced lens performance always has some limits and tradeoffs. It is not a constructive discussion to put on blinders, dwelling on one aspect of lens performance.
  • You get what you pay for: those wishing for better but still imperfect correction for SLOCA should acquire the Otus lenses and Zeiss 135/2 APO-Sonnar which have no peers.
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