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Zeiss 15/2.8 Distagon Q&A — Question List

Just prior to the formal introduction of the Carl Zeiss 15mm f/2.8 Distagon, I had the pleasure of discussing the 15/2.8 Distagon and related topics with Carl Zeiss Staff Scientist Dr. Hubert Nasse.

We covered the Zeiss 15mm f/2.8 Distagon in detail, as well as some sagittal topics, a few of which are tangential, but all of which are interesting.

The original written questions below formed a basis for discussion; they are recorded below as a point of interest only, the following 8 pages capture the phone discussion.

Carl Zeiss ZF.2 15mm f/2.8 Distagon T*


Below are the original questions which I posed via email prior to the phone discussion.

#1 Overall design

The 15/2.8 Distagon appears to be a landmark design for a 15mm lens; I understand that the 15/2.8 is a brand-new design from the ground up. Its imaging quality exceeds any 15mm lens with which I am familiar.

The 15/2.8 is also a lens that I had personally desired for some years now, to complement the 21/2.8 Distagon. If anything, it has exceeded my expectations in terms of its superlative sharpness and contrast, as well as its unusually low and “friendly” distortion shape, and just as impressive, very low flare and field curvature. The ability to take front filters is an unexpected bonus.

  • What were the design goals for the 15/2.8 Distagon? (performance, market goals, etc)?
  • With all its strengths, could you discuss any less optimized behaviors that might intrude (even if minor) into real photography?
  • It would also be interesting to hear about what special lenses/glasses are involved, as well as whether the 15/2.8 Distagon requires any special assembly precision to achieve its high performance.
  • Please share any other general thoughts on the 15/2.8 that you think might be of interest.

Computers now aid lens design greatly, but there is still an art to it, and a judgment and practicality to making real lenses to be mass produced.

  • What insights went into the 15/2.8 Distagon design? Anything special to share here, a special personal or professional pride in the resulting design or challenges that were overcome?

The f/2.8 aperture of the 15mm Distagon is an aggressive lens speed for a 15mm lens, but a very welcome one.

  • How did the f/2.8 aperture influence the optical design, and can you speak to the relative image quality of f/2.8 vs f/5.6— the MTF charts suggest that the 15/2.8 is already near-optimal wide open.
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#2 Behavior at different focusing distances.

  • What kind of performance differences (if any) can be expected at infinity focus, mid-range and close-up? (sharpness/contrast and/or distortion)
  • How does the 15/2.8 Distagon behave in terms of maintaining a true focal length of 15mm when focus is varied from near to far (focus breathing)?

#3 Durability in the field for a complex design

  • With its complex 15-element design, is the 15/2.8 Distagon (or similar lenses like the 21/2.8 or 35/1.4) subject to any special tender loving care in the field to maintain performance (e.g., bumps and bangs, vibration, etc)?

#4 Lens advances elsewhere in the lineup

Clearly the performance of the 15/2.8 Distagon was held to very high standards.

  • What kind of advances might be possible at a similar price point elsewhere in the Zeiss lens lineup? (not asking for future specific lenses here, must general ideals on what Zeiss might consider in the future).
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#5 Design goals with digital sensors.

  • Have the design goals for the ZF.2 / ZE lens lines changed given the dominance of high-resolution digital sensors?
  • For example, might some design parameters like field curvature or “breathing” now be given more attention than in film days? Does the 15/2.8 represent any new thinking in this regard?

#6 What is good enough?

The recording medium and subject matter (portraits vs landscape vs documentary vs 4K/8K video) determine what is “good enough”, but ultimately it is a judgment call based on the photographer’s own work. The 15/2.8 Distagon and 21/2.8 Distagon and 35/1.4 Distagon suggest that Zeiss is raising the bar on full-frame image quality.

  • With today's 36 megapixel DSLRs and demanding video applications, have the Zeiss design goals for image quality shifted, especially with regard to image quality to the edges of the frame and corners?

#7 MTF — corners

The 15/2.8 MTF chart is superlative even forgetting that this is a 15mm f/2.8 lens, and field shots confirm the exceptional contrast. However, the MTF chart shows far corners with lower contrast at f/5.6 than at f/2.8, which seems highly unusual. I have made certain field shots which show corner blur which looks a bit odd in context of the rest of the frame. Corners can also be a consideration for astrophotography, e.g., blur in a starry sky, which might draw unwanted attention.

  • What is going on in the corners and why would f/2.8 outperform f/5.6 there? Also, what does Zeiss suggest for situations where maximum sharpness into the far corners is desirable?

#8 System tolerances

  • To what extent does Zeiss see the precision of the camera lens mount and sensor alignment and focusing screen optical path to be an issue in achieving the peak lens performance provided by top-performing Zeiss lenses? (e.g. not only parallelism but focus accuracy and system tolerances).
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