A Nikkor Z Lens Design Interview

Xitek last month published an interview with four Nikon engineers about Nikkor Z lens development. In particular, the 50mm f/1.8 S and 85mm f/1.8 S were discussed. A few interesting points were dropped:

  • Precise focus methods (moving of elements) differ between various Z lenses and are always optimized to each new lens design (though they all use a stepper motor design). That "always optimized" thought partially explains the one-motor/two-motor differences, as well as some other things I've noted in the optical designs.
  • Nikon understands the demand for less expensive lenses than the S-line. [It appears two such prime lenses are in the roadmap; though without specifications it's difficult to tell what the real goal of those are.]
  • Nikon started with f/1.8 prime lenses instead of f/1.2 because they wanted to provide small with high quality first, and they felt that f/1.8 was the right compromise for that. 
  • The rearward out-of-focus area is considered more than the forward area in optical design. This should be a bit obvious when you consider photographic practices, but it's good to see it acknowledged.
  • Chromatic aberration and coma suppression in the Z lenses is partly done to improve the quality of the out-of-focus areas, while spherical aberration is allowed (or at least controlled). As was noted, high resolution sensors have this tendency to look unnatural in the transition from focus to out-of-focus, and part of this is that artifacts produced by the lens are easier to see at 45mp than 24mp.
  • The focus methods used within the Z series lenses also attempt to reduce focus breathing [I'd note that this is sometimes also done in conjunction with the Auto Distortion Control corrections].

While the Nikon lens engineers continue to talk consistently about the focus-to-out-of-focus transition—something they've done since the 58mm f/1.4G introduction—they've yet to quantify how they judge this. There's a lot of mumbo-jumbo around "pleasing" or "natural" or "beautiful." I'll jump in here and say this: what I've seen in recent Nikkor designs is that there's a strong emphasis on the out-of-focus transition not calling attention to itself. I use the terms "busy" versus "quiet." A busy bokeh simply calls attention to itself, and I can't think of a photograph I want to make where I want to call attention to the out-of-focus area specifically. Out-of-focus areas tend to support your composition in terms of color, structure, and help in isolating the subject. You don't want them doing something odd or directly noticeable in any of those areas.

Update: a reader who's first language is Chinese (thanks WL) offers this translation of critical part of the interview:

A (Ishigami): In order to provide the smallest possible lenses with high image quality, we developed the f/1.8 lenses first. As f/1.8 prime lenses, you may think that they are a bit pricey and bulky, but we designed them in a way that they can be used at the maximum aperture in various scenes. As an analogy, you can think of it this way. Compared with the F-mount f/1.4 lenses, they [the f/1.8 Z prime lenses] are more compact and lower-priced, and they perform better. The f/1.2 [prime lenses] will be launched later, so stay tuned.

Q: It is understandable that high-performance lenses are more costly than ordinary ones, but the f/1.8 lenses' price tags are higher than those expected by customers. Why does Nikon do this? Why are they more costly than expected?

A (Ishigami): The Z mount lenses are graded to S-Line. [Translator's note: the meaning of the original text is unclear. The literal translation will give something like: "the grade of Z mount lenses are set to S-Line".] We wish to provide a product that is as small in size as possible and has the performance that deserves the S-Line name, that is this [Translator's note: It is unclear if "this" refers to a lens or the S-Line as a whole.] We understand that some users want to have lower-priced lenses and they may consider other lines of products from us.

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