S Dilworth
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 p.6 #3 · Zeiss: new lenses at photokina | |
Thinking some more about this lens, I’m reminded of the super-speed cine lenses Glatzel developed under his concept of ‘relaxation.’ Those lenses, also branded Distagons, were retrofocus derivatives, but they didn’t look much like simple reversed telephotos of the Horace Lee or Pierre Angénieux variety.
Optical relaxation distributes optical problems throughout several lens groups, with little regard for the resulting physical length of the lens. This gives the designer additional degrees of freedom to correct aberrations. Obviously it’s only used if size, weight, and cost are secondary to optical performance.
If Zeiss wanted to make a splash with a new high-performance lens range, an f/1.4 normal would be a good place to start. Typically the 50 mm f/1.4 at full aperture has the worst image quality of any lens in a manufacturer’s catalogue. The shorter f/1.4 lenses (24–35 mm) are very sophisticated designs with better full-aperture performance, and the longer f/1.4 lenses (75–85 mm) are easier to design because they have a narrower angle of view, so they too tend to have better full-aperture performance.
The double-Gauss type has been incredibly successful for fast normal lenses, because it’s small, relatively inexpensive, and offers very high optical performance at typical working apertures. For low-cost 50 mm lenses of around f/2 it offers overwhelming advantages.
But for lenses of f/1.4 or faster, there are very considerable problems. First, these lenses don’t naturally provide the 38 mm back-focus needed for SLRs. This has to be forced on them at the optimisation stage, which is why SLR fifties look and perform slightly differently to rangefinder fifties.
Second, spherical aberration becomes an extreme problem, such that the designer is forced to split the rear element into two (Planar, Canon) or three (Nikkor) elements of less curvature. Also essential are glasses of higher refractive index to allow elements of less curvature, again to contain spherical aberration. High-index glass can be very expensive, so f/1.4 fifties are typically much more expensive than f/1.8 or f/2 lenses.
Third, severe vignetting is both unavoidable and desirable to reduce aberrations, but obviously undesirable for many photographs.
And fourth (this isn’t comprehensive, so I wish I hadn’t started numbering these), distortion control becomes problematic, often resulting in distortion greater than the 1% that’s often considered unnoticeable.
Let’s make it five: lateral chromatic aberration, almost invisible in f/2 double-Gauss lenses, becomes clearly visible in f/1.4 lenses.
A Distagon with Zeiss’ optical relaxation could deliver substantially better performance in these areas, depending on where the design priorities lie.
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, can they find a better engineer to do the job?
I don't know either. Maybe it is their way of saying "photographers who look in the corners of their photos".
How difficult can it be.