Zeiss 15/2.8 Distagon Q&A — Tolerances
Related: MTF and Micro Contrast, wide angle, Zeiss, Zeiss 15mm f/2.8 Distagon, Zeiss Distagon, Zeiss DSLR Lenses
This is one of several pages resulting from the March 13, 2012 discussion with Senior Scientist Dr. Hubert Nasse. See the original list of questions.
These pages are a summary of the discussion based on notes taken, and as reviewed by Dr. Nasse. Bracketed comments [ ] are editorial in nature.
The advanced design of the 15/2.8 Distagon requires exacting assembly tolerances. It is also for this reason that a good-size sample of the first production run of both ZF. 2 and ZE lenses are carefully checked for variation and MTF, to verify that construction tolerances are being fully met. Only once those pre-production lenses are fully tested is production for public sale commenced.
Zeiss also checks each production lens for MTF. Dr. Nasse states:
We see a large number of samples (a statistically significant quantity) from series production per mount type. And they are checked deeply, i.e. at several f-stops and by measuring a complete field curve which needs at least 6 sampling points.
This is a time-consuming procedure, and of course in regular production a somewhat simpler procedure is applied, but every production sample is MTF-checked before being sold to the customer.
MTF-metering machines at the production facility are Oberkochen-made and are serviced by CZ-staff.
Mechanical precision and precision of assembly must necessarily be high, but each batch of glass has to be carefully checked for its refractive properties, because no batch is quite the same. Small deviations cannot be tolerated if peak performance is to be reached. The lens design must then be tweaked to accommodate slightly different refractive properties for the actual batches of glass used. Lenses are assembled in production runs accordingly.
Once assembled and meeting specifications, ZEISS expects its lenses to hold that performance for many years, even in the face of some abuse.