As for your more pointed question - well... I think it would be a fairly simple thing to adjust a lens formulation for a drifting refractive index... I mean - just change the curvature of the corresponding element and run the calculations through the whole design and go back and forth until you have a winner. I don't think there's any huge mystery to the science and art of lens design. All I'm trying to say is that they probably have a strategy for dealing with aberrations (no pun intended!) in their supply chain. And they're probably also highly motivated to cut costs and improve profit margins. They're a company after all. Is that not what companies DO? It seems reasonable to me.
Ok, I have to chime in on this one.
What you say is ridiculous, to anyone who has manufacturing experience. I have experience in both the production side and the QC side.
You betray your ignorance in these statements. You think that it's as simple as just recalculating and just changing the curvature of an element: what do think that costs?
I have seen a lot of expensive manufactured products get scrapped because they are out of spec. Sometimes a product can be reworked until it is in spec., sometimes it can get a variance, but usually it is scrapped. It's just not worth it to mess with it. It's quicker, easier and simpler to remake the product, than to mess with fixing discrepant product. To start custom making batches of parts to match discrepant material is to introduce variables no one wants. It introduces uncertainty and creates hassles. Why spend a bunch of time and effort to make something that's not as good as it's supposed to be?
I've scrapped orders with 2% bad parts, because the time taken to sort good from bad cost more than they were worth.
I have seen instances where discrepant materials have had to be used for some reason, and I mean material out of spec enough that the difference was not just academic. Every time was a huge hassle, and every time was due to a failure somewhere. It usually resulted in making little or no profit or losing money on the order. It was always the result of the failure of someone to do their job. It was chronic in poorly managed companies, and non-existant in well managed ones.
I once was ordered by my boss to make an adapter for several thousand knobs that a purchaser had ordered without realizing they wouldn't fit. The adapters could not exceed 7 cents cost because that was the price differential between the right knobs and the wrong ones. All my other work came to a halt while I came up with something that would work, along with an engineer to okay it, and all this was under the table. No part number, no job number, setup time slopped into other jobs, material provided by purchasing who stole it here and there from other jobs. I produced the adapter, the purchaser and his boss got their asses covered, my boss and the engineer got a favor to call in on purchasing, and I got the fun of busting my ass to make up for lost production.
That's no way to do things. And that was a poorly managed company.
Any time you change one thing it changes other things. And no one wants those headaches. If refraction is out, what about dispersal, and every other characteristic?
Materials are seldom a major part of the cost in manufacturing an object. Overhead and labor will typically represent a far greater amount of the cost of a product. The labor and delay involved in recalculating and resetting a machine will exceed the savings from using the discrepant glass. I would think, but don't know, that rejected glass could be remelted and brought to spec, at least in some cases.
Here is what would be required at a minimum to use the discrepant material, and don't forget every stage is going to require paperwork to be generated, reviewed, handled, signed off on, and archived.:
>QC discovers the glass is discrepant, and rejects it.
>If not scrapped outright by glass manufacturing, it goes to a Material Review Board, usually composed of representatives of manufacturing, engineering and QC, to determine cause and course of action.
>MRB (or whatever Zeiss calls it) decides to use the material.
>Engineering recalculates optical formula and produces new production spec.
>Spec. is reviewed and approved by optical and mechanical engineering, then checked and approved by quality control. Then checked and accepted by manufacturing. Then signed off on by management of those departments, then by higher management, possibly to VP level.
(This a minimum. There are actually quality engineers, manufacturing engineers, production planners, and others whose input and work will be needed. The delay may create a bottleneck in getting other materials produced, and may make the whole idea unworkable. Recalculating takes computer time, in addition to engineering time, away from other projects, and disrupts projects underway.)
>Manufacturing modifies machine setups to produce parts to modified spec.
First article part (aka "First good part) is produced for each phase of production. First articles are checked and rejected or accepted.
>After acceptance, the limited run begins, each step in turn. At the conclusion of the run, with parts accepted to modified spec, all machines will have to be set up to original spec. if more of the same part are to be made right away. Unless of course, the next batch of glass is somehow out of spec, and the process will have to be repeated for that batch. Unless it's identical to the first out of spec batch.
Zeiss has been making optical glass for a very long time. They know how to do it. They know how to hold tolerances. They know how avoid problems. They did not get where they are by screwing around with non-conforming materials.