The lens is not mated to a shutter, all modern shutters are equal, what is mated is a front cell to a rear cell, for each pair of cells there is an optimal lens spacing, not because the threads of the cells are not done equal, but because glasses are not all equal, refraction index of a batch may vary slightly (say an 1 per 1000), thermal issues during manufacturing, etc who knows... but optical design may allow a better compensation of aberrations (front+rear) by shimming.
So if replacing a shutter we can use a caliper to check that the new one is same thickness, but the important thing it will be reusing the shimming that mates the front cell to the rear one for best performance.
Papi, Boyer recomputed lenses for each batch of glass. My friend Eric Beltrando sometimes did that work for his friend M. Kiritsis, Boyer's last owner.
Just an speculation... if a design may make spherical and chromatic aberrations more or less proportional in the front cell depending on glass, and the same in the rear cell but with aberrations compensating in the other sense... then inter-cell spacing would adjust how big the aberrations are when rays arriving to the rear cell, in order to get an optimal cancellation.
What I say is that if a design is sound enough perhaps shimming could compensate glass variability without recomputing the lens and messing in serial production. Perhaps in most modern computerized designs a priority was alowing a clean compensation of glass variability by a simple shimming... I repeat , just speculating... just realizing the value of that possibility...
As Bob has posted endlessly over the years, the factory does it using elaborate optical measuring equipment. That's why it's important to transfer the original shims when moving cells into a replacement shutter. An independent optical facility could perform similar optimization if 'loose' cells need placement in a shutter or if re-shuttering cells where it's not known whether original shims existed or have been changed.
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