2395317 T, 2135127, 2134948, 1826639, 1754317, 1661030, 1607140 -> Oldest lens on the right, newest lens on the left, the serial rises from right to left
I noticed something with my 2135127 and 2134948 pair. Both are from the same batch.
2134948 aperture ring moves past f11
2135127 aperture ring moves past f1,5
I would interpret it as that both lenses reach f16 but the aperture ring of 2135127 is not aligned correctly. So the aperture ring has been changed in this batch (37th) already. So from my list I would say they changed it after batch 28 and at least in batch 37.
Back to the original question from
@dexdog. Why this change? Well I can not answer this with 100% certainty.
It is not the first time Zeiss did this. They did a similar thing in batch 6 (starting 1415701). In batch 6 they started with lenses that where engraved with max f8 and could only move to f8. Within 300 copies they changed it to engraved f8 and move to f9,5 and then engraved an unlabeled f9,5 line and move to f9,5. The reason for what happened in batch 6 was that this batch already uses the Berteles second optical calculation (v2). The first calculation (v1) only allowed f8 as maximum aperture. The second calculation (v2) is used later with a max aperture of f11. I would interpret it like that the department building the barrel had no idea of this change and just used the old barrel design until someone mentioned the mistake and they just changed the movement of the aperture ring to correct for it and later put a small line as an indicator of the new max aperture at the lens. Even this is highly debatable because it is still unclear if those lenses really use the second or the first optical calculation.
So why does Zeiss change the aperture ring of the Sonnar 5cm F1,5 to reach f16? Well maybe it is the same story.
Bertele created a third optical calculation (v3) in 1935. In theory batch 28 is the first one that uses the third optical calculation. In theory because that is what is written down in Mr. Thieles Fabrikationsbuch. In reality I have doubts about this. I have not written about this yet but I doubt that Berteles second calculation was used in batch 26 and 27. And that is the reason why I have Sonnars from batches 26, 27 and 28. But this aside the answer to your question might be in a little detail most people will not notice. The barrel of the Sonnar 5cm f/1,5 has been changed in batch 33. It is only the mounting part that has been changed cosmetically but this might be the point where they also changed the aperture ring. So in combination with Bertele's 3rd optical design (that allowed a new max aperture of f16) they lightly changed the barrel design and the movement of the aperture ring. Sounds plausible but needs confirmation.
Why did they not engrave a f16 at the aperture scale? Maybe because they did not felt confident about the usability of f16? We can only speculate. I doubt that it is because of spare parts. It would be easy to just engrave an additional number at the scale. So the reason is that they did not want to engrave the f16 at the lens.
