Leica APO-SUMMICRON-M 35 F/2 ASPH's optical design is particularly difficult to manufacture.
From their optical diagram, the third group from the front is a cemented triplet triplet, which is really difficult to manufacture. Vast majority of modern lens only uses cemented doublet design.
Is a cemented triplet triplet really that difficult to manufacture? How has Sigma done it occasionally without fanfare and with even more steeply convex/concave elements involved?
Look at the diagram of the discontinued Sigma 50mm f/1.4:
And the Sigma 28mm f/1.4:
There may be more - I didn't search that intensively.
