Originally posted by Henrico The second diagram would never result in sharp images, so this is clearly a mistake. My opinion is that both lenses share exactly the same formula, only the coatings and the layout differ somewhat.
It could be accurate, although it would require a few changes in the optical construction of the lens since it is a retro-focus design. The Only example of this I can think of right now is the contrast between the early 7 and 8 element 50mm f/1.4 takumars. The lens design was reduced in its complexity - one cemented group of different refractive indices was replaced with a larger solid element with recalculated geometry and the optical design changed to compensate.
In this case I would say - judging from the optical schematics presented here, there is no evidence that suggests pentax has changed the optical formula to compensate for the removal of an element from the design, it could very well be a mistake.
---------- Post added 2021-05-25 at 10:20 PM ----------
Originally posted by house I thought anomalous dispersion was more expensive and rare than ed glass but what do I know.
They are both expensive, depending upon quality and the exact chemistry needed* to fit the manufacturers specifications. With exotic glass types factors such as the ability to be made in various sizes and in large batches without material defect, the ability to be formed and annealed without dimensional instability and ability to be ground to specific tolerances and polished to a high finish: taking these factors into account high grade ED glass is the more difficult of the two materials to work with as it is quite fragile.
* Lens elements made for the Early Leica 50mm f/1.2 Noctilux ASPH had to be made in crucibles made of
solid platinum as it was the only material inert enough at those temperatures to not affect the chemistry of the molten glass. A lot of glass types are made these days, and glass manufacturers like Schott and Ohara have expansive catalogues of glass types and seldom have to develop an entirely new glass types from scratch. Lenses these days are designed around what is available and easy to source.
Last edited by Digitalis; 05-25-2021 at 04:59 AM.