Second brand-new sample of Pentax-D FA 55mm f/2.8 has same hazing and scratch-like marks
Related: medium format, Pentax, Pentax 645 D FA 55mm f/2.8 AL[IF] SDM AW, Pentax 645D, Pentax DSLR, Pentax Lenses, Pentax medium format
See yesterday’s coverage of the Pentax-D FA 645 55mm F2.8 AL[IF] SDM AW Lens.
I received a second brand-new sample of the 55/2.8 today. This second sample is certainly not better than the first, and might be worse, with the same scratch-like marks visible on lens elements, and the same hazy look from microscopic dust. The haze can't possibly help contrast, and the fine scratch-like marks seem dubious for shots with the sun in the image.
As I still have not received the Pentax 645D, I can’t make any pictures with either lens just yet.
According to Pentax:
As far as we are aware of, PENTAX Japan has not confirmed this to be an abnormal trait for this lens.
An optical engineer who contacted me speculates:
This is a simple failure to properly finish the fine grinding phase of lens batch production. If the grit is insufficiently selected or insufficiently sufficiently homogeneous, there will be deep scratches that no amount of final polish will eliminate. This is the only logical explanation for scratched elements. There is a scratch-dig spec (Normally 20-50 scratches/digs per mm^2) that was not honored or detected. Precision use such as visual optics (Mirrors/flats) often specify 10-5 scratch/dig.
The problem is to get that level of precision (10-5), a great deal of extra work must be performed with many mounts and dismounts from polishing machines. There are many more opportunities to damage the optic and or the precious tooling. Photo applications are far less stringent than visual uses so a simple 50-20 spec is called out. BTW the more elements the more this limiting spec effects the final result exemplified by haze or veiling...especially into the sun.
I also bought from KEH.com a used (Ex+) Pentax-A SMC 75mm f/2.8. The classic build is easily superior to the new plastic 55/2.8, and the manual focus throw and feel is no contest in favor of the 75/2.8 (a bargain at $159). And while used, the glass is crystal clear and shows no hazing like the brand-new 55/2.8! Go figure. But the proof is in pictures, and that will have to wait until the camera arrives.
