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I had the A* and the K 2.5 and the 2.8 - and the Spiratone Plura-Coat (as it seems, branded Porst 1.8/135 in Germany).
I collected all lens Tests form the late 70's to the early 90's and remember many 135mm-comparison Tests.
The Porst is weak at 1,8 to a degree, that you wouldn't use it instead of making soft pics.
Beginning at 2.8 it starts to produce sharp pics - in the Center with the edges falling off.
The A* is a bit soft (less contrast) wide opended with quite good sharpness in the center but light fall-off to the edges.
Stopping down this fall-off was reduced to Zero and sharpness war VERY good from 2.8 on.
The A* is simply sharper than the K 2.5 at same apertures.
The K 2.5 sharper than the 2.8/135 from Pentax.
Some Things that are very special with the A*:
As those 3 *-lenses it has a FREE-design (fixed rear lens element). Due to this design it is very good even at very close distances - where most tele lenses lack a bit.
I used it for some macro-works and the results outperfomed those from true macro lenses (Pentax and Tamron).
It is therefore very special for portrait work too.
The focussing ring is much smoother and with more degree than the competition (not knowing the Samyang though) and therefore much easier and more precisly to focus.
The A* can be used with TC with very good results - you cannot say that for every lens, esp. not for all 2.8-Zooms! There have been tests with regard to the use of TCs and even the 2.8/135 were not really good with them (but the Zeiss 2.8/135 was!)
The price that was posted by Beholder is ridiculous:
Even the F/FA 2.8/135 go more more.....
As for the competition:
In those days even the Zeiss 2.0/135 was not really better in the comparison than the A* - and the CaNikon rivals left behind ....
If sharpness is everything that counts you have to go with other lenses.
If rendering and macro abilities count and you love to focus manually (!?!) the value of the A* is really high.
- and the A* is much easier to handle than any 2.8/70-200 lens!
I'm still very sad that I sold my 3 A*-lenses due to the need of money.
@Beholder
why do you always call for 36 MP-pics?
Not all of us are using those fullframe-bodies!
... with good reason!