Prospective collector buyers of the Meyer-Optik Domiplan 2.8/50 M42 mount automatic lens should be aware of a problem with these lenses that hampers their usefulness. So far, I have purchased a number of these lenses in one way or another, and of these only one was even partially functional. In examining the lenses, one finds that the problem is a design flaw in the automatic diaphragm mechanism. The problem involves the failure of the spring mechanism (which would normally push the diaphragm pin back up and open up the diaphragm) to work properly.
The same lens in Exakta mount may not have exactly the same problem since the diaphragm mechanism connection appears to be different. However, I have been surveying eBay offerings and many of the Exakta mount lens also appear to have some kind of problem. This lens was standard on middle-period East German cameras (e.g., Praktica L) as the cheapest kit lens. They are not "rare" and usually the shipping will be higher than the cost of the lens, but there is a reason they are cheap. The ones I have purchased have mostly been the so-called "zebra" focusing and aperture rings (as shown), but this problem seems to have persisted into the later all-black models. The reason I am writing this guide is that nearly every example of this lens I bought was described by the vendor as "working," often with specific reference to the diaphragm "smoothness." I don't believe that the vendors were purposely misleading, but I do think that many people testing a lens simply see whether the aperture ring turns easily--they are not looking into the lens to see if the blades are moving or are returning to position. If you do bid on one of these, please have the vendor specifically test whether the automatic diaphragm is really, really functional or not by pushing in and releasing the pin on the base of the lens and by trying different aperture settings. Since this guide was originally posted, I have noted a trend toward more accurate descriptions of this lens when offered for sale.

As noted, I  have identified at least a couple of the problems of this poorly designed lens. The main areas where the connections between the various moving parts are made are

1) where the red arrow is pointing, where a pin is shown peeking up, the pin can be bent and not move the aperture blades.
and
2) where the blue arrow is pointing, there are problems in the connection between the external diaphragm pin and the internal mechanism that has to translate vertical motion into rotary motion. This design is what is called a "kludge" and it is incredibly inelegant. While I have identified the problem, so far I have not been able to get the mechanism to function properly on reassemble. At least one person has simply removed the diaphragm and converted the lens into a soft-focus lens using new baffles, etc. This may not be a bad idea. At its best, the lens is mediocre in optical terms.
By the way, if disassembly is tried, it is crucial to photograph every step. The re-intertwining of the diaphragm blades is a wonderful hobby task for cold winter nights.

revised 10 Sept 2008