Originally posted by deadwolfbones The stains are only really apparent at 100% zoom and when using an aperture that's smaller than the diffraction limit. Your camera would be perfectly usable in most situations, stains and all.
(And anyway, the stains are simple to clone out in post.)
With all due respect, this is exactly what annoys me. Some (many) people have legitimate problems. Then someone with several thousand posts shoots from the hip and makes slight of the issue. Some people are simply inadequately informed and generalize the issue to fit their personal perspective. Each stain issue will have its own qualities to a lesser or greater degree. Some will have visible areas of air pockets/bubbles, some will have the characteristic spots and some will have both.
It is at times difficult enough to clone/heal out dust bunnies in the sky, where subtle gradations are present, but "dust" is easily controlled by sensor cleaning/maintenance.
Sensor stains cannot be removed by the user; plus the PP removal process is time consuming, especially if the spots are guaranteed to be there. The stains may develop belatedly and/or become more visible/prominent as the fluid/solvent/residue between the sensor and glass-plate/filter (or between glass-plates/filters) evaporates due to the heating of the sensor. This phenomena may affect a substantial number of early production K-5's. How many of these are still in the supply-chain is unknown. There are some reports that K-5's marked as "Stain Free" by the gray circle on the packaging have developed stains.
In general we don't know much about the assembly/manufacturing process of the sensor module and who is to blame. But one can maybe deduce a couple scenarios. The affected glass-plates/filters may have been subject to cleaning with a solvent/cleaning fluid that was contaminated or not allowed to desiccate prior to assembly. The moist residue would eventually be triggered to evaporate by a warming sensor and surface tension and irregularities in surface smoothness would randomly distribute the residue as "Strings of Pearls". Maybe it is the sensor that is subject to some cleaning process and had residue, or both.
I want to re-iterate that Pentax/Hoya has acknowledged the issue and they are working quite hard to sort it out. It is not the end of the world, but armed with a better understanding, we as consumers can make better and more timely choices.