Every manufactured thing has tollerances. Nothing is ever exact. This applies to the performance of a lens. Way, way way back in the days when lens making for cameras began to become an important industry the question arose as to when was a lens in focus? What kind of tollerance could be assigned to this lens performance property so that lenses could be manufactured to satisfy reasonable customer expectations? This question was settled by focus groups sometime around the year 1900. A reasonable range of focus was determined by allowing randomly selected groups of ordinary persons to examine normal pictures of normal objects at a normal viewing distance. The pictures were taken with a standard lens set at both sides of the actual distance to varying degrees. From these results it was determined that a lens was "in focus" when it resolved a mathematical "point" at an actual diameter of 0.26mm in a picture of normal consumer size. So a lens is considered to be in focus if it will produce an image on a negative that is "in focus" as determined by these 1900 focus groups when the negative is enlarged to 3.5" X 5" it has a smallest image point of 0.26mm in diameter.
But, we know this is all nonsense. What is "in focus" to one person is fuzzy to another. And all of us who have done darkroom work and made reasonably sized enlargements know that a negative with a smallest image element producing a 0.26 mm dot when enlarged to consumer picture size makes mush enlargements.
I have seen enough 50mm f1.5 Sonnar lenses to be able to make one definitve statement about it regardless of when it was made and it is simply that this lens has "ZERO" depth of field at f1.5. Zeiss said is had some depth of field at this aperture, but then they had to, but it wasn't true then and it isn't true now.
The quest for sharpness is a never ending one. Photographers will always seek the perfect lens with perfect sharpness and contrast. But unfortnuately, Zeiss made lenses that have a range of normally manufactured sharpnesses and contrasts. If this was not the case then the Linhoff company would not have obtained the right to select out superior Zeiss lenses for its cameras out of the normal production run of lenses. So when you buy a Zeiss lens you are taking a chance. In general it can be said that Zeiss lenses are in all ways superior to those made by other manufacturer's of the time. My feeling is that I would rather have a "Zeiss Ordinary" 50mm f1.5 Sonnar than have a "Leitz Best" 50mm f1.5 Summitar any day. But in my practice I see a great many Zeiss lenses and can tell you that some are to die for they are so good, and others are merely "Zeiss Ordinary". Just what the market will give you is something we all find out by chance.
I do my best to obtain the best lenses for my customers. But due to the constraints of time and the market cannot be expected to obtain anything specific in the way of resolving power or contrast. I buy based upon my experience and get the best I can. But as the old saying goes, they aren't making them any more and we all have to live with what is available.