OK, I'm SATISFIED ! Thanks Ghammer, Giordano, Jaap, I see my initial doubt is not significant in practice: even applying the "f related formula" the difference in DOF between 50 and 135 at 1:2 ratio and f11 is in the range of 10^-4 mm ... the CoC value is anyway the driver... the simply act of focusing on the Viso screen makes all this irrilevant, even admitting that the superb Leitz extension rings have surely screwing tolarences UNDER 10^-4 mm
... (not to mention the alternative of Summicron DR with std. RF focusing...
)