Canon Serenar 50f1.8 focus shift?

[Greg_E]

I was just fooling around with a new to me Serenar 50f1.8 lens and noticed that with an object distance of about 8 feet I get a shift in focus of almost 2 feet between f1.8 and f4. This is looking through the EVF on my Fuji Xpro1. None of my other lenses shift this much with aperture change making me think this lens has been modified for focus at wide open, or something else. The previous owner said that no one has ever done anything to the lenses and he was the original owner. My Serenar 50f1.9 doesn't shift this badly so I have to assume it is something done to this lens. Any ideas where I should look?

 

[rover]

How does it work on a film camera?

 

[Greg_E]

The camera that it came with isn't in good enough shape to run film through yet, needs to be cleaned and try to kill the fungus that was in all the lenses and needs the rangefinder cleaned so I can see through it better.

I just checked several other lenses including my collapsible Serenar 50f1.9 and my Serenar 35f3.2 and Serenar 28f3.5, all set to f4 and all of them hit (within reason) the same spot on the scale when focused at the same target. I guess this means the 50f1.8 is working the way it should, but the shift is definitely more than I expected based on my other Serenar lenses. The 50f1.9 shifts a little less and in a more predictable manor, also all in one direction where the 50f1.8 seems to front focus at less than f4 and back focus at greater than f4.