ZM 50mm C Sonnar focus shift

[FrozenInTime]

There's a note on popflash saying that post 2007 50mm Sonnars are optimized for f/1.5 on film cameras:

http://www.popflash.com/zeiss-lenses/zeiss-50mm-f/1.5-zm-c-sonnar-t-lens-black-usa-new/

All production since 1997 is focus optimized at f/1.5 for film based cameras to a FFD distance at 27.86mm.
this adjustment works well for film based cameras, but causes a noticeable shift on digital rangefinder cameras - since their FFD is longer than film-based M-mount cameras (basically they do not meet the standard M-mount spec). When the optimized lenses are tested with digital rangefinder cameras at a close focus distance of 0.9m and at f/1.5, they front focus by about 2cm
because of the differences between film and digital rangefinder cameras - which we cannot control - it is impossible to have a focus optimized C Sonnar for both systems.
for customers who want their lens adjusted for digital rangefinder cameras, Zeiss Germany have set the FFD to a distance of 27.90. This provides good results on digital cameras, but will cause a focus shift on film cameras for the reasons mentioned above
If an individual customer wishes to have their C Sonnar optimized for use with a digital camera, the customer can send the lens into Zeiss Germany for adjustment.
( I assume 1997 is an error and should be 2007)

But there are a lot of older threads suggesting otherwise.
I've not seen any tests done with film cameras, just the M8.
It was raining all day, so I thought I would do a test with the new lens I got from Santa - S/N 156032XX and PlusX M6x0.85.

All at 0.9m - the focus should be on issue 60 - the 10cm mark on the metal plate and the corner of the film box.
The scans are of the whole frame including rebate, but I lopped off the top third to save download time.

@f/1.5

@f/2.0

@f/2.8

@f/4.0

@f/5.6

@f/8.0

At f/1.5 the focus is to the front - issue 54
It looks like mine if optimal at f/2.0-2.8 - issue 58
At f/4 the focus has slipped a little back - issue 62
At f/5.6 is spot on issue 60.

I also did some 1.5m and 2m tests, but have yet to pull them together.

 

[mfogiel]

The focus shift does not float back - it is either optimal on issue 60 at f4.0, or on issue 62 at f5.6. These photos are not enlarged enough to discern. Look here:
http://www.rangefinderforum.com/for...51742&highlight=comparative+focus+test+sonnar

 

[FrozenInTime]

mfogiel, thanks for the link : comparing with your results, it looks like my lens is notionally optimized for f/2.8, which is in contradiction to the popflash statement.
The lens has the newer bayonet with the machined recess ring.
It looks like it's a mystery how or if Zeiss have changed the lens over the years.

Here are the close in shots, which probably show more clearly the center of sharpness shifting backwards as the lens stops down.

@f/1.5

@f/2.0

@f/2.8

@f/4.0

@f/5.6

@f/8.0

 

[Chriscrawfordphoto]

Yours is obviously one of the older ones that came optimized for f2.8, like the one I have. Your test results looks similar to mine. I prefer the 2.8 version as it is more usable at commonly used apertures than the f1.5 version. I mostly use mine at f2.8-f8 and the focus is very good at all those apertures on mine.