ZM C Sonnar focus shift tests: variations in 1.5 optimized lenses

Richard G · 04-28-2012

See my test on photo.net and comparison with m.fogiel's here on RFF. I seem to have fluked a copy that is useable across wider apertures, whereas Marek's 1.5 optimized lens was back focussed by 1cm at 1.5 and by 6cm at f4.

http://photo.net/leica-rangefinders-forum/00aKSW

BobYIL · 04-28-2012

I once have tested both my chrome and black C-Sonnars for focus shifting; both are optimized for f1.5. Both backfocused by the same amount.

The amount of backfocusing is related to the focusing distance. I have no idea what focusing distance Marek chose but his lens backfocuses 4.5cm between the extremes; my lenses too backfocused about 5cm when focused at around 1m (can't remember the exact distance as the point in my test was to compare the two versions on the same tripod spot).

Now the comparison: Mine was 5cm for a little longer than the min. focusing distance. Marek's lens shows 6-1.5 = 4.5 cm between the extemes. And finally to my eyes your lens show a shift from 18.5cm to around 23cm.. again close to 4.5cm.

Optimization aperture (as a physical rule) can not have anything to do with the amount of focus shift due to spherical aberrations; it only "locates" the reference distance the focusing roller to follow on the focus cam on the back side of the lens; i.e. the location of the whole glass group with respect to the focusing ring of the lens. Actually what does Zeiss AG do to optimize at f1.5 of a lens a optimized for f2.8? Nothing but adding a thin shim ring on the back side of the lens group. The formula and the characteristics never change.

Richard G · 04-28-2012

Thanks BobYIL. Marek focussed at one meter in his tests. His 1.5 optimized lens started back focussed slightly and moved further back. Mine has maximum focus in front of the rangefinder focus point at 0.9m and moves backward as i stop down. As you say, the range is the same, but the usefulness of straddling the rangefinder focus point is probably why I have had no problems with the lens in practical use.

My previous experiments were too inexact to draw any conclusions from but this one is useful to me. I guess anyone owning this lens who has noticed problems might benefit from a similar examination. If there are no problems in practical use I wouldn't recommend looking into it.