My Sonnar ZM shows a little bit of front focus but with some practice the results are stunning. I would like to thanks mfogiel for the help and advices and I agree with willie 901. I like mine as it is. Here the answer arrived today from Germany to my question about recalibrate or not the lens:
If the C Sonnar T* 1,5/50 ZM should predominantly be used at full aperture, in a few cases a recalibration of the lens to f/1.5 might be necessary to minimize focus shift at full aperture. Together with this calibration to f/1.5 comes an increase of the focus shift at f/2.8 and smaller, but this will be covered by the depth-of-field in most cases.
Because the results of the focus shift of the C Sonnar in practical use depend on different factors (e.g. on the calibration of the flange focal distance and rangefinder of the camera body in use, the preferred distances and f stops, the 3-dimensionality of the subject etc), we strongly recommend to make individual test shots with a C Sonnar before you decide to send it in for recalibration.
In most cases, a recalibration of the lens is not necessary.
If you´d like to get your lens recalibrated, you have the following alternatives:
-return it to the dealer where you purchased it, so he can send in the lens to us
-send it to our distributor in the country of purchase
-send it to us (Carl Zeiss Germany), if you have an European warranty card that came with your lens
The recalibration is free of charge as long as the lens is covered by warranty. Therefore, the filled out original warranty card and a copy of the invoice have to be provided.
If you´d like to send the lens directly to us, please fill out the following form and add it to your shipment...