Leica S2 Instructions Manual page 85

AF mode
In AF mode, the AF/AE lock button can be assigned the
following functions:
AF-L - Autofocus lock (AFs / AFc)
AE-L - Exposure value lock
AF-L on / AE-L - Autofocus (AFs / AFc) and exposure value
lock
8. Optimization of AF performance - dynamic AF metering
field width
We have been able to further increase the metering accuracy
of the autofocus function. In particular, very small image
details can now be focused better. The metering point is
located at the center of the viewfinder cross.
Firmware Update 1.0.0.24 / 5