  There are different techniques to sense the wavefront phase-distortions due
to atmospheric turbulence. Curvature sensors are practical in their sensitivity
being adjustable to the prevailing atmospheric conditions. Even at the best
sites, the turbulence intensity has been found to vary at times over only a few
minutes and regularly over longer periods. Two methods to automatically adjust
the sensitivity of a curvature sensor are proposed: First, the defocus distance
can be adjusted prior to the adaptive-optics (AO) loop through the acquisition
of a long exposure image and can then be kept constant. Secondly, the defocus
distance can be changed during the AO loop, based on the voltage values sent to
the deformable mirror. We demonstrate that the performance increase - assessed
in terms of the image Strehl-ratio - can be significant.
