  Context. Solar Shack-Hartmann wavefront sensors measure differential
wavefront tilts as the relative shift between images from different
subapertures. There are several methods in use for measuring these shifts.
Aims. We evaluate the inherent accuracy of the methods and the effects of
various sources of error, such as noise, bias mismatch, and blurring. We
investigate whether Z-tilts or G-tilts are measured. Methods. We test the
algorithms on two kinds of artificial data sets, one corresponding to images
with known shifts and one corresponding to seeing with different r_0. Results.
Our results show that the best methods for shift measurements are based on the
square difference function and the absolute difference function squared, with
subpixel accuracy accomplished by use of two-dimensional quadratic
interpolation. These methods measure Z-tilts rather than G-tilts.
