Phase estimates in adaptive-optics systems are computed by use of wavefront sensors such as Shack-Hartmann or curvature sensors. In either case the standard error of the phase estimates is proportional to the standard error of the measurements; but the error-propagation factors are different. We calculate the ratio of these factors for curvature and Shack-Hartmann sensors in dependence on the number of sensors, n, on a circular aperture. If the sensor spacing is kept constant and the pupil is

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... larged, the ratio increases as n^0.4. When more sensing elements are accommodated on the same aperture, it increases even faster, viz. proportional to n^0.8. With large numbers of sensing elements this increase can limit the applicability of curvature sensors.