CCD-shutters take some time to open and close. This results in a difference between the recorded and real exposure times. The pixel-to-pixel pattern δ(x, y) in which this difference varies depends on the characteristics of the device. These timing errors affect photometric measurements in two ways: directly, since the recorded exposure time is not the real one, and indirectly, via the flatfielding process, because the shutter effect is mixed up with the flatfield pattern. The direct effect may be negligible when exposure times are long. The indirect effect is present irrespective of the exposure times of the frames, if flatfield exposure times are short.