... Although the simulations have been performed for WFPC II and FOS, the results are expected to apply to the FOC and GHRS, respectively. ... The variation of Strehl ratio with focus for eight wavelengths varying from 125 nm to 700 nm for the cameras is plotted in Fig. ... Fig. 4 gives the tolerable focus error for the cameras as a function of wavelength (using the 20%Strehl ratio degradation criterion), and Fig. 5 the tolerable focus error for the spectrographs. ...