Telescope mirrors suffer the least loss of quality by elastic fiexure if the supporting forces are balanced against each other in an optimum manner. There are unresolved differences of opinion whether favorable support conditions for large mirrors should be synthesized from requiring minimum rms values of wave aberrations or of angular ray deviations. Results are presented of a comparative theoretical study for mirrors on radial supports. The two hypotheses are checked against modulation transfer curves. Even for nondiffraction4imited mirrors up to 7-rn diameter, the wave-optical approach leads to a better balance of the supporting forces than assessment by angular ray deviations. Similar conclusions are likely to apply to axial supports if the corresponding fiexure is of the same order of magnitude. An estimate is included of the effect of air turbulence on the assessment of various types of fiexure. It serves for determining misas- sessment factors for both approaches. This is no substitute, however, for more elaborate tests of final image quality.