An exact expression is derived for the Strehl ratio as a function of the mi
nimum root-mean-square aberration value that may produce it. The result is
applicable to any aperture and is shown to imply a similar expression for t
he Strehl ratio as a function of the minimum amplitude range of the aberrat
ion. (C) 2000 Optical Society of America [S0740-3232(00)00302-1] OCIS codes
: 080.1010, 110.3000.