This paper disccuses the general formulae and normalisation factor of line spread function and the edge. An appsoximation equation specified by Ω

which is a function of the wavefront variation

of the Strehl ratio of the LSF

is given. The relationship between wavefront coefficients and Ω which is divided info even part Ω

E

and odd part Ω

0

for the symmetrical aperture and for rotational symmetrical optical system

especielly for circular aperture

is investigated. Then use of Ω

E

to determine the best focal plane and Ω

0

to get the position of the maximum value of LSF is described. Finally some examples about the aberration tolerances in some cases are showen.