Working F/# is defined as,W= 1/(2n sinθ) -1$z=,q'
where θ is the real marginal ray angle in image space and n is the index of refraction of image space. The marginal ray is traced at the specified conjugates. Working F/# ignores surface apertures but considers vignetting factors. sJQ~:p0e
Read the manual,you will learn more.