Working F/# is defined as,W= 1/(2n sinθ) 9'M({/7y
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. #a|5A:g%
Read the manual,you will learn more.