Working F/# is defined as,W= 1/(2n sinθ) 7h/{F({r=  
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. O4 	Y;  
Read  the manual,you will learn more.