The image space f/# is the f/# of the system for a paraxial infinite conjugate -- that is for an object at infinity and the image plane at the focal length. For a system imaging finite conjugates the f/# differs. For an object at a finite distance the image side cone angle of the marginal rays decreases and the effective f/# increases. This is the working f/# for the system as-used. It is a better measure of the system as-used. You can find a good discussion of this is you follow the link in the help system for WFNO, which can be accessed conveniently by clicking the ? in the merit function editor tool bar. The help system is your friend!