The image space f/# for a lens requires the object to be at infinity; and is defined as 1/(2n*sin(θ))=1/(2*NA)=EFL/EPD where n is the refractive index in image space, θ is the paraxial marginal ray angle and NA is the numerical aperture of the lens. The working f/# is meant to be used for finite conjugate systems, where the object is not located at infinity. The definition of the working f/# looks very similar, working f/# = 1/(2n*sin(θ)); however, since the incoming rays are not collimated, the angle will depend on the defined conjugates. It is also important to note that, for the working f/#, θ is the angle of the real marginal ray in OpticStudio. Sometimes, you may find people use the paraxial ray angle in the definition (more correctly called the paraxial working f/#), and because there is some variation in how the term “working f/#” is used, it is always good to check the exact definition that is being used. In OpticStudio, you can always refer to the Conventions and Definitions section of the help file to be sure of the exact definitions used by the software.