Focal Length and f/Number
For gratings (or grating systems) that image as well as diffract light, or disperse light that is not collimated, a focal length may be defined. If the beam diffracted from a grating of a given wavelength λ and order m converges to a focus, then the distance between this focus and the grating center is the focal length r'(λ). If the diffracted light is diverging, the focal length may still be defined, although by convention we take it to be negative (indicating that there is a virtual image behind the grating). Similarly, the incident light may diverge toward the grating (so we define the incidence or entrance slit distance r(λ) > 0) or it may converge toward a focus behind the grating (for which r(λ) < 0). Usually gratings are used in configurations for which r does not depend on wavelength (though the focal length r' usually depends on λ).