Fraction of the total source flux density that can be detected on long baselines for the photon ring model described in Section 3.1, shown here for a ϑ = 40 μas diameter. The gray curve shows Equation (17), and the dashed black line tracks the envelope of this function. The red curve shows a running average of the gray curve across a 2% fractional observing bandwidth, and the dashed blue curve shows Equation (19). The vertical cyan lines show the resolution criteria used for the shadow (θ_r = ϑ) and for the first two orders of photon ring (θ_r = 2w_n−1).