Top: comparison between visibility functions for Gaussian, shown as a dashed curve, and power-law distributions where red, green, and purple curves correspond to the cases of R_1/2 = 6, 2, 1.3 R_in, respectively. For comparison, the visibility function of a thin ring with radius being equal to the Gaussian HWHM (i.e. R_1/2) is shown as a dotted curve. Bottom: the ratio of Gaussian HWHM to R_1/2 is plotted against the same spatial frequency in each case. The vertical dot-dash line corresponds to the spatial wavelength of 4.5 R_1/2.