Optimal aperture radii, R_opt, versus the F475W cluster magnitude measured in the B0 field. Top panel: R_opt for measuring the F475W magnitude. Middle panel: F336W−F475W. Bottom panel: F475W−F814W. Results are shown for the B3 field using three combinations of geometric parameters, r_c and γ (see legend, where approximate median half-light radii, R₅₀, are also included for reference). Markers show the R_opt median values. Error bars indicate the 16th–84th percentile ranges. Dashed lines represent second-order polynomial least-squares fits.