Illustration of the curve-of-growth analysis described in Sect. 2.2, here applied to the 2MASS H-band image of simulated galaxy TNG 000001 in orientation O1. The 2D surface brightness distribution is fitted with a series of concentric ellipses, and the total integrated flux within each ellipse is determined. The effective radius is the semi-major axis of the ellipse that contains half of the total flux.