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Writing an isophote's radial residual about its fitted ellipse as $\delta r=a_4\cos 4\theta +\cdots$, positive $a_4$ extends the major and minor axes and gives a pointed, disky shape; negative $a_4$ extends diagonal directions and gives a boxy shape. The sign stated here is for radial residuals, not an unconverted brightness residual. Massive slowly rotating elliptical galaxies commonly have boxy isophotes, while embedded rotating disks commonly make lower-mass elliptical galaxies disky.

An isophote is a contour of equal surface brightness in an astronomical image. Deviations from fitted ellipses encode additional information about a galaxy's projected stellar structure.

Surface brightness is received flux per unit solid angle on the sky. It describes the brightness of an extended source independently of a particular aperture's area. An isophote is a contour of constant surface brightness; central surface brightness sets the available photons for resolved galaxy spectroscopy.