Of course, the brightness differences will depend on how broad the line is and how accurate the red-shift actually is. The usual way of displaying magnitude differences is to produce a plot of magnitude difference against some reference magnitude. The reference magnitude should not be biased thus it is usual to use an average magnitude involving the magnitude differences. Fixed aperture photometry is generally preferred for this analysis as it is less biased than isophotal magnitudes. Smaller apertures give more sensitivity to faint objects but increase the errors.
Large apertures can give inaccurate results in crowded fields.
A 6 pixel aperture was used, corresponding to about twice the seeing disk. There are several errors which occur in the estimate of the magnitudes which are vital to knowing how real any difference might be. However, there are extra errors which often dominate the Poisson noise. Secondly, the process of deblending which allows Sextractor to work so well can add an extra error. It is therefore wise to assume that the errors given by Sextractor are accurate.