1. A method for optimizing a mode of an inspection tool, comprising: obtaining a first image or signal for each of a plurality of first apertures of the inspection tool, wherein each first image or signal pertains to a defect area; for each of a plurality of combinations of the first apertures and their first images or signals, obtaining a composite image or signal; and analyzing each composite image or signal to determine an optimum one of the combinations of the first apertures based on a defect detection characteristic of each composite image.