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 by combining the first images or signals obtained for each individual aperture in such combination; 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, wherein determining an optimum one of the combinations of the first apertures includes selecting a set of one or more individual apertures that result in the highest contrast for the defect area; and setting the optimum combination of the first apertures on the inspection tool and inspecting a sample using such optimum combination of the first apertures.