A method for maintaining focus of an optical system on a target surface is provided. The method includes directing at least one beam of light through an objective lens to a sample, wherein the at least one beam of light is associated with at least two effective numerical apertures and receiving at least one reflected beam at the objective lens. The at least one reflected beam is associated with the at least two effective numerical apertures. The method also includes determining at least one image based on the at least one reflected beam, determining a baseline of a feature for each of the one or more objects, and when the feature differs from the baseline, adjusting a working distance between the objective lens and the sample.