The magnifying lens which is situated at the base of a microscope tube immediately above the specimen. Objectives may be described in terms of lens quality (e.g. achromat, apochromat, or fluorite), magnification (which may be 5×, 10×, 20×, or larger), and numerical aperture (NA) which is a measure of the amount of light converging on the specimen through the lens. Its value is normally between 0.1 and 0.9.