Numerical aperture (abbreviated N.A. or NA) is a number which refers to an objective lens’s ability to gather light, and to its resolution – its ability to clearly show fine details in objects being viewed.
In general, the higher the numerical aperture, the brighter and clearer the images the objective will produce. This is only generally true, because other components of the microscope also affect brightness and resolution, including the type of illumination used, and components used to transmit the illumination to the specimen and to the objective lens.
Because of these different factors, it is possible for a microscope to have an objective with a very high numerical aperture, but to be unable to make effective use of this otherwise desirable feature. Therefore it is important that an instrument’s various components be well integrated – that is, that they will work well together to provide the best possible viewing.
Many higher-magnification objectives are designed to be used with oil immersion, effectively increasing their numerical aperture. Oil immersion means that a small drop of a special oil is placed on the coverslip, directly over the specimen to be viewed. The objective lens is carefully brought into contact with the oil, so that the oil fills the gap between coverslip and lens. Because of the oil’s optical properties, a higher numerical aperture is obtained than the objective would have if no oil were used.
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