The objective lens is a critical part of the microscope optics. The microscope objective is positioned near the sample, specimen, or object being observed. It has a very important role in imaging, as it forms the first magnified image of the sample.

• The objective forms the first magnified image of the sample

The numerical aperture (NA) of the objective indicates its ability to gather light and largely determines the microscope’s resolution, the ability to distinguish fine details of the sample.

Objectives are categorised into performance classes on the basis of their transmission, chromatic correction, planarity, and others. From the point of view of the International Organization of Standardisation (ISO), there are three groups of objective classes differing in quality of chromatic correction (Achromats, Semi-Apochromats and Apochromats).