OBJECTIVE (OPTICS)

An 'objective' in optics is the lens or mirror in a microscope, telescope, camera or other optical instrument that receives the first light rays from the object being observed. The objective is also called the 'object lens', 'object glass', and 'objective glass'.
Microscope objectives are typically designed to be parfocal, which means that when one changes from one lens to another on a microscope, the sample stays in focus. Microscope objectives are characterized by two parameters, namely, magnification and numerical aperture. The former typically ranges from 5X to 100X while the latter ranges from 0.14 to 0.7, corresponding to focal lengths of about 40 to 2 mm, respectively. For high magnification applications, oil-immersion objective has to be used. The objective is specially designed and has to be immersed in refractive index matching oil (an index-matching material). Photographic zoom lenses may also be parfocal, so that one can change the magnification without having to refocus.
they also focus in on the sample