Object lens

In optics, an objective is the lens or mirror in a microscope, telescope, camera or other optical instrument that gathers the light coming from the object being observed, and focuses the rays to produce a real image. 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 5× to 100× 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, an oil-immersion objective or water-immersion objective has to be used. The objective is specially designed and refractive index matching oil or water must fill the air gap between the front element and the object to allow the numerical aperture to exceed 1, and hence give greater resolution at high magnification.
Numerical apertures as high as 1.6 can be achieved with oil immersion. To find the total magnification of a microscope, one multiplies the magnification of the objective lenses by that of the eyepiece.