Objective (optics)

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 has to be used. The objective is specially designed and refractive index matching oil must fill the air gap between the front element and the object to allow numerical aperture to exceed 1, and hence give greater resolution at high magnification.