The objective lens is the main microscope lens used to magnify tiny objects or examine minute specimens that are a part of larger objects that are otherwise invisible to the naked eye. Other lenses simply provide additional fine focus or illumination.
Most microscopes have four objective lenses, and each provides a varying level of magnification. The shortest objective possesses the least power (4X), and is called the scanning objective. It is followed by the low power objective (10X) and the high power objective or "high-dry" objective (40X). The longest objective, which is also the strongest, is the oil immersion objective (100X). The maximum magnification potential of an objective lense is typically determined by its distance from the image plane and the specimen that's being observed.