What is the function of a high power objective of a microscope?

Most standard compound microscopes use a combination of an eye piece lens and objective lenses in order to magnify an object. In the majority of these microscopes, the eye piece lens will have a magnification factor of 10 x, or ten times the original size, and the objective lens system will consist of three lenses of varying focal lengths, described as low, medium and high power objective lenses.

The magnification factors (MF) of the three objective lenses are usually as follows:

• Low Power Objective (LPO) Lens, MF = 4 x
• Medium Power Objective (MPO) Lens, MF = 10 x
• High Power Objective (HPO) Lens, MF = 40 x

The total magnification possible with a standard compound microscope is the multiple of the magnification factor of the eye piece lens and the objective lens in use. In other words, total magnification using the LPO lens would be 40 times the original size, the MPO lens would achieve a total magnification of 100 times, while the HPO lens will achieve a total magnification of 400 times the original size of the object to be viewed.

The HPO lens is longer, and therefore closer to to the object viewed, resulting in a narrower field of vision, making it difficult to study living, fast moving organisms. It does, however, make it possible to see details which are not visible on lower magnification on non-living specimens or slow moving living organisms. The HPO lens allows users to zoom in on specific areas of an object to be viewed and identify details not visible on lower magnifications.

The function of the HPO lens on a microscope is therefore to provide maximum possible magnification in order to view and identify details which can not be discerned while viewing the object through an objective lens with a lower magnification factor.