Page "Optical microscope" Paragraph 32

At the lower end of a typical compound optical microscope there are one or more objective lenses that collect light from the sample.

The objective is usually in a cylinder housing containing a glass single or multi-element compound lens.

Typically there will be around three objective lenses screwed into a circular nose piece which may be rotated to select the required objective lens.

These arrangements are 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.

Objective lenses with higher magnifications normally have a higher numerical aperture and a shorter depth of field in the resulting image.

Some high performance objective lenses may require matched eyepieces to deliver the best optical performance.