In its classic version, the microscope is composed of 3 optical elements: the objective, the eyepiece and the field lens.
The objective collects the light diffused by the image and forms an intermediate image. This image is a magnified image of the object, containing the details to be observed.
The eyepiece serves as magnifying lens, helping the eye to see the intermediate image produced by the objective.
With these two lenses alone - the objective and the eyepiece - the microscope already carries out its functions. But one problem still remains. Some of the rays coming from the peripheral areas of the object (the "field" to be observed) are lost, not intercepted by the eyepiece or by the pupil of the eye. The useful field remains too small and is not well lighted. (Fig. 2)
The field lens serves the function of correcting this problem, by deviating the rays so that they are all intercepted by the eyepiece, and enter into the pupil of the eye. (Fig. 3)