A field lens or collective lens is a convex lens that can be attached in optical instruments with an intermediate image at the location of the intermediate image in order to image the exit pupil of the previous stage onto the entrance pupil of the following stage. This means that it is not necessary to increase the diameter of the subsequent step in order to avoid vignetting or even a restriction of the field of view . The image is not changed by the field lens if it is exactly at the location of the intermediate image.
Multi-level systems with an intermediate image are, for example, telescopes and microscopes . They combine the field lens and the eyepiece lens (the next step) into one component, the eyepiece , which was first carried out around 1850 with the development of the Kellner eyepiece . The condenser lens is a special field lens. It shows the illuminated surface of the lamp enlarged on the lens that projects the object (for example a slide ). As a result, its entrance pupil is enlarged, which improves the image brightness.
The illustration shows an astronomical telescope with and without a field lens. In both cases, the beam paths of the top and bottom just visible image points are shown. Without a field lens, only a small circular area of the intermediate image is fed to the eye, which is also vignetted (not sharply delimited). He has the impression of tunnel vision. In order to make the entire intermediate image accessible to the observer, a field lens is attached to the intermediate image. As a result, rays from the outer part of the intermediate image are also directed in the direction of the eyepiece and ultimately enlarge the field of view.
A telescope with a field lens and low magnification is called a periscope . It is used in submarines as a periscope in order to be able to look undetected over the water surface during the dive. With the help of field lenses it is achieved to keep as large a section of the horizon as possible in the field of view.