Field lens

Multi-stage systems with intermediate image are as telescopes and microscopes. In these field lens and eyepiece lens are combined ( the next stage ) to a component, the eyepiece. The condenser is a special field lens. It is the illuminated area of the lamp increases from the lens that projects the object (for example a slide). Thus the entrance pupil is enlarged, so that the image brightness is improved.

The figure shows an astronomical telescope with and without a field lens. In both cases, the optical paths of the top and bottom just barely visible pixels are drawn. Without field lens of the eye, only a small circular area of the intermediate image is fed to the vignetted also (not sharply defined ) is. He has the impression of a tunnel vision. To make the entire intermediate image to the observer accessible, a field lens is placed at the intermediate image. Characterized also radiation from the outer part of the intermediate image are directed in the direction of the eyepiece, and eventually increase the field of view.

A telescope with a field lens and low magnification called periscope. It takes place in submarines as a periscope use to look unnoticed during the dive over the water surface can. With the help of field lenses is achieved thereby to retain as much of the horizon in view.