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Field Lenses

Definition: lenses in imaging instruments which affect the field of view

German: Feldlinsen

Category: vision, displays and imaging

How to cite the article; suggest additional literature

If in an imaging system a lens is used to increase the field of view, that lens is called a field lens. That term does not imply any special form of lens, but only refers to its function.

Example: Field Lens in a Telescope

We first consider a Keplerian telescope, which consists of two lenses: an objective and an ocular (eyepiece). It provides some angular magnification for viewing distant objects according to the ratio of focal lengths of objective and ocular. For a large magnification, one will thus need to use an ocular with small focal length, which will in practice limit the size of the ocular, since the f-number cannot be arbitrarily small. The problem is then that the diameter of the ocular limits the field of view. Even for moderate viewing angles, some of the rays cannot enter the ocular any more, so that one has some vignetting, as can be seen in Figure 1.

Keplerian telescope
Figure 1: A Keplerian telescope with input rays from two distant objects: one on the optical axis (blue lines) and one which is slightly off-axis. In the latter case, some of the rays cannot enter the ocular any more, so that there is a vignetting effect. The simulation has been done with the software RP Resonator.

This problem can be cured by inserting an additional lens (a field lens) in the intermediate image plane (see Figure 2). The rays for the on-axis object point are not affected by the field lens, because they go through its center. For the off-axis object point, the rays are bent towards the ocular – all by the same amount, because they all go through the same point on the field lens –, so that they can now get through the ocular. Thus, the vignetting effect is avoided. The field lens can help to expand the field of view of the telescope.

telescope with a field lens
Figure 2: Modified telescope with a field lens, which bends the rays from off-axis object points such that they can get through the ocular. Effectively, the field of view is increased.

Interestingly, the field lens (when placed in the intermediate image plane) does not modify the magnification. Also, it can be shown that the combination of field lens and ocular lens has the same focal length as the ocular lens alone.

A general problem with a field lens arises from the fact that it is placed in an image plane: its details can then appear in the generated image. For example, scratches on a lens surface or dust particles will be visible – while for other lenses (e.g. the outer surface of the objective) they may not have any noticeable effect on the image, because they are far away from an image plane. In order to mitigate that problem and/or in order to place some other optical element (e.g. a reticle) in the image plane, a field lens is sometimes slightly displaced from the image plane. There, it may still perform essentially the same function, while still not having a substantial influence on the image magnification.

Another problem with field lenses can be that they reduce the eye relief, i.e., the distance of the exit pupil from the eye lens.

The field lens can be integrated into the ocular. The second lens of the ocular may then be called the eye lens.

Field Lens in an Illumination System

In the illumination system of an overhead projector, for example, one often uses a condenser lens just before the object to be imaged (e.g. a slide). While that lens does not significantly modify the diameter of the illuminated object area, it serves to bend the light rays coming from the light source such that they stay within the imaging system, and a brighter image is obtained. The condenser lens is then also called a field lens.

See also: imaging, lenses, field stops
and other articles in the category vision, displays and imaging

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