field of view (original) (raw)

Author: the photonics expert (RP)

Acronym: FOV

Definition: the angular extent of the field which can be observed with an optical instruments or the eye

Alternative term: angle of acceptance

Category: vision, displays and imaging

Related: imagingtelescopesmicroscopesfield stopsfield lensesvignetting

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DOI: 10.61835/459 Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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Contents

What is a Field of View?

The field of view of a telescope or a photo camera, for example, is usually understood as the range of angular directions in which objects can be observed for a fixed orientation of the instrument. (One may of course observe a wider range by combining images from different instrument orientations.) It can be quantified in different ways:

• One may specify the full angle (or half angle) of view in the horizontal, vertical and diagonal directions. Note that the field of view can have different shapes, e.g. circular or rectangular.
• Alternatively, one may specify a solid angle.

The reference point for the mentioned angles is the center of the entrance pupil.

The term field of view is not only used for that field itself, but also for its magnitude. In some cases, for example for telecentric lenses, it is used for the viewed area on the object plane.

In some cases, the term angle of acceptance is used instead of field of view — particularly for non-imaging optical instruments.

The field of view of an optical imaging instrument is often limited by an intentionally created field stop. This is an optical aperture, e.g. in the form of a diaphragm, which is placed in an image plane or close to such a plane, such that the edges of the field of view are sharply defined. However, in some cases one obtains vignetting effects, i.e., a gradual decrease of image brightness towards the edges of the field of view. This happens when the field of view is limited by an aperture which is not in an image plane. An example of that situation can be found in the article on field lenses, where vignetting in an optical telescope is explained. In cases with vignetting, one may define different values for the field of view — for example, the field without any vignetting, the field up to the point of half-vignetting or (as the very maximum) up to the point where the intensity vanishes.

The field of view of the human eye is also not precisely defined. Maximum image resolution is only achieved in the central area, and the peripheral regions exhibit a substantially lower image quality. This is largely compensated by rapid eye movements for covering a wider angular range and accurately viewing objects of particular interest.

As mentioned above, the field of view of an instrument is often intentionally limited. This is often not motivated by its application, but rather because the image quality would be too much degraded by optical aberrations when permitting a larger field of view. Advanced optical designs, e.g. based on aspheric lenses or more refined combinations of lenses, can offer a wider field of view with good image quality. However, they are not always employed, e.g. for reasons of higher cost, size or weight of the instruments.

Field of View of Photo Cameras

The field of view of a photo camera depends on both the used photographic objective and the size of the photographic film or the image sensor. This is explained in Figure 1, which shows the optical configuration in a greatly simplified way: The objective is represented by a single lens, although it is usually a system containing multiple lenses. One can simply consider rays coming from object points and going through the center of the lens, where no ray deflection occurs.

Figure 1: The field of view depends on the sensor size and the focal length of the lens.

For small angles, the full-angle field of view in radians is approximately the sensor diameter divided by the focal length; for obtaining a value in degrees, one has to multiply that by ($180\textdegree / \pi$). For wide angle cameras, one has to use a more accurate formula: {\rm{FOV}} = 2\;\arctan \frac{{w/2}}{f}$$

where ($w$) is the sensor width.

The field of view gets particularly small when using a tele-objective, while wide-angle objectives are by definition made for a large field of view. Extreme versions are called fish-eye objectives; they produce substantial image distortions, which are hardly avoidable in that regime.

Standard photographic objectives are made such that their field of view is similar to that of the human eye — with a full horizontal angle around 50°, when considering the range with reasonably sharp imaging.

When changing the zoom setting of an objective, the field of view will change, of course.

Field of View of Telescopes

Telescopes provide some magnification for viewing distant objects. The higher the magnification, the smaller the field of view typically is. However, there are optical designs which provide a larger field of view for given magnification. For example, a simple Keplerian telescope has a small field of view, which can be expanded by inserting an additional field lens.

A large field of view is particularly relevant, for example, for astronomical telescopes which are used in stellar surveys.

Field of View with Viewing Aids and Eye Protection

The field of view of the eye may be restricted by various kinds of devices, for example by correction glasses of small size or by magnifying glasses. In some cases, the peripheral view is completely blocked, e.g. with some laser safety glasses; that can introduce additional hazards, e.g. of bumping into items which could not be seen.

Frequently Asked Questions

What is the field of view?

The field of view of an optical instrument, such as a telescope or camera, is the range of angular directions in which objects can be observed. It is typically quantified with an angle or as a solid angle.

How is the field of view of a camera determined?

A camera's field of view depends on the focal length of its objective and the size of its image sensor. A shorter focal length or a larger sensor results in a wider field of view.

What limits the field of view in an optical instrument?

The field of view is often intentionally defined by a field stop to create sharp edges. In practice, the usable field of view is frequently limited by optical aberrations, which degrade image quality at large angles.

How are magnification and field of view related in a telescope?

For a telescope, there is typically an inverse relationship between magnification and field of view. The higher the magnification used to view distant objects, the smaller the resulting field of view.

What is vignetting?

Vignetting is the gradual decrease of image brightness towards the edges of the field of view. This effect occurs when the limiting aperture is not located in an image plane.

What is an angle of acceptance?

The term angle of acceptance is often used as a synonym for field of view, particularly for non-imaging optical instruments, where it defines the angular range from which the instrument can accept light.

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