Angle of view

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Angle of view 2.png

The angle of view is the decisive variable for the visual perception of the size or projection of the size of an object.

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Angle of view and perception of size

Angle of view 1.png

The perceived size of an object depends on the size of the image projected onto the retina. The size of the image depends on the angle of vision. A near and a far object can appear the same size if their edges produce the same angle of vision. With an optical device such as glasses or binoculars, microscope and telescope the angle of vision can be widened so that the object appears larger, which is favourable for the resolving power of the eye (see visual angle) [1] [2]

Angle of view in photography

A camera's angle of view can be measured horizontally, vertically, or diagonally. Angle of view.svg
A camera's angle of view can be measured horizontally, vertically, or diagonally.

In photography, angle of view (AOV) [3] describes the angular extent of a given scene that is imaged by a camera. It is used interchangeably with the more general term field of view.

It is important to distinguish the angle of view from the angle of coverage, which describes the angle range that a lens can image. Typically the image circle produced by a lens is large enough to cover the film or sensor completely, possibly including some vignetting toward the edge. If the angle of coverage of the lens does not fill the sensor, the image circle will be visible, typically with strong vignetting toward the edge, and the effective angle of view will be limited to the angle of coverage.

In 1916, Northey showed how to calculate the angle of view using ordinary carpenter's tools. The angle that he labels as the angle of view is the half-angle or "the angle that a straight line would take from the extreme outside of the field of view to the center of the lens;" he notes that manufacturers of lenses use twice this angle. Angle of View F V Chambers 1916.png
In 1916, Northey showed how to calculate the angle of view using ordinary carpenter's tools. The angle that he labels as the angle of view is the half-angle or "the angle that a straight line would take from the extreme outside of the field of view to the center of the lens;" he notes that manufacturers of lenses use twice this angle.
In this simulation, adjusting the angle of view and distance of the camera while keeping the object in frame results in vastly differing images. At distances approaching infinity, the light rays are nearly parallel to each other, resulting in a "flattened" image. At low distances and high angles of view objects appear "foreshortened". Camera focal length distance house animation.gif
In this simulation, adjusting the angle of view and distance of the camera while keeping the object in frame results in vastly differing images. At distances approaching infinity, the light rays are nearly parallel to each other, resulting in a "flattened" image. At low distances and high angles of view objects appear "foreshortened".

A camera's angle of view depends not only on the lens, but also on the sensor. Digital sensors are usually smaller than 35 mm film, and this causes the lens to have a narrower angle of view than with 35 mm film, by a constant factor for each sensor (called the crop factor). In everyday digital cameras, the crop factor can range from around 1 (professional digital SLRs), to 1.6 (consumer SLR), to 2 (Micro Four Thirds ILC) to 6 (most compact cameras). So a standard 50 mm lens for 35 mm photography acts like a 50 mm standard "film" lens on a professional digital SLR, but would act closer to an 80 mm lens (1.6 x 50mm) on many mid-market DSLRs, and the 40 degree angle of view of a standard 50 mm lens on a film camera is equivalent to an 80 mm lens on many digital SLRs.

Calculating a camera's angle of view

For lenses projecting rectilinear (non-spatially-distorted) images of distant objects, the effective focal length and the image format dimensions completely define the angle of view. Calculations for lenses producing non-rectilinear images are much more complex and in the end not very useful in most practical applications. (In the case of a lens with distortion, e.g., a fisheye lens, a longer lens with distortion can have a wider angle of view than a shorter lens with low distortion) [5] Angle of view may be measured horizontally (from the left to right edge of the frame), vertically (from the top to bottom of the frame), or diagonally (from one corner of the frame to its opposite corner).

For a lens projecting a rectilinear image (focused at infinity, see derivation), the angle of view (α) can be calculated from the chosen dimension (d), and effective focal length (f) as follows: [6]

represents the size of the film (or sensor) in the direction measured (see below: sensor effects). For example, for 35 mm film which is 36 mm wide and 24 mm high, mm would be used to obtain the horizontal angle of view and mm for the vertical angle.

Because this is a trigonometric function, the angle of view does not vary quite linearly with the reciprocal of the focal length. However, except for wide-angle lenses, it is reasonable to approximate radians or degrees.

The effective focal length is nearly equal to the stated focal length of the lens (F), except in macro photography where the lens-to-object distance is comparable to the focal length. In this case, the magnification factor (m) must be taken into account:

(In photography is usually defined to be positive, despite the inverted image.) For example, with a magnification ratio of 1:2, we find and thus the angle of view is reduced by 33% compared to focusing on a distant object with the same lens.

Angle of view can also be determined using FOV tables or paper or software lens calculators. [7]

Camera focal length vs crop factor vs angle of view.svg
Log-log graphs of focal length vs crop factor vs diagonal, horizontal and vertical angles of view for film or sensors of 3:2 and 4:3 aspect ratios. The yellow line shows an example where 18 mm on 3:2 APS-C is equivalent to 27 mm and yields a vertical angle of 48 degrees.

Example

Consider a 50 mm camera with a lens having a focal length of F = 50 mm. The dimensions of the 35 mm image format are 24 mm (vertically) × 36 mm (horizontal), giving a diagonal of about 43.3 mm.

At infinity focus, f = F, the angles of view are:

Derivation of the angle-of-view formula

Consider a rectilinear lens in a camera used to photograph an object at a distance , and forming an image that just barely fits in the dimension, , of the frame (the film or image sensor). Treat the lens as if it were a pinhole at distance from the image plane (technically, the center of perspective of a rectilinear lens is at the center of its entrance pupil): [8]

Lens angle of view.svg

Now is the angle between the optical axis of the lens and the ray joining its optical center to the edge of the film. Here is defined to be the angle-of-view, since it is the angle enclosing the largest object whose image can fit on the film. We want to find the relationship between:

the angle
the "opposite" side of the right triangle, (half the film-format dimension)
the "adjacent" side, (distance from the lens to the image plane)

Using basic trigonometry, we find:

which we can solve for α, giving:

To project a sharp image of distant objects, needs to be equal to the focal length, , which is attained by setting the lens for infinity focus. Then the angle of view is given by:

where

Note that the angle of view varies slightly when the focus is not at infinity (See breathing (lens)), given by rearranging the lens equation.

Macro photography

For macro photography, we cannot neglect the difference between and . From the thin lens formula,

.

From the definition of magnification, , we can substitute and with some algebra find:

Defining as the "effective focal length", we get the formula presented above:

where .

A second effect which comes into play in macro photography is lens asymmetry (an asymmetric lens is a lens where the aperture appears to have different dimensions when viewed from the front and from the back). The lens asymmetry causes an offset between the nodal plane and pupil positions. The effect can be quantified using the ratio (P) between apparent exit pupil diameter and entrance pupil diameter. The full formula for angle of view now becomes: [9]

Measuring a camera's field of view

Schematic of collimator-based optical apparatus used in measuring the FOV of a camera. FOV test Optics apparatus.svg
Schematic of collimator-based optical apparatus used in measuring the FOV of a camera.

In the optical instrumentation industry the term field of view (FOV) is most often used, though the measurements are still expressed as angles. [10] Optical tests are commonly used for measuring the FOV of UV, visible, and infrared (wavelengths about 0.1–20 μm in the electromagnetic spectrum) sensors and cameras.

The purpose of this test is to measure the horizontal and vertical FOV of a lens and sensor used in an imaging system, when the lens focal length or sensor size is not known (that is, when the calculation above is not immediately applicable). Although this is one typical method that the optics industry uses to measure the FOV, there exist many other possible methods.

UV/visible light from an integrating sphere (and/or other source such as a black body) is focused onto a square test target at the focal plane of a collimator (the mirrors in the diagram), such that a virtual image of the test target will be seen infinitely far away by the camera under test. The camera under test senses a real image of the virtual image of the target, and the sensed image is displayed on a monitor. [11]

Monitor display of sensed image from the camera under test FOV Target on Monitor.svg
Monitor display of sensed image from the camera under test

The sensed image, which includes the target, is displayed on a monitor, where it can be measured. Dimensions of the full image display and of the portion of the image that is the target are determined by inspection (measurements are typically in pixels, but can just as well be inches or cm).

= dimension of full image
= dimension of image of target

The collimator's distant virtual image of the target subtends a certain angle, referred to as the angular extent of the target, that depends on the collimator focal length and the target size. Assuming the sensed image includes the whole target, the angle seen by the camera, its FOV, is this angular extent of the target times the ratio of full image size to target image size. [12]

The target's angular extent is:

where is the dimension of the target and is the focal length of collimator.

The total field of view is then approximately:

or more precisely, if the imaging system is rectilinear:

This calculation could be a horizontal or a vertical FOV, depending on how the target and image are measured.

Lens types and effects

Focal length

How focal length affects perspective: Varying focal lengths at identical field size achieved by different camera-subject distances. Notice that the shorter the focal length and the larger the angle of view, perspective distortion and size differences increase. Focal length.jpg
How focal length affects perspective: Varying focal lengths at identical field size achieved by different camera-subject distances. Notice that the shorter the focal length and the larger the angle of view, perspective distortion and size differences increase.

Lenses are often referred to by terms that express their angle of view:

Zoom lenses are a special case wherein the focal length, and hence angle of view, of the lens can be altered mechanically without removing the lens from the camera.

Characteristics

For a given camera–subject distance, longer lenses magnify the subject more. For a given subject magnification (and thus different camera–subject distances), longer lenses appear to compress distance; wider lenses appear to expand the distance between objects.

Another result of using a wide angle lens is a greater apparent perspective distortion when the camera is not aligned perpendicularly to the subject: parallel lines converge at the same rate as with a normal lens, but converge more due to the wider total field. For example, buildings appear to be falling backwards much more severely when the camera is pointed upward from ground level than they would if photographed with a normal lens at the same distance from the subject, because more of the subject building is visible in the wide-angle shot.

Because different lenses generally require a different camera–subject distance to preserve the size of a subject, changing the angle of view can indirectly distort perspective, changing the apparent relative size of the subject and foreground.

If the subject image size remains the same, then at any given aperture all lenses, wide angle and long lenses, will give the same depth of field. [17]

Examples

An example of how lens choice affects angle of view.

28 mm lens, 65.5deg x 46.4deg Angleofview 28mm f4.jpg
28 mm lens, 65.5° × 46.4°
50 mm lens, 39.6deg x 27.0deg Angleofview 50mm f4.jpg
50 mm lens, 39.6° × 27.0°
70 mm lens, 28.9deg x 19.5deg Angleofview 70mm f4.jpg
70 mm lens, 28.9° × 19.5°
210 mm lens, 9.8deg x 6.5deg Angleofview 210mm f4.jpg
210 mm lens, 9.8° × 6.5°

Common lens angles of view

This table shows the diagonal, horizontal, and vertical angles of view, in degrees, for lenses producing rectilinear images, when used with 36 mm × 24 mm format (that is, 135 film or full-frame 35 mm digital using width 36 mm, height 24 mm, and diagonal 43.3 mm for d in the formula above). [18] Digital compact cameras sometimes state the focal lengths of their lenses in 35 mm equivalents, which can be used in this table.

For comparison, the human visual system perceives an angle of view of about 140° by 80°. [19]

Focal length (mm)Diagonal (°)Vertical (°)Horizontal (°)
0180.0180.0180.0
2169.4161.1166.9
12122.090.0111.1
14114.281.2102.7
16107.173.995.1
2094.561.982.4
2484.153.173.7
3563.437.854.4
5046.827.039.6
7034.419.528.8
8528.616.123.9
10523.313.019.5
20012.36.8710.3
3008.254.586.87
4006.193.445.15
5004.962.754.12
6004.132.293.44
7003.541.962.95
8003.101.722.58
12002.071.151.72
24-72mm zoom demo.jpg
Five images using 24, 28, 35, 50 and 72 mm equivalent zoom lengths, portrait format, to illustrate angles of view [20]
24-72mm zoom demo horizontal.jpg
Five images using 24, 28, 35, 50 and 72 mm equivalent step zoom function, to illustrate angles of view

Sensor size effects ("crop factor")

As noted above, a camera's angle of view depends not only on the lens, but also on the sensor used. Digital sensors are usually smaller than 35 mm film, causing the lens to usually behave as a longer focal length lens would behave, and have a narrower angle of view than with 35 mm film, by a constant factor for each sensor (called the crop factor). In everyday digital cameras, the crop factor can range from around 1 (professional digital SLRs), to 1.6 (mid-market SLRs), to around 3 to 6 for compact cameras. So a standard 50 mm lens for 35 mm photography acts like a 50 mm standard "film" lens even on a professional digital SLR, but would act closer to a 75mm (1.5×50 mm Nikon) or 80mm lens (1.6×50 mm Canon) on many mid-market DSLRs, and the 40 degree angle of view of a standard 50mm lens on a film camera is equivalent to a 28–35 mm lens on many digital SLRs.

The table below shows the horizontal, vertical and diagonal angles of view, in degrees, when used with 22.2 mm × 14.8 mm format (that is Canon's DSLR APS-C frame size) and a diagonal of 26.7 mm.

Focal length (mm)Diagonal (°)Vertical (°)Horizontal (°)
2162.9149.8159.6
4146.6123.2140.4
7124.693.2115.5
9112.078.9101.9
1296.163.385.5
1487.255.776.8
1679.649.669.5
1776.247.066.3
1873.144.763.3
2067.440.658.1
2458.134.349.6
3541.723.935.2
5029.916.825.0
7021.612.118.0
8517.810.014.9
10514.58.112.1
2007.64.26.4
2107.34.06.1
3005.12.84.2
4003.82.13.2
5003.11.72.5
6002.51.42.1
7002.21.21.8
8001.91.11.6

Cinematography and video gaming

Ratio1080p resolutionCommon nameVideo format / lens
32:271280x1080p DVCPRO HD
4:31440x1080p
16:91920x1080pWidescreen
2:12160x108018:9 Univisium
64:272560x1080pUltra-Widescreen Cinemascope / Anamorphic
32:93840x1080pSuper Ultra-Widescreen Ultra-Widescreen 3.6 / Anamorphic 3.6

Modifying the angle of view over time (known as zooming), is a frequently used cinematic technique, often combined with camera movement to produce a "dolly zoom" effect, made famous by the film Vertigo . Using a wide angle of view can exaggerate the camera's perceived speed, and is a common technique in tracking shots, phantom rides, and racing video games. See also Field of view in video games.

See also

Notes and references

  1. Light microscopy online theory and application - Optics page 24}}]
  2. Georg Eisner: Perspektive und Visuelles System page 134]
  3. Tim Dobbert (November 2012). Matchmoving: The Invisible Art of Camera Tracking, 2nd Edition. John Wiley & Sons. p. 116. ISBN   9781118529669.
  4. Neil Wayne Northey (September 1916). Frank V. Chambers (ed.). "The Angle of View of your Lens". The Camera. Columbia Photographic Society. 20 (9).
  5. "Canon EF 15mm f/2.8 Fisheye Lens Review". The-Digital-Picture.com. Archived from the original on 7 August 2017. Retrieved 1 May 2018.
  6. Ernest McCollough (1893). "Photographic Topography". Industry: A Monthly Magazine Devoted to Science, Engineering and Mechanic Arts. Industrial Publishing Company, San Francisco: 399–406.
  7. CCTV Field of View Camera Lens Calculations Archived 2008-08-22 at the Wayback Machine by JVSG, December, 2007
  8. Kerr, Douglas A. (2008). "The Proper Pivot Point for Panoramic Photography" (PDF). The Pumpkin. Retrieved 2014-03-20.
  9. Paul van Walree (2009). "Center of perspective". Archived from the original on 30 April 2009. Retrieved 24 January 2010.
  10. Holst, G.C. (1998). Testing and Evaluation of Infrared Imaging Systems (2nd ed.). Florida:JCD Publishing, Washington:SPIE.
  11. Mazzetta, J.A.; Scopatz, S.D. (2007). Automated Testing of Ultraviolet, Visible, and Infrared Sensors Using Shared Optics. Infrared Imaging Systems: Design Analysis, Modeling, and Testing XVIII,Vol. 6543, pp. 654313-1 654313-14
  12. Electro Optical Industries, Inc.(2005). EO TestLab Methadology. In Education/Ref. "Archived copy". Archived from the original on 2008-08-28. Retrieved 2008-05-22.CS1 maint: archived copy as title (link).
  13. Ray, Sidney F. (1 May 2018). Applied Photographic Optics: Lenses and Optical Systems for Photography, Film, Video, Electronic and Digital Imaging. Focal. ISBN   9780240515403 . Retrieved 1 May 2018 via Google Books.
  14. Lynne Warren, Encyclopedia of 20th century photography, page 211
  15. Langford, Michael (1 May 2018). Basic Photography. Focal Press. ISBN   9780240515922 . Retrieved 1 May 2018 via Google Books.
  16. 1 2 "Your Site". www.photographywebsite.co.uk. Retrieved 1 May 2018.
  17. Reichmann, Michael. "Do Wide Angle Lenses Really Have Greater Depth of Field Than Telephotos?". Archived from the original on 2011-06-10. Retrieved 2011-07-08.
  18. However, most interchangeable-lens digital cameras do not use 24×36 mm image sensors and therefore produce narrower angles of view than set out in the table. See crop factor and the subtopic digital camera issues in the article on wide-angle lenses for further discussion.
  19. Kollin, Joel S. (1993). A Retinal Display for Virtual-Environment Applications. Proceedings of Society for Information Display. XXIV. p. 827. Archived from the original on 2013-07-04. Retrieved 2014-04-27.
  20. The image examples uses a 5.1–15.3 mm lens which is called a 24 mm 3× zoom by the producer (Ricoh Caplio GX100 Archived 2009-06-01 at the Wayback Machine )

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For many cameras, depth of field (DOF) is the distance between the nearest and the farthest objects that are in acceptably sharp focus in an image. The depth of field can be calculated based on focal length, distance to subject, the acceptable circle of confusion size, and aperture. A particular depth of field may be chosen for technical or artistic purposes. Limitations of depth of field can sometimes be overcome with various techniques/equipment.

The focal length of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system's optical power. A positive focal length indicates that a system converges light, while a negative focal length indicates that the system diverges light. A system with a shorter focal length bends the rays more sharply, bringing them to a focus in a shorter distance or diverging them more quickly. For the special case of a thin lens in air, a positive focal length is the distance over which initially collimated (parallel) rays are brought to a focus, or alternatively a negative focal length indicates how far in front of the lens a point source must be located to form a collimated beam. For more general optical systems, the focal length has no intuitive meaning; it is simply the inverse of the system's optical power.

Camera lens

A camera lens is an optical lens or assembly of lenses used in conjunction with a camera body and mechanism to make images of objects either on photographic film or on other media capable of storing an image chemically or electronically.

Wide-angle lens

In photography and cinematography, a wide-angle lens refers to a lens whose focal length is substantially smaller than the focal length of a normal lens for a given film plane. This type of lens allows more of the scene to be included in the photograph, which is useful in architectural, interior and landscape photography where the photographer may not be able to move farther from the scene to photograph it.

Optical telescope Telescope for observations with visible light

An optical telescope is a telescope that gathers and focuses light, mainly from the visible part of the electromagnetic spectrum, to create a magnified image for direct view, or to make a photograph, or to collect data through electronic image sensors.

Field of view Extent of the observable world seen at any given moment

The field of view (FoV) is the extent of the observable world that is seen at any given moment. In the case of optical instruments or sensors it is a solid angle through which a detector is sensitive to electromagnetic radiation.

Fisheye lens

A fisheye lens is an ultra wide-angle lens that produces strong visual distortion intended to create a wide panoramic or hemispherical image. Fisheye lenses achieve extremely wide angles of view. Instead of producing images with straight lines of perspective, fisheye lenses use a special mapping, which gives images a characteristic convex non-rectilinear appearance.

Eyepiece Type of lens attached to a variety of optical devices such as telescopes and microscopes

An eyepiece, or ocular lens, is a type of lens that is attached to a variety of optical devices such as telescopes and microscopes. It is so named because it is usually the lens that is closest to the eye when someone looks through the device. The objective lens or mirror collects light and brings it to focus creating an image. The eyepiece is placed near the focal point of the objective to magnify this image. The amount of magnification depends on the focal length of the eyepiece.

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A digital single-lens reflex camera is a digital camera that combines the optics and the mechanisms of a single-lens reflex camera with a digital imaging sensor.

APS-C Image sensor format

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Crop factor

In digital photography, the crop factor, format factor, or focal length multiplier of an image sensor format is the ratio of the dimensions of a camera's imaging area compared to a reference format; most often, this term is applied to digital cameras, relative to 35 mm film format as a reference. In the case of digital cameras, the imaging device would be a digital sensor. The most commonly used definition of crop factor is the ratio of a 35 mm frame's diagonal (43.3 mm) to the diagonal of the image sensor in question; that is, CF=diag35mm / diagsensor. Given the same 3:2 aspect ratio as 35mm's 36 mm × 24 mm area, this is equivalent to the ratio of heights or ratio of widths; the ratio of sensor areas is the square of the crop factor.

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Image sensor format

In digital photography, the image sensor format is the shape and size of the image sensor.

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Sigma 8-16mm f/4.5-5.6 DC HSM lens

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