Ghost spot

Ghost spots in a dusty attic (flash shot). Focal length 7 millimeters, sensor diagonal 12.7 millimeters, f-number 2.4

Ghost spot in the Bruchwald (daylight photo)

Dark ghost spots caused by flying insects at different object distances that absorb the blue of the sky and are blurred.

Ghost spots are diffuse , glowing, more or less circular discs in photographic images . In English-speaking countries, these spots are often referred to as orbs . It is not a question of an imaging error in the imaging system, but rather a blurring artifact , mostly when taking pictures with flash .

description

The light of the flash is of particles between motif and camera float, scattered and partly reflected back. The blurring caused by proximity and focus results in coarse disk-shaped light images. The image of the particles is much larger than the particle itself. The shape is based on the shape of the respective aperture . The actual cause of these scattering centers is often dust , but the effect can also be caused by raindrops , snowflakes , insects or other small objects near the camera.

The greater depth of field required in digital compact cameras due to the smaller image sensors compared to the analog 35mm format and the associated shorter focal lengths results in images in which these particles are depicted with a sufficiently small circle of confusion and thus bright enough. In full-format or medium-format photography , the images of the scattering centers are often so large due to the shallow depth of field that they can hardly be seen due to their low brightness.

The following observations are typical for ghost spots:

• The closer the scattering centers are in front of the camera or the more blurred the image, the larger the lenses become.
• The ghost spots become smaller, the smaller the aperture or the focal length of the camera is set, corresponding to the increased depth of field associated with it.
• As a rule of thumb, the brightness of the ghost spots is largely independent of the distance in the case of quasi-punctiform scattering centers, since the brighter scattered light from closer scattering centers is distributed over a larger, blurred area. This contradicts the usual calculation, according to which the brightness of objects irradiated with flash light decreases quadratically with their distance.
• When the focal length is doubled, the diameter of the ghost spots becomes four times as large with the same distance between the scattering center and the same aperture.
• When taking pictures without a flash, the scattering centers are usually not recognizable, as flash is the most common cause. Longer exposure times without artificial lighting reduce the probability, although other light sources can also be considered as secondary causes, such as lamps, the sun, etc.
• The use of lens hoods cannot reduce the occurrence of ghost spots .
• The smaller the image or the image sensor in a camera, the smaller and brighter the image of a ghost spot will be with the same image angle (image section) and with the same f-number .

Theoretical background

Relationship between the circle of confusion diameter Z in the image plane (violet), diameter D of the entrance pupils (black), focal length f (blue rays for objects at infinity) and image distance b (green rays for objects with finite object distance)

The size of a ghost spot can be estimated very easily when the camera is focused at infinity; the object distance of objects depicted in point form is then also infinite. The diameter Z of the circle of confusion from a point imaged from the finite is obtained with the help of the lens equation and the geometric relationship

${\displaystyle {\frac {Z}{b-f}}={\frac {D}{b}}}$, where f is the set focal length , D is the size of the entrance pupil and b is the image distance ,

then to:

${\displaystyle Z={\frac {f^{2}}{k\cdot x}}}$, where k is the set f-number and x is the distance from a point that is out of focus.

A point-like light source at a distance x in front of the main plane of the imaging lens is thus imaged as a luminous disk with a diameter Z, the diameter of which is inversely proportional to the distance from the scattering center - if the distance from the scattering center is ten times greater, the ghost spot becomes ten times smaller under otherwise constant conditions .

The diameter of the corresponding ghost spots increases quadratically with the focal length used and linearly with increasing aperture (smaller f-number). Under otherwise constant conditions, ghost spots become four times as large at double the focal length and twice as large at half the f-number (twice the diaphragm diameter).

• 

  Ghost spots from rain (wide-angle photo, focal length 7 millimeters, sensor diagonal 14.1 millimeters, aperture 2)

• 

  Ghost spots from rain (zoomed in, focal length 21 millimeters, sensor diagonal 14.1 millimeters, aperture 2.7)

Distance law

The diameter of the ghost spots increases in inverse proportion to the distance from the scattering centers. As a result, the area of ​​the ghost spots, i.e. also the area over which the amount of light from a scattering center is distributed, increases inversely square to the distance from the scattering centers. On the other hand, according to the law of distance, the illuminance of the scattering centers decreases quadratically with the distance of the flash from the scattering center. These two effects compensate each other so that all ghost spots with scattering centers of the same size in the image have the same brightness; More distant scattering centers are shown with their total amount of light as smaller ghost spots with a correspondingly higher luminance , but are also less strongly illuminated by the flashlight to the same extent.

Sample calculations

Small camera

A small digital camera with an image sensor size of half an inch (this corresponds to an image sensor diagonal of about 12.7 millimeters) takes a picture with a set f-number of 2.4 and a focal length of 7 millimeters with activated flash, while at a distance of about 40 millimeters A speck of dust is floating in front of the lens (compare picture above right). This is on the image sensor with a diameter of

${\displaystyle Z_{1}={\frac {(7\,\mathrm {mm} )^{2}}{2{,}4\cdot 40\,\mathrm {mm} }}={0{,}5\,\mathrm {mm} }}$

pictured. In relative terms, this diameter corresponds to 4 percent of the sensor diagonal. Therefore, the resulting orb takes 4 percent of screen sizes and is at sufficient brightness clearly visible in the image as slices.

Great camera

With a larger camera with an image size in small format , the image diagonal is 43.3 millimeters. A focal length of 24 millimeters is required to take a picture with the same angle of view as with the small digital camera described above. With the same f-number of 2.4 and the same distance between a speck of dust in front of the lens of 40 millimeters, the result is a circle of confusion of:

${\displaystyle Z_{2}={\frac {(24\,\mathrm {mm} )^{2}}{2{,}4\cdot 40\,\mathrm {mm} }}=6\,\mathrm {mm} }$

In relative terms, this corresponds to 14 percent of the screen diagonal.

comparison

The relative areas of the two circles of confusion under consideration and behave like the squares of the relative diameters and thus like: ${\displaystyle A_{\mathrm {rel} ,1}}$ ${\displaystyle A_{\mathrm {rel} ,2}}$ ${\displaystyle Z_{\mathrm {rel} ,1}}$ ${\displaystyle Z_{\mathrm {rel} ,2}}$

${\displaystyle {\frac {A_{\mathrm {rel} ,1}}{A_{\mathrm {rel} ,2}}}={\frac {Z_{\mathrm {rel} ,1}^{2}}{Z_{\mathrm {rel} ,2}^{2}}}={\frac {(4\,\%)^{2}}{(14\,\%)^{2}}}=0{,}08}$

As a result, the brightness within the circle of confusion with the larger camera is only 8 percent of that with the small digital camera. However, it may be too dark to be recognized in the recording. To put it the other way round, the brightness of the ghost spot on the small digital camera is about twelve times greater than on the larger camera, so that it can be seen much more clearly in the recordings of the small digital camera.

example Pictures

Examples of ghost spots caused by solid or liquid, illuminated scattering centers:

• 

  Ghost stain through dust

• 

  Ghost spots from dust

• 

  Dense coal dust

• 

  Coal dust floating in the air

• 

  Ghost spots in the forest

• 

  Ghost spots from rain with coma tail

• 

  Enlargement of the upper right corner from the previous image with visible chromatic aberration

application

Knowledge of the mechanisms involved in the formation of ghost spots enables artistic use, for example through different aperture shapes or artificial production. Lens reflections and blurring of point-shaped light sources or the sun create similar fragments and image artifacts (see also: Bokeh ).

literature

• Gary E. Schwartz & Katherine Creath (2005): Anomalous Orbic "Spirit" Photographs? A Conventional Optical Explanation , in: Journal of Scientific Exploration, Vol. 19, No. 3, pp. 343-258. PDF online
• Klaus Heinemann & Dr. Míceál Ledwith (2007): "The Orb Project" ISBN 1582701822 , ISBN 978-1582701820 German title (August 2008): "The Orb Project - In Search of Energy Phenomena with Digital Photography"
• Winrich Neumann (2009): "ORBs - A picture book of illusions", ISBN 3868054774 , ISBN 978-3868054774 Publisher: Pro Business; 1st edition (October 7, 2009)
• Ed Vos (2010): "Orbs and other light phenomena" ISBN 978-3-89060-551-7 , Neue Erde GmbH
• Ernst Laschan von Solstein (2011): "Orbs - gateway to another world?" with a preface by Dieter Broers. ISBN 978-3-942408-21-9 , Kollateral Verlag GmbH

Web links

Commons : Ghost spots  - collection of images, videos and audio files

• Depth of Field (English technical article on the subject of depth of field, including the chapter "Flashlight and ghosts")
• The Orb Zone - Orbs explained
• The Secret of the Orbs ; Der Tagesspiegel, September 23, 2008
• Physical explanation "Orbs are quite normal" ; Focus Online