Orb (optics)

Ghost spots are appearing diffuse, glowing, more or less circular discs in photographic images. In the English-speaking world, these spots are often referred to as Orbs. In esoteric circles they are considered paranormal phenomena. This is not an aberration of the imaging system, but often a blurring artifact, usually when shooting with flash.

Description

A flash of lightning is by particles suspended between the image subject and the camera, scattered and partially reflected. As a result of due to the proximity and the focus blur caused roughly disk-shaped light images. The mapping of the particles is much larger than the particle itself, the form is based on the shape of the respective panel. In the actual cause of these scattering centers are frequently to dust, but the effect can also be caused by raindrops, snowflakes, insects and other small objects near the camera.

The digital compact cameras format and focal length- related greater depth of field often makes images that depict these particles still sufficiently sharp. In analogue photography, the scattering centers of the spirits spots are due to the shallower depth of field usually not or hardly recognizable.

The following observations are typical of spirits stains:

• The closer the scattering centers are located in front of the camera or per blurred is the image, the greater the spreading discs.
• The ghost spots are smaller according to the associated increased depth of field, the smaller the aperture or focal length of the camera is set.
• The brightness of the ghost blotches, one hand rule in quasi- point-like scattering centers largely independent of the distance, because the lighter scattered light is distributed among a total of scattering centers closer to a larger focus area imaged. This contradicts the usual calculation after which the brightness of the irradiated with flash objects decreases quadratically with the distance.
• With a doubling of the focal length of the diameter of the ghost spots are four times as great at the same distance of the scattering center and at the same aperture.
• When shooting without flash, the scattering centers are not usually recognizable because Flash is the most common cause. Longer exposure times without artificial lighting to reduce the likelihood, although other light sources are a possible cause secondary question, such as lamps, the sun, et cetera.
• The occurrence of ghost spots can not be reduced by the use of baffles.
• The smaller the image, respectively, of the image sensor in a camera is, the smaller and brighter the image of a ghost spot at the same angle of view (picture detail) and at the same f-number.

Theoretical background

The size of a haunted spot can be estimated very simply, when the camera is focused at infinity; the object distance of point-like objects is shown then also infinite. The diameter of the Z circle of confusion of an imaged from the finite point results using the lens equation and the geometric ratio

Then:

A point source of light at a distance x in front of the main plane of the imaging lens is thus depicted as a luminous disk with a diameter Z whose diameter is inversely proportional to the distance of the scattering center - at ten times larger distance of the scattering center of the orb is under otherwise constant conditions, ie ten times smaller.

The diameter of the spots corresponding ghost takes the square of the focal length and magnifying linearly with aperture used to be (smaller f-number). Under otherwise constant conditions are ghosts spots at twice the focal length four times as great and at half the f-number ( double aperture diameter ) is twice as large.

Ghost spots by rain ( zoomed recording, focal length 21 mm, sensor diagonal 14.1 mm, aperture 2.7 )

Distance law

The diameters of the ghost spots takes inversely proportional to the distance from the scattering centers. Accordingly, occupies the area of ghost spots, including the surface on which the light amount is distributed by a scattering center, reverse square of a distance from the scattering centers. On the other hand, following the law of distance, the light intensity of the scattering centers the square of the distance of the flash light from the scattering center from. These two effects compensate each other, so that all spirits stains with scattering centers of the same size in the picture have the same brightness; more distant scattering centers are indeed ready with their total amount of light than smaller ghost spots with a correspondingly higher luminance, but less strongly lit by the flash and to the same extent.

Example 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 ) during a set f-number of 2.4 and a focal length of 7 millimeters with flash is activated, an image, while in about 40 millimeters from front of the lens a speck of dust floats (see picture above right). This is on the image sensor with a diameter of

Mapped. This diameter corresponds to 4 percent in relative terms, the sensor diagonal. Therefore, the resulting orb occupies 4 percent of screen sizes and is clearly recognizable as slices with sufficient brightness in the image.

Great Camera

With a larger camera with an image size in small picture format, the image can be is 43.3 millimeters. Receiving a record having the same angle of view as a small digital camera described above, a focal length of 24 millimeters is required. At the same f-number of 2.4 and the same distance from a speck of dust on the lens of 40 mm results in a Zerstreungskreisdurchmesser by:

This corresponds to relative terms, 14 percent of the image diagonal.

Comparison

However, the relative areas of the two circles of confusion under consideration and behave like the squares of the relative diameters and, hence, as:

As a result, the brightness within the circle of confusion in the larger camera is only 8 percent of those in the small digital camera. Under certain circumstances, however, this is too dark to be at all recognized in the recording. Stated conversely, the brightness of the ghost spot in the small digital camera approximately twelve times larger than at larger camera so that it can be seen more clearly in the images of the small digital camera.

Sample Images

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

Ghost spots caused by dust

Poet coal dust

In the air floating coal dust

Ghost spots in the forest

Ghost spots by rain coma tail

Enlargement of the upper right corner of the preceding recording with visible chromatic aberration

Application

Knowledge of the mechanisms involved in the formation of ghost spots allows an artistic use, such as different panel models or artificial production. Lens reflections and blur point light sources or the sun form similar fragments and image artifacts (see also: bokeh).