Globe effect
The globe effect, sometimes called the rolling ball effect or the spinning globe effect, is an optical
Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...

 phenomenon that occurs with visual optical instrument
Optical instrument
An optical instrument either processes light waves to enhance an image for viewing, or analyzes light waves to determine one of a number of characteristic properties.-Image enhancement:...

s, in particular binoculars
Binoculars, field glasses or binocular telescopes are a pair of identical or mirror-symmetrical telescopes mounted side-by-side and aligned to point accurately in the same direction, allowing the viewer to use both eyes when viewing distant objects...

 and telescopes
Optical telescope
An optical telescope is a telescope which is used to gather and focus light mainly from the visible part of the electromagnetic spectrum for directly viewing a magnified image for making a photograph, or collecting data through electronic image sensors....

, that are designed to be free of distortion. When these instruments are panned, the moving image appears to roll over a curved, convex surface. In 1949, Horst Koehler at Zeiss (Jena) suggested adding some pincushion distortion to the optical design to eliminate the globe effect. August Sonnefeld conducted experiments with volunteers, which supported the claim that a supplementary distortion could improve the imaging of visual optical instruments. Since that time, most binocular manufacturers have followed Zeiss's example and added pincushion distortion to their optical design.

The origin of the globe effect initially remained unclear after its discovery in the first half of the past century. Koehler speculated about an "unnatural perspective
Perspective (visual)
Perspective, in context of vision and visual perception, is the way in which objects appear to the eye based on their spatial attributes; or their dimensions and the position of the eye relative to the objects...

 generated by the binocular while panning over a three dimensional scenery", thereby ignoring the fact that the globe effect was observable at the night sky as well, where perspective distortions were absent.

Recently, the globe effect has been linked to the peculiar properties of human visual perception
Visual perception
Visual perception is the ability to interpret information and surroundings from the effects of visible light reaching the eye. The resulting perception is also known as eyesight, sight, or vision...

, which adds a certain amount of barrel distortion
Barrel distortion
In geometric optics and cathode ray tube displays, distortion is a deviation from rectilinear projection, a projection in which straight lines in a scene remain straight in an image...

 to the visual field
Visual field
The term visual field is sometimes used as a synonym to field of view, though they do not designate the same thing. The visual field is the "spatial array of visual sensations available to observation in introspectionist psychological experiments", while 'field of view' "refers to the physical...

. The amount of barrel distortion is subject to individual differences, which explains the fact that the perceived intensity of the globe effect varies significantly between different observers.

An alternative approach for explaining the globe effect comes from the technical journalist and optics specialist Walter E. Schön. He states that the observed effect is in fact not that of a rolling globe but that of a vertically rotating cylinder. The globe shape of the illusion seen by most observers is only due to the fact that the field of view through the optical device is circular. This illusion of a rotating cylinder during panning is caused by the horizontal movement of the image being (due to the angular magnification of the device) faster and more uniform (with less parallax) compared to the naked eye and also not corresponding to the felt rotational speed of the observers head. When the brain tries to integrate these conflicting signals, it creates the perception that the image is moving slower at the left and right edges than in the middle, giving the illusion of a rotating cylinder.
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