Pincushion distortion
The pincushion distortion is a distortion of the image geometry of the picture tube of a computer monitor or television , which can be recognized by the fact that a rectangle has pointed corners at the corners, which looks like a (eponymous) pillow .
In the case of camera lenses , in particular telephoto lenses, one speaks of pincushion distortion. You can recognize them by the fact that straight lines at the edge of the photo appear curved. In contrast, barrel-shaped distortion occurs with wide-angle lenses.
Emergence
Pincushion distortions arise because the beam path is longer at the edges than in the middle due to the flat curved screen.
Elimination
In the case of black and white tubes, these distortions can be compensated for by correcting magnets attached to the deflection unit.
This is no longer possible with color picture tubes. Therefore you need the east-west correction (horizontal correction) there to compensate for the lateral distortions and the north-south correction (vertical correction) for the distortions above and below. Since the electromagnetic north-south correction is very complex, the deflection unit is designed and wrapped in such a way that the pillow tips are pushed horizontally; The tips cannot be completely avoided. Therefore, without correction, the image is displayed as in the photo next to it. As a result, the horizontal and vertical deflection fields - viewed across the surface - are no longer entirely orthogonal . The east-west correction is a barrel-shaped amplitude modulation of the horizontal deflection current, which precisely counteracts the pincushion distortion. The modulator is usually a transducer with only one winding, whose frame-frequency barrel-shaped control current is derived from the vertical output stage.
With camera lenses, the pincushion distortion occurs regularly when the lens hood is attached behind the lens (this is the case with many single-lens lenses ( monocles )). This can be remedied by using multi-lens lenses (in the simplest case a periscope (lens) ) with a diaphragm arrangement between the lenses
See also
Individual evidence
- ^ Ronny Puchner: Abbildfehlerv <Physics <Basics <Photo school ;. Retrieved June 19, 2018 .