Although distortion can be hit and miss or follow many patterns, the most common distortions run into are radially symmetric, or something like, taking place from within the symmetry of a
lens. Radial distortion can normally be listed as one of two main categories:
Barrel distortion, happens where image magnification lessens as it becomes more distant from the optical axis. The perceptible consequence is that of an impression which was charted around a ball. A
fisheye lens, that has a hemispherical view, utilizes this kind of distortion as a way to chart a never ending wide object surface into a predetermined image section.
Pincushion distortion occurs, when a magnified
image increases when moving away from its visual axis. The optical consequence is that shapes that do not move through the center of the impression are bowed toward the inside, in the direction of the center of the image. Many times this
aberration is seen in out of date or bargain
A combination of both types, sometimes labeled moustache distortion, not quite as common but they do exist. It starts out as barrel distortion near the center of the image and progressively transforms to pincushion distortion on the way to the outer edges of the image. It happens with a a small number of retrofocus lenses, and also in newer super-zooms
Nikon 18–200mm and Sigma 18-200mm lenses.
Distortion is the easiest to recognize optic
aberration as it entirely deforms the image. In
particular the straight lines inside the object space become curved lines on the image sensor, the label curvilinear distortion ioften comes to mind . Straight lines moving through the center of the image remain as straight lines and a concentric circle within the image center continues to be a circle, even though there is a result on the radius.
Wineglass distortion simulation
Barrel distortion simulation
Pincushion distortion simulation
Calibrated systems function from a lens/camera table of transfer designs: PTlens is a plugin for Photoshop and also a standalone application designed to correct complex distortion in images. DxO Labs' Optics Pro was created to rectify complex distortion, and even takes the focus distance into account
Manual DistortionManual systems permit physical adjustment of distortion boundaries:
Adobe Photoshop Elements (beginning with version 5 on) and Photoshop CS2 incorporate a manual Lens Correction filter designed for simple (barrel/pincushion) distortion GIMP includes correction of manual lens distortion (starting with version 2.4). The software Hugin can also be used to fix distortion, although that is not Hugin's foremost application
Radial distortion is a lens failing to be rectilinear: when not formed with or based on straight lines. If an
image is not shot straight-on even when an exacting rectilinear lens is used, rectangles will be seen as trapezoids: these lines are still imaged as lines, however the angles in between are not maintained (tilt is not a conformal chart). This effect can be managed by employing a lens that has perspective control, or modified in post-processing.
Because of perspective, cameras view a cube as a square frustum (truncated pyramid, containing trapezoidal sides) the distant end is not as large as the near end. This creates perspective, at similar rate at which the scaling occurs (how fast more distant objects get smaller) creates a feeling of a setting being either deep or shallow. This cannot be altered or repaired by a simple change of the resulting image, as it requires knowledge of 3D information, more specific the depth of objects in the setting. The resulting consequence is commonly referred to as
Radial distortion is fashioned by a behavior D(r) which has an affect on every point v in the anticipated plane in relation to the major point p, where D(r) is usually a non-linear scalar behavior and p is near the middle point of the projected likeness. Barrel projections are distinguish by a positive pitch of the distortion behavior, whereby pincushion as a negative one.
Those distortions can be corrected in software by curving the image using an opposite distortion.
Updated Dec 29, 2010