Geometric Lens Distortion: a common problem in photography

What is Lens Distortion and what causes it?

All lenses produce a certain amount of geometric lens distortion. This is caused by a range of factors, including the curve of the lens, the focal distance, and the angle at which the photo was taken. It affects the image in a variety of ways and is especially apparent when straight lines at the edges of the frame are no longer parallel. Geometric lensdistortions primarily occur when the focal distance is short (barrel distortion), long (pincushion distortion), or when the photo is taken at a low angle (keystone distortion) with a fish-eye lens.

Distortion can make straight lines in an image no longer appear parallel, especially on the edges of the frame.

Paris, France

Shooting settings:

Camera: Canon EOS 5D Mark II
Parameters: ISO 100 - 1/40s - f/22 - 15 mm

Image processing:

Correction optique DxO

How does DxO correct lens distortions?

To measure the degree to which a lens produces geometric distortion, DxO’s experts use a combination of different focal lengths and distances. They photograph a reference image that contains a dot plot laid out on a very precise grid. Depending on the lens and camera combination being used, this process can require 100 to 500 reference images.

The extent of the distortion is then precisely calculated in each position of the image field. With this series of measurements, the algorithm can generate an average and maximum level of distortion. These values are arranged on a precise scale. Below 0.3%, the distortion is not perceptible to the untrained eye. Between 0.3% and 1.0%, it is visible. Any value above 1.0% is considered problematic.

The calibration data obtained by the analysis are compiled in a correction file called the lens module.

The degree of geometric distortion in a lens is evaluated using a reference image containing a dot plot laid out on a very precise grid.

Better lens distortion corrections than any other software

Correcting geometric lens distortions is much more than a matter of simply straightening lines. DxO’s algorithms even correct the most complex distortions, including the keystone effect. This occurs when a large subject is photographed from a low angle with a wide-angle lens. It is a type of perspective distortion in which the converging lines make the subject appear larger on the bottom than on the top.

To achieve even better results when correcting the geometric distortions that occur with wide-angle lenses, DxO algorithms also removes volume anamorphosis, which means objects located at the edges of the frame retain their original shape.