Curved Images of straight Lines – What is distortion?
Find out more about the different types of distortion found in wide angle and telephoto lenses. Discover what causes barrel and pincushion distortion and step back in time to see how artists prevented distortion in the 16th Century. Learn how to interpret manufacturers’ data charts for lens distortion.
Have you ever looked at a photo of a subject that had straight lines only to find they look curved on the screen? The reason for this distortion is due to the physical properties of your lens. A lens that exhibits distortion produces slightly warped images of lines that do not pass through the centre of the lens.
Distortion can be a creative tool and can be desirable for some shots according to your personal style. It is generally causes most problems in wide-angle portraits of people, where those on the edge of the frame can end up with distorted “eggheads” and shots with architectural subjects that the viewer knows should have straight lines.
Types of Distortion
Pincushion Distortion – Pincushion distortion is where the straight lines bend inwards or “pinch” towards the centre of the frame, due to higher magnification at the edges. It happens most often when using the telephoto end of a zoom lens.
Barrel distortion – This is where straight lines bend outwards from the centre of the image. It is most common in wide-angle lenses or telephoto lenses used at their widest focal length.
The diagrams above show the subject, a grid, on the left, the lens in the centre and the corresponding image on the right. The top diagram shows extreme pincushion distortion, the middle diagram shows barrel distortion, and the bottom a lens design with no distortion.
Distortion happens when the image scale is not constant throughout the entire image field. The image may be in perfect focus throughout but some areas of the picture will have a slightly different focal length and will be magnified more than others, leading to distortion. In other words: the focal length of a lens showing distortion changes with the distance of each point of the image from the optical axis.
Imagine you are taking a picture looking straight at a brick wall with a wide-angle lens (or a zoom lens at its shortest focal length). The distance from the edges of the field of view to the lens is significantly further than from the centre of the field of view to the lens and the light does not pass through the exact centre of the lens. The centre of the image is magnified more than the edges and it causes the “barrel-type” illusion that the brick surface forms a dome towards the observer.
Correcting perspective in art
Artists throughout the centuries have had the task of mapping a three-dimensional image onto a two-dimensional surface. In the early 16th Century techniques for reproducing correct perspective began to develop. In the woodcut below, the artist is using a frame with a grid as a projection surface to then correspond with a grid on his drawing surface. The artist’s viewpoint is fixed by using the tip of a rod. This single point of view corresponds to the aperture of a camera.
Measuring Lens Distortion
Camera lens manufacturers release charts that show the amount of distortion of their lens at different distances from the central axis. This is commonly expressed as a % Radial distortion or the exact amount of distortion in millimetres.
Download the full article What is distortion? by B. Hönlinger and H. H. Nasse which explains in detail how to interpret these charts. It also explains how the lens designs of the ZEISS Biogon and ZEISS Planar 1.4/85 exhibit virtually no distortion.