April 03, 2008
This is part of an ongoing series of short articles about lenses and how they are used on Digital SLR cameras. The series currently contains the following articles:
Lenses with focal lengths that are less than normal are refered to as wide angle lenses. These lenses are best suited for capturing large areas and are preferred by landscape photographers who desire to capture stunning vistas. As lens manufacturing technology has improved, extremely wide angle lenses have been created, as well as fish-eye lenses that give a nearly 180° field of view. The wider a lens is, the more noticable the barrel distortion that these lenses create becomes. Many photographers use this distortion to artistic effect (whether you appreciate the art is another matter).
Wide angle lenses create images that have a lot of depth to them - they exagerrate the apparent distance between foreground and background. This tends to introduce distortion into images in a couple of ways.
Perspective distortion is a function of how close to the camea the subject is. The closer the subject is, the larger it appears in the image. Try this without a camera. Find a small object and hold it up close to your eyes. Note how large it appears.
Conversely, objects that are far away appear much smaller than they are. The moon and the sun both appear to be about the same size in the sky to our naked eyes, but we know that the sun is very much larger than the moon. And we also know it is many times farther away than the moon.
Back to that small object you were holding up close to your eyes again. Notice that when it is up close you can’t focus on anything much farther away than the object, or if you do, the object goes out of focus. This is because at this focusing distance our eyes have a very narrow depth of focus. Wide angle lenses have a greater depth of focus than our eyes, and are able to keep both foreground and background in focus.
Wide angle lenses also have a larger field of view, and thus are able to “see” more of that background, and they have short focal lengths, which means the camera can get very close to the subject and still be able to focus on it. The large depth of focus, large field of view, and short focal lengths all work together to exaggerate the apparent difference in sizes between foreground and background objects, leading to the distortion.
Barrel Distortion is a function of the lens. Most wide angle lenses are designed to provide more or less rectilinear images—that is straight lines are straight. The wider the lens, the more likely it will be to produce some barrel distortion. This is because the light is bent more near the edges of the lens than it is in the center. This magnifies the edges more than the central part of the image, resulting in an image that looks as if it were mapped on a sphere.
Lens manufacturers have worked to mitigate the effects of barrel distortion on their lenses, so that save for on the very wide end of the scale, or fish-eye lenses, most of the distortion you see will be due to perspective distortion. Using the lens on a cropped sensor will help even more since the effects of barrel distortion have the greatest effect near the edges of the image, which is cropped out by these smaller sensors.