Sunday, February 6, 2011

Distortion and Perspective

A while back a colleague posted:
Eliminate wide angle distortion with Panoramic photosHave you ever wished that you could capture a whole scene in one frame without having the trees/light poles/buildings slant towards the center?  You can!  
And then, unthinkingly, proceeded to confuse perspective and distortion as well as the readers.
In this post I would like to take up the first part: distortion.
Most of tzoom lenses on popular cameras present some barrel distortion on the low end of the focal length range. What is barrel distortion? Here is a photo taken with the lens covering about 76 degrees, the equivalent of a 28mm lens on a 35mm camera.
LJK_1894-PSP-600This is the inside of the famous Grill in Athens, Georgia. There was no way to step back far enough so I used the shortest focal length setting – 18mm. Note the curved ceiling. This is distortion. Here an exaggerated version with more barrel distortion to illustrate the effect.
The lens barrel distortion is pretty well gone at longer focal length settings. So I took several overlapping exposures of the same wall and stitched them into a panorama with Windows Live Photo Gallery. To show the stitching I did no cropping:
ICEWell the barrel distortion is well reduced but now there is obvious perspective distortion. Live Photo Gallery does not provide any controls for the stitching process, but Microsoft Image Composite Editor, “ICE”, does. ICE allows shifting the combined image to achieve perspective control. I will have more on this in another post. For now here is the resulting image, again without cropping from ICE:
So it is possible to reduce wide-angle lens barrel distortion by using tiled exposures obtained at focal length settings that do not show the distortion.
The slanting tress/light poles/buildings is, however, a perspective problem. More on that in another post.
Grill-ICE-2 (600x196)

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