Figure 1. Photo taken at 16mm. Click to Enlarge
There is a discussion happening on the Bushwalking Tasmania about how to take good panorama images. At the request of one of the forum members, I am writing a little bit about why the focal length is important when composing your panorama shot. When I take panorama images, I always use a 50mm lens of greater.
What most people seem to forget with cameras is that you are taking an image from the lens that is circular, and you are putting it on a rectilinear sensor or film. as your angle of vision increases (usually at smaller focal lengths), the more likely you are to experience Barrel Distortion, particularly with fisheye lenses. Barrel Distortion will cause you problems when you start stitching together your panorama.
Figure 1 was taken with a Sigma 10-20mm Lens, at 10mm. Taking into account the Canon crop factor of 1.6 on APS-C Sensor, it works out to 16mm. Here you can see that the stitching tool is showing you how much distortion it needs to crop from the image to make it a panorama, and you will also note that there is still significant distortion on the vertical edges resulting in obvious stitching joins. You can also see that you lose some 30-40% of the image’s height due to the crop.