>
> As the angle varies from straight ahead, the focal distance increases
> and the image gets smaller. The effect is not particularly obvious with
> many subjects. With a small, even, linear band like the sea in this
> image, it becomes very obvious, appearing larger in the middle and
> tapering off in both directions.
>
I do not agree.
It is true that a single wide-angle lens that is corrected to provide a
linear image increases the focal length the farther from the center of the
image. This is very apparent with an ultra-wide lens.
However, in the case of a panorama camera with rotating lenses AND a curved
film plane, there is no increase in focal length other than the minor change
from top to bottom. But the laws of projection would nullify any
focal-length change when the sensor or film plane rotates equally. If the
film plane is flat and the lens rotates, then focal-length increase occurs.
When this happens, you also get extreme "ripping" distortion the closer to
the edge and corners as is so common with extreme wide-angles.
When you do a panoramic photograph with a standard digital (or film) camera
and you attempt to rotate the camera-and-lens around the rear nodal point,
again there is no increase in focal length as the sensor/film is rotating
equally with the lens. You will always get some distortion between
overlapping frames, but this is the fault of the lens being used and is why
it is best to use as close to a "normal" focal length as possible when
taking the individual photographs used in a stitching.
A variation on this theme is when you use lens-shift instead of rotation for
shooting the frames used in the panorama. As a lens is shifted to the side,
the greater the effective focal-length.
I maintain that the reason why you see the bulge in the middle of panoramas
is that the subject is usually closest in the middle. For example if you
are standing on the curb of a street and have the camera pointed directly
across the street, the distance to the other side may be only 25 feet, but
you might be seeing a mile or more down the street each way. Of course the
cars, trees and pedestrians are going to "bulge" at you.
The only way to avoid the bulge (and linear distortion of straight lines) in
these photographs is to assemble and display the panorama in a curved plane
around the viewing position. This is NOT a capture issue, but a display
issue.
AG
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