I think it is the horizontal format that messes it up - don't have the issue when I do verticals, no matter what lens I use
The "issues" are the very same, no matter in which orientation the frames are in the composition.
However, the degree of the "distortion" (which is none) depends on the angle of view and the projection. When you put a landscape oriented frame horizontally not in the center, then it has to be warped more than one in portrait orientation, because the former covers a wider area
horizontally. The same with a wider lens.
The warping is particularly large with rectilinear projection far from the center. The reason, that the rectilinear projection is limited to a horizontal angle of 180° (of the total composition) is, that at that angle the vertical edges would have to be increased to
infinity. In practice the limit is around 120-140°.
The rectilinear projection's result looks like a single shot with a rectilinear lense (as opposed to a fisheye). Imagine, how the scenery would look like at a very wide angle: objects at the edges appear smaller than in teh center. When you make separate shots with much smaller focal length, the frame depicting the "edge" covers a much smaller area than the edge of a single frame with large angle of view. Therefor the far edge has to be stretched - that's what you think of as "distortion".
The warping is linear with rectilinear projection, but it becomew curved with the other projections. For example with cylindrical projection the top and bottom edge of the frames have to be warped in barrel-like shape, the degree depending on the angle of view.
You may profit from going through following demonstrations, which I created just to explain some closely related subjects:
http://www.tawbaware.com/forum2/viewtopic....t=4700&start=12http://forums.dpreview.com/forums/read.asp...essage=24047035