Thanks for reading!
-Tentus
In this rant I'm going to talk conceptually about taking photos for panoramas.
Imagine the line of the horizon going through the center of your vision, like this:
Now, if we overlap three images by a little, we get this:
Except that the angle has changed a little, so we really something get this:
Notice that the horizon is still going straight through all the images, even when composited.
The change in angle means that the ending image area is going to be a little contracted, since we don't want funky white triangles in our final image. The purple here shows what the final image dimensions would be:
Now, if we're looking down even a little, the horizon starts to curve. It looks something like this:
See how the horizons stop matching up? You get a bumpy image. We can compensate by doing this:
But notice how the final area shrank again, and now the edges of the horizon are falling down into the corners of the image. This is generally an unwanted side effect, that we can avoid by keeping the viewing plane level.
It is worth mentioning that you can compensate for angle change with clever camera tilting. This is extremely difficult, so I personally would not recommend it.