There are two primary uses I make of stitching images. One is simply to extend the natural view of the camera and lens by panning or shifting left and right to take supplementary shots to widen the scope of an image. If shot carefully, the images can be quite easily stitched in Photoshop to produce an extra-wide landscape.
The other use I have for stitching is when creating portrait format images of architecture and interiors. When shooting architecture, I use a tilt/shift lens, also known as a perspective correction lens. While it’s perfectly possible to spin the lens 90º and use the shift function in portrait format, I frequently find that this doesn’t give me as much width as I’d like. I basically prefer a squatter rectangle for portrait images. So I tend to leave the camera and lens set up in landscape position, and use the shift function to take in more of the scene above and below the perspective corrected frame. Because these supplementary images won’t be perfectly perspective controlled in camera, there is a certain amount of correction in post-production, but this is mostly at the corners of the image and, with care, is barely noticeable.
In the example below, the central unshaded section represents the part of the image shot with full perspective control, while the paler sections above and below it are the supplementary images used to augment the image and create the portrait format picture.
The images didn’t align perfectly, but selective use of the free transform and warp tools produced a good result. You have to make a case-by-case judgement on when to free-transform across an entire layer and when to work with just a section. In the top section, for example, I was able to leave the central section of the wall painting almost untouched by working on the corners separately. I worked on the central window independently too. In the bottom section, the shadows were able to absorb a lot of the discrepancies between the alignment of the images.