Understanding Aspect Ratios & The Art of Cropping, Michael Reichmann's article, is as informative now as it was eleven years ago.
The simplest way to obtain an aspect ratio different from the 35mm 2x3 is to shoot wide and then crop down. Doing so involves throwing away pixels, which I hate doing doing just on general principle. A better alternative is to stitch multiple frames and then crop to the desired format. The end result can have more pixels than any of the individual 35mm files—sort of a poor man's large format.
An efficient way of taking multiple frames for stitching is to use shift-tilt lenses. With the camera body in landscape orientation, it is possible to shift side to side for a 1x2 panorama without going anywhere near the edge of lens travel. Two frames will suffice. Similarly, shifting vertically with the camera in landscape orientation will enable cropping to square to a variety of rectangles, all without having to shift to the extreme edges of lens travel.
There are difficulties with this approach, the most obvious of which is the subject must be fairly static. Another, which will become quickly apparent at the beginning, is the need to mentally see in aspect ratios other than the standard 2x3 ratio. This may take some practice.
On the other hand, merging the shifted frames when processing is a snap. Photoshop's tools work perfectly fine in my experience.
I've used Canon's tilt-shift lenses for years. The new versions have improved controls and, more importantly, better optics at the extreme limits of shift travel. Especially the 50mm over the old 45mm, which was never a stellar performer. A recent shifted pano -
http://www.pbase.com/image/167798014/original And a recent two-shot vertical shift -
http://www.pbase.com/image/167598158/original
