Why use a sunshade on a lens...

[pluton]

What would be the rules, if any, for the size of a lens hood or compendium to keep out unwanted light? Where would I get, let's say, a compendium that would fit a 77mm thread?

The rule would be to bring the edges of the compendium shade as close as possible to edge of the frame without being visible in finder or in the shot.  It helps to stop the lens down and check at both minimum and maximum focus distances, as the lens field of view can(and usually does) slightly change through the focus range.  I have a couple of the mentioned Mamiya shades($20-$30 at the bargain photo swap meet), and they are square.  So, when I used them on lenses attached to a Nikon FX or DX camera, I added black tape to the top and bottom of the square front opening of the shade, so as to create a 1.5:1 aspect ratio on the front edge of the shade that matches the shape of frame that the FX or DX camera is shooting.
Does this make sense?

[Akira]

To avoid the flare when it is obvious where the light source is (like the sun or a street light), I sometimes hold the camera with the right hand only and hold my left hand a little away from the lens and make the shade for the front of the lens, so that my hand doesn't cause vignetting.  It is pretty doable when the shutter speed is fast enough to avoid the camera blur.

[David H. Hartman]

The Good (20/3.5 AI), The Bad (I'll have to think about it) and The Ugly (24/2.8 AI). The 20/3.5 AI as very low flare and I don't know if I can make mine ghost. The 24/2.8 AI flares generously in unfavorable light and if the sun is included in the corner there will be 12 or 13 small, hot ghosts; 12 if no filter and 13 with. Being both AI Nikkors the difference must be in the optical design as the coatings are of the AI era.

I don't think we should loose sight of the OP's point. Lens hoods matter. Those for 50mm and up excluding zoom lenses are mostly very effective.

Dave Hartman who is probably repeating himself in this post, in this thread.

[David H. Hartman]

To avoid the flare when it is obvious where the light source is (like the sun or a street light), I sometimes hold the camera with the right hand only and hold my left hand a little away from the lens and make the shade for the front of the lens, so that my hand doesn't cause vignetting.  It is pretty doable when the shutter speed is fast enough to avoid the camera blur.

This technique is frequently required with my 15/5.6 AI and 24/2.8 AI.

Dave who is again surly repeating himself.