When a light or another bright object is within the field of view of the lens (and not close to the edge) then the light rays from that object are close, or relatively close, to the optical axis of the lens. With the result being that most of those rays become image forming light rays.
Whereas with a bright object that is very close to the edge of the field of view of the lens a large portion of the light rays are not able to contribute to the formation of the image but are still striking the lens. Because of the higher angle of incidence, they produce a lot of reflections off of the surfaces of the lens elements, with the result becoming a wash of non-image forming light---->Flare.
Well, this sorta sounds good, doesn't it??
Oh, and you really can't avoid it--except by not having a bright light/object close to the edge of the lens field of view. Multicoating was created to help control this.
I have this feeling your explanation is right on, David. The thing that amazes me is that small lights (say in a night shot) positioned just out of view can result in such a narrow beam of flair coming in from the side of the frame. I've got a great example of this in print which I'll try to upload to the tech gallery over the next few days.
I think that's the clue: Bright lights just outside the film frame will 1) be far off the optical axis, and 2) have a good chance of reflecting off any internal surfaces in lens and camera.
I have a beautiful example of this, where the sun was just outside the frame. Since the film was Konica IR 720, I was also able to prove that multicoating is not optimized for IR light... I would never have thought that there were quite so many surfaces in a Zenzanon PE 40mm...
For me, it's always seem that the more perpendicular a light source is to the front lens element, the less flare and the more parallel the light source is, the worse. And then there is the "the flatter the glass, the more flare" too.
Seems like the "light piping" theory might apply here...can we say fiber optic properties?
For weddings (receptions) I usually put fresnels in front of my strobe heads, so if I get a strobe in the shot, it becomes a star instead of flare.
For the best flare protection, "light trap" methods are the most effective. See lens hoods for movie production and Arri cameras. The hood looks more like a barn door that is near fully closed (the outer tips of the flaps point inward instead of outward). I modified a small barndoor with a Cokin Bay60 ring and a ton of hot-melt glue to make mine. Never get much flare at all.
I think in a lot of ways, flair is one more expressive tool that, can be understood and used for creative expression. I just got out of the darkroom - spending some time with a neg that I made on a moutain top where the sun was just behind a tree directly in from tof the camera. I was shooting a Rollei SL66 - using movements and a 50mm Distagon with no filter or shade. I was going for a very gritty look in the pine needles with the way the morning light shone off of them and I got a very interesting effect. 60% of the neg has exactly that gritty shine I was looking for but the area of the print where the sun was blazing around the tree, created a softness that set up real contrast to the sharp rest of the image - It will give me something to really toy with in the coming week. -Frank