The other night I was shooting an elevated section of freeway in foggy, misty conditions. Here's the setup:

-Shooting 4x5 HP5
-Fujinon-C 300mm lens @ f22 1/2
-No direct light sources in the shot itself, brightest element the foggy night sky illuminated by off-frame street lights.
- 4 Exposures of 12, 15, 20 & 25 minutes

Now to the question:

After development, the negatives had what looked like the equivalent of chemically induced fogging that increased proportionally to the longer exposure times, only I know it has nothing to do with development because the unexposed edges of the film are clear, as they should be. Even the deepest shadows of the negative that had gotten the longest exposure had a base density that takes about 2 1/2 times the time to print through to attain black compared to the shortest exposure shot. Could this be due to stray light from the large Fujinon-C image circle bouncing around for a longer period of time within the camera? Is this something that wouldn't have happened if I'd been shooting, say, 8x10?

As it turns out, perfectly exposed shots with great contrast and shadow detail and all that bore me (I never met a light leak I didn't like), so I'm not too disappointed with the results, but I'm still curious as to what happened here.

Any ideas?