I want you to do an experiment, if you can.
Find a bright background (daytime sky, very brightly lit wall). Make a pinching motion right in front of your eye so you can just barely see the background past it—and focus your eye on the background.
Just as your fingers are a hairs-breadth apart... do you see a bridge of shadow between them?
- What does it look like?
- Does your camera see the same thing?
- Why the heck is it there?
I have a few possible explanations for this shadow-bridge:
1. Light blurring an actual overlap
2. Overlapping shadows
3. Diffraction
These would show up in different situations. It's just hard to distinguish them because in the time it takes to refocus my eyes, my fingers can move slightly!
1. Light blur:
If my fingers are actually overlapping along my line of sight, then if they were perfectly in focus, I would see them appear to be actually touching. (One finger's almost-contact-point slightly behind the other's.
If that is blurred, the light from either side of the almost-contact might create the appearance of a blurry column of shadow.
I could try to falsify this by using a set of carefully placed objects that are very stable but allow fine adjustment.
2. Overlapping shadows:
When my fingers are almost touching and there's a visible gap, the halo of blurred silhouette around each one overlap with each other. This could cause the appearance of a lenticular shadow between them. I'm not sure why it would cause smaller, shorter lenticular shadows above and below that one, though.
3. Diffraction:
...I'm really hand-waving on this one, but I wonder if diffraction across the fingerprint ridges could cause the weird zigzag shadow I sometimes observe.
Maybe I could try replicating with various smooth vs. ridged objects.
Huh, that's weird...