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Black Drop Effect
Now bearing in mind that the black drop effect can be seen, and photographed, and that this can be done without anything of an astronomical scale, it seems that the black drop effect is some sort of optical phenomenon. There are other things like this, for example diffraction, Young's Fringes, etc. However, the black drop effect has traditionally been associated with astronomical events such as the transit of Venus.
In this photo, the black drop effect can be seen between thumb and forefinger, which I know were not touching at the time. It looks as if the shadow has bridged the gap, leaving an apparently inexplicable dark region inbetween.
Here's a larger version of the picture...
Now let's zoom in and take a closer look at the crucially important piece of that picture...
Quite distinctive. It looks like a drop of black gooey liquid, like crude oil, stuck like a meniscus between the digits. However, there is no such thing present, and it is something purely visual or an effect to do with optics.
Or to put it another way, the lightbulb is real, the finger and thumb are real, but the black drop is something that can be seen even though it is not physically there. It is, however, visually observable, and its image can be captured on camera.
Now let's see if there's anything to do with perception and contrast. The eye directly seeing the effect has something like ALC automatic level control, so it's possible to read by sunlight or by moonlight, a factor of a million different in intensity. What happens is, for each scene, the eye adjusts. Digital cameras also have to do this, in fact even more so, because digital cameras have a smaller dynamic range than the eye, or even than old-style film. Digital cameras adjust automatically to the scene, to maximise the amount of usable image.
What if we blank out the lightbulb and see what the camera has picked up in close-up...
Just blanking-out the lightbulb, you can now see the anatomy of the shadow bridge in higher detail. Interestingly, it is not totally dark, but has some colouration similar to the finger and thumb. The partial shadow is still quite dark, but the mid part of it has some light in it.