It seems to me that the subject is oscillating that's the only real way
you'd get that kind of fall-off?
(Unless there's optical issues like
the edge of the image circle or vignetting)
I'm figuring the
subject was about zone VIII-ish because of the way it's staying visible
even into the short exposures it must be up near the shoulder of the curve
(A grey square would disappear into black)
The other plausible
possibility is the speed of light theory... Do you have grey-square
accelerator?
