>The photos shown above look a lot more spherical >than a liquid, since the greatest light absorption is in the >center, where a liquid would be the least in the center and >furthest boundaries.
Stan, you raise an interesting point, but depending on the composition of the viscous fluid, it may or may not behave as you state.
For instance if you scroll down to page four of this PDF it seems that all the spread patterns are contrary to what you state. And I believe that a viscous lubricant used in mechanisms such as these would have a higher viscosity leading to a drop form more toward glycerin in figure two than would the ethylene glycol or propylene glycol. But in all cases, the equilibrium state is densest in the middle.
So my spots seem to be more bead like in that wider apertures, enough light gets underneath them to make them less visible, but from about f/7 and smaller they become more pronounced. Whereas dust which is more irregular in shape, light can get under it until the more narrower apertures of f/11 or smaller.
But I’m no chemist, so it’s only a layman’s observation and opinion.