70-300 mounted plus reverse ring plus reversed 50mm f/1.8.
Focal length = .5-3cm. Focal depth = maybe 1mm.
This is a backlit water drop on cotton. I don't really know how big it is, but I can tell you that with this setup, the circular end of a standard safety pin fills the frame.
I took a cotton ball, stretched it out a bit, then sprayed it with water. Set it up in front of a clip-on light, then started searching for droplets through the viewfinder.
Post-processing consisted primarily of sharpening the droplet, enhancing the overall contrast of the image, adding vignetting, and colorizing.