For water-drop shots, autofocus usually isn’t the best approach. The drop is tiny, moving, and low-contrast, so AF often won’t lock. Instead:

1. Switch to manual focus.
2. Put a small object exactly where the drop hits the water—such as a pencil, bolt, or similar narrow object.
3. Focus manually on that point, ideally using live view for better precision.
4. Leave focus unchanged and shoot when the drops fall.

A coin at the bottom of the tray isn’t ideal because you want focus at the impact point on the water surface, not below it.

Also check whether you’re closer than your lens’s minimum focus distance. If so, move the camera back and crop later, or use close-up gear such as a true macro lens, extension tubes, reversing rings, close-up filters, or a teleconverter.

Setup tips: fill the tray close to the top, and consider placing another tray underneath to catch overflow. The water depth can affect the splash shape; around an inch of depth can work well. The key is making sure the drop lands in the same spot every time, then pre-focusing there manually.