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Photographing falling stuff

Details

Food colouring dropped in milkI wanted to capture the splash that objects make the moment they fall into a liquid, but this is pretty hard to do if you have to operate the shutter release by yourself: you need a lot of luck to capture the right moment, and a lot of time trying to.

The drop from the point where the object is released till the surface of the liquid or the point where it has to cross the lens, is in this case about forty centimetres (16") and only takes a few hundred milliseconds. You'd have to take into account the delay of the camera, the available light, etcetera.

I figured I needed some electronic assistance. So I took to the drawing board and used a combination of hardware and software to help me out.

The set

Two chairs were placed back-to-back on a table. A plank sitting horizontally on the chair's seats functions as a starting point for the drop: in this way the height above the surface is always the same. Setup for photographing falling stuff

The optical 'gate' consists of a transmitter (a laser diode) 'T' pointing at a receiver (photo resistor) 'R', each one fixed to the leg of a chair with a tie-wrap.

The point where the object is to cross the laser beam is marked by a plumb line, 'P'. You can see the red dot of the laser on it. The plumb line is only usedto determine the point from where the object should be dropped to cross the laser beam. This point is then marked on the plank.

The camera 'C', mounted on a tripod, is pointed at the place where the object will hit the liquid, and pre-focused (autofocus set to manual). Flash 'F' illuminates the object and it is controlled by the electronical system 'A', consisting of an Arduino microprocessor with my own software.

The electronic setup and the software I wrote are described on this page.

The LED lights on top of the left chair are used to make small adjustments to the set, when the room is darkened.

How it works

This is what I did to take the pictures of drops of liquid falling in a bowl of water or milk:

  • set up the camera on a tripod. I used a 90 mm macro lens, which I prefocused on the point where I expected the drops to hit the liquid in the bowl (this is also where the plumb line came in very handy)
  • set up the flash, and the bowl of liquid
  • align the laser with the photo transistor and determine the place from where to drop stuff
  • darken the room
  • open the camera shutter
  • drop the object through the laser beam; I used a thick drinking straw to hold just enough liquid for one (or a few) drops
  • let the electronics trigger the flash at the right time
  • close the shutter

I was really pleased with the results. Not all drops were successful: now and then I had to adjust the flash timing, and often they did not cross the laser beam in the right place, or missed it completely. But that was to be expected with the primitive way I used to measure them out and drop them ;)

The images that did work out looked just like what I'd seen in magazines or TV documentaries. Quite beautiful, I think. Some of them are included below. More can be seen on the dedicated gallery page.

   
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