Recent cold Memphis weather (weird, right?) has forced me to stay indoor. I decided to take out my fish tank, fill it out with water and play with food coloring. It's a very simple setup and there is nothing artistic about the picture but I hope some of you will find the information useful.
I used a set of LED Christmas lights. One row of the lights wasn't enough so I put a whole bunch of them about 2-3 feet (60-90cm) behind the fish tank. At first, they were just behind the fish tank but even at f1.4, there wasn't enough bokeh effect so I had to put them further.
You can see in the diagram that I used 3 light sources (Christmas lights as a background and 2 speedlights) and a light reflector. A Nikon SB-700 was pointed to the fish tank from the left. I position a light reflector on the other side so that some of that light could hit back and illuminate the other side. I put Nikon SB-900 with a grid on the floor and position it in the middle of the shot to illuminate a bit the background. You can probably see that the background (besides the Christmas lights) is not evenly lit - in fact a chair to which the lights were attached is visible to the right. I don't mind it at all, actually i think it looks much better with this uneven background. I used Phottix Odin TTL to trigger flashes but you could do it easily in manual mode with any other flash-trigger system.
I knew from the very beginning I wanted a very shallow depth of field and f1.4 was as low as I could get. Focus can get tricky. I put the camera on a tripod and I put a ruler in a water at the spot where I wanted to drop the food coloring. I manually focus on the ruler and I marked on a side of a fish tank where the ruler was.
Measuring and exposure
Before doing anything, I turned on the Christmas lights and I exposed for the lights in manual mode. This way I made sure that the Christmas lights will be nicely visible in a picture. Then I turned on the speedlights. The first picture I took was too bright so I dialled down both speedlights (the background speedlight by 1 stop and the left speedlights by 2 stops).