Soap film interference. I've done it before (using my 18-55mm) but this is the first session with my macro lens. Using a basic wire frame, I've wrapped a portion of it with a pipe-cleaner so that more detergent/water mix collects. The wire frame is slightly angled to avoid reflection of the lens. Using my external flash (430exii) with stoben diffuser and piece of white paper for additional flash diffusion - I have the flash set at 1/4 power.