I had the notion to light the background for a pleasing color. In the first failed try, two Nikon SB-800 speedlights were used, one as main light close to the milk drop, and one on the background. The timer triggered the background light directly, and then its flash optically triggered the foreground main light in SU-4 mode. The main light was at minimum power to be fast to stop the milk drop fall. The background light was much farther back, but much stronger (and slower), which was a dumb idea, because the slower background light ruins the shot, but in an interesting way.
The milk drop has fallen 12 inches here (24 inches in the final test of all lights), which computes about 8 feet per second velocity. The main light was at 1/128 power in all frames, and it did stop the milk drop well, clear and sharp, until the slower background light (higher power setting) continued to add effect, much more than I expected from its distance.
What appears as a shadow under the milk drop is NOT a shadow at all. It cannot be a shadow, because the main light path and camera lens path makes an X, not pointing the same direction, which is seen in the highlights on the milk drop. Turns out that the false "shadow" is the dark side of the milk drop, illuminated afterward from the rear by the continuing reflection of the slower light on the background as the drop falls farther (drop itself is blocking the light on the background). It was very visible when the main light was faster than the background light. The only benefit of that dark "shadow" is that it does clearly represent the difference in speed of the two lights.
You can click each image for a larger view. The only change in all frames is that background light was changed to be the power level specified below:
|1/32 power||1/16 power||1/8 power|
|1/4 power||1/2 power||full power|
The main light at 1/128 power was about 11 inches in front of the milk drop, and the front of the camera lens was about four inches. Background was at seven feet behind, and its light was four feet in front of it. That seemed far enough back to not expect any ill effect. But it had effect, not due to the light levels, but due to flash speed.
This test sequence was taken to try to resolve what that dark shadow was, since no shadow was possible here (certainly not this shadow). Then the truth was easily revealed. These photos show the speed difference very clearly (due to difference of two power levels). The fast flash in front cleanly stopped the falling milk drop, but the slower light on the background was present a longer time, and was blocked from the camera lens for a longer time by the moving milk drop. This really surprised me! It wouldn't have mattered on a motionless opaque subject, but it was necessary to abandon the pretty background light for these shots.
Some shots would be with the AlienBees B800 at full power from about 15 feet, and some with the Nikon SB-800 at less than one foot, so if I could not light the background independently, then I felt it necessary for the milk drop and background to be very near each other, to keep the background from appearing bright at high power, but dark at low power. In the final run, I put the background up within a couple inches of the drop (to illuminate it), which still shows a slight difference at lowest power. But this lost my planned deep red color. And then it was necessary to work around the real drop shadow on the background (a small angle fixed it). The final results are here.