Using flash guns to capture the perfect water droplet photograph is one of the best ways to get the result you want. Flash provides plenty of light to use a small aperture giving a good depth of field and it reduces the effects of motion blur.
However it is very important to realise that it is the duration of the flash that freezes the image and removes the motion blur NOT the shutter speed, all of these images were taken with the camera in bulb mode and an exposure around 1/4 second. The amount of motion blur that is acceptable is down to the photographer but the higher the magnification being used the more motion blur becomes apparent.
This page attempts to illustrate the problem with motion blur and the ways to combat it.
The set up was a Canon 580Ex flash gun positioned 150mm away from the droplet which was in free fall approximately 200mm below the release solenoid valve. The timing to position the droplet was achieved using a Cognisys StopShot system . If you look at the position of the droplet against the ruler in each shot you will see it is gives a very accurate and repeatable droplet position, ideal for timing of water droplets.
Starting with the flashgun, in manual mode, on 1/1 (full power) each photograph ‘halves’ the flash power so the sequence is 1/1, 1/2, 1/4, 1/8, 1/16, 1/32, 1/64, 1/128 of full power. I have altered the aperture during the sequence in order to get a set of usable images.
The best way to observe the motion blur is to compare the highlights in the droplets, as the power reduces the length of the highlight shortens until around 1/32 power it is just a dot.
From the results of this experiment I rarely shoot droplets with the flash above 1/32 of full power to minimise the blur. At power levels higher than 1/16 the blur is clearly visible.
To do this the flashguns are positioned very close to the subject so I can use small apertures and a low iso setting.
Canon 580Ex flash gun 1/1 Power
Canon 580Ex flash gun 1/2 Power
Canon 580Ex flash gun 1/4 Power
Canon 580Ex flash gun 1/8 Power
Canon 580Ex flash gun 1/16 Power
Canon 580Ex flash gun 1/32 Power
Canon 580Ex flash gun 1/64 Power
Canon 580Ex flash gun 1/128 Power