Many modern external flash units have a “stroboscopic” flash mode, allowing you to use multiple bursts of flash in a single photograph. On a Nikon push the mode button until “RPT” appears. On a Canon push the mode button twice until “Multi” appears. Using this mode you can fire off a number of flashes in one exposure. This technique is great for capturing a sequence of movement in one photograph, such as a dancer in motion. It works best in a darkened room with a dark background behind the subject, where the flash is the main light source. If the background is too light, then it tends to overexpose with the cumulative exposure of all the flashes, overpowering the subject.
Once you have selected this mode on your flash you need to select a power output (1/1, 1/2, 1/4,1/8, 1/16, 1/32, 1/64, 1/128) – most flash units will only allow you to select 1/4 power or below in this mode. You then need to enter the number of times the flash needs to fire in succession and the hertz rate.
The hertz rate determines in what space of time the series of flashes will be discharged. Let’s say for example that you set the number of flashes to 5 – a high hertz rate would mean that the 5 flashes are fired in very quick succession, whereas a low hertz rate means that the 5 flashes are fired over a longer period of time. There is a relationship between the power output and the number of flashes you can fire in quick succession. If the frequency and the power level are set too high, then the flash will not have a chance to recharge between bursts. Further information about these correlations can be found in the flash manual. It is also important to check how many bursts you can do in a row, as the flash needs to cool down.
After you have decided on the flash settings you need to get the exposure right. Set the camera to Manual (M) mode and estimate how long your shutter needs to stay open to capture the full range of the movement.
Your aperture setting is determined by the power output you selected and how far the subject is from the flash. The only way to determine this accurately is to use a hand-held flash meter.
We recommend using the LCD as a guide and adjusting the power output of the flash if the subject is too bright or too dark. If you are not able to get the desired aperture at the highest power setting for the number of flashes you want, then you can either increase the ISO or move the camera closer to the subject. Learn more on this and other flash techniques on our Mastering Flash Photography Course.