by Mickey Rountree
When using flash, we normally have to shoot at the camera’s “synch speed” shutter speed or slower. For most cameras the synch speed will be 1/200 sec, but check your manual to be sure. A shutter speed faster than the synch speed does not allow the shutter curtains to be fully open, but instead the curtains open as a slit that moves across the sensor. The higher the shutter speed, the narrower the band becomes. So using flash at a shutter speed faster than synch speed will result in a band of underexposure, and the band gets larger as the shutter speed gets faster.
Normally at “synch speed” or slower, the shutter curtains open completely, the flash fires, everything is lit evenly, and then the shutter closes. High speed synch mode allows us to use flash at shutter speeds up to the fastest speed of the camera. It requires a flash that is compatible with the camera, and has high speed synch (HSS) mode. It works by continually firing low power flashes at a fast rate, so that the entire image is evenly illuminated. The higher the shutter speed, the more rapid the pulses, and the lower the power output of each flash. These pulses may be several thousand times per second and a speedlite can’t recycle fast enough at full power.
So why do we need HSS? The main use is in shooting with flash outdoors where bright sunlight requires a faster shutter speed. While it may be possible to stop down the aperture and achieve a proper exposure at synch speed, depth of field will be great. Many portrait and glamour photographers prefer to shoot at wide apertures for shallow depth of field, and also slightly underexpose the background which requires even faster shutter speeds.
What are the disadvantages of HSS? First the speedlites (or now even larger strobes) must be specific to the camera brand, and they generally cost more than speedlites without HSS. Second, HSS decreases the output of the flash, so it must be close to the subject, and it may even require multiple speedlites. The higher the shutter speed, the lower the flash output, so you will need to adjust flash output with every shutter speed change. Third, even though the light output is low, those multiple flashes consume a lot of power, and battery life is shortened. And last, but not least, all of those fast pulses can cause a speedlite to overheat. The better speedlites will shut down when they overheat; strobes without good heat protection may self-destruct, or worse yet, catch on fire.
Speaking of battery life, if you work with speedlites a lot, it pays to invest in good rechargeable batteries and a good quality charger. I have found Eneloop to be the best batteries, and I use a Powerex 8-cell charger.
Here I’ll show you some examples of using HSS to capture a portrait with shallow depth of field to isolate my model in bright sunlight. I had my model under shade so there wouldn’t be harsh shadows on the face. I used a Magmod softbox with two Yongnuo YN600EX-RT speedlites off camera and a YN-E3-RX controller on the camera.
It should go without saying that I am in manual mode so that I have complete control of aperture and shutter speed and also adjust my flash power manually. The first shot is at 1/200 sec, f/16 with no flash. This gives me the slightly dark background I want to help separate my subject. I find it best to turn off the flash and set the background exposure first. Without flash the subject is totally silhouetted.
The shot below is at 1/200 sec (my camera’s synch speed), and f/16. It’s nicely exposed and balanced, but at f/16, the background is too sharp for the effect I’m after. Also even at f/16, the two speedlites were only ay about ½ power.
For the next shot I switched to HSS, 1/4000 sec and f/2.8. Notice how soft and out of focus the background is. The two speedlites were at full power to provide enough light at f/2.8.
Below is a similar series. The first shot is at 1/200 sec and f/16 and the second shot is HSS, 1/3200 and f/2.8. Again it took two speedlites at full power to light the shot with HSS at f/2.8.
