The problem with the shutter becoming a slit that travels across the sensor for a period of time is that if you fired a traditional flash at any period during that transit, the resulting picture would simply show a brightly exposed strip of illuminated scene corresponding to the area of the sensor where the shutter was when the flash fired.
If you want to use flash, then, you’ve got to stay within the sync speed capabilities of the camera. The problem arises when you want to use wider apertures to produce shallower depth of field, or if and when you’d like to use your flash to freeze superfast moving action. In these cases, high speed sync comes in very handy.
First, some terminology. Canon calls this capability High-Speed Sync, but Nikon calls it Auto FP. Either way, they work the same and accomplish the same great result. It all starts with a flash capable of high-speed synchronization. Canon’s 600EX-RT and Nikon’s SB-5000 are both well capable of performing high-speed sync magic.
Setting Auto FP is done on Nikon camera menus, while Canon Speedlites like the 600EX-RT have a high-speed sync icon (a lightning bolt with an H next to it) on the back of the flash LCD. To engage high-speed sync on a 600EX-RT, click the Sync button on the right to toggle from normal mode to high-speed sync (and on to rear curtain sync). With high-speed sync engaged, a little lightning/H icon will appear on the back of the flash’s LCD.
When set to high-speed sync, the flash will fire several thousand very short bursts of light for as long as it takes the slit-sized shutter to transit the sensor. These multiple flashes essentially increase the duration of the flash, allowing it to continue to cover the entire frame.
The problem with high-speed sync and these several thousand small flashes is that they are much lower output than a typical flash. This might be no problem if you’re using high-speed sync to enable shallower depth of field and therefore wider apertures, because at f/2 you’re going to need less light to illuminate the subject.
However, if you’re using high-speed sync in order to try to overpower the sun’s ambience, you’re going to need to get the flash as close as humanly possible.
For instance, if you want to create the “day for night” effect and make an otherwise normal daylight background appear dark and dramatic, you need to underexpose that ambient image. That can be accomplished by, say, increasing the shutter speed or shrinking the aperture. At ISO 50, for instance, an exposure of 1/2000th of a second at f/11 on a slightly overcast day will deliver a background that is at least two stops underexposed. With a flash set to high-speed sync, and positioned as close as possible to the subject (likely just out of frame), you’re maximizing the flash’s output in order to correctly illuminate the subject while allowing the background to dramatically fall out of the area of illumination and into darkness.
Notice, too, that I described a fairly bright ambient scenario in which this technique can take place? Where might this technique work even better, and give you more options for flash output and placement? A low light situation—like a sunrise or sunset, when the flash doesn’t have to work as hard to match the ambient light.
Whether your goal is to use the flash for faster shutter speeds that stop fast-moving action, or because you simply want to use fill flash when making a shallow depth-of-field portrait, high-speed sync is an invaluable flash too.