If you have read our previous post titled Understanding Flash Sync Speed, you probably know what high speed sync (HSS) is. If not, or you need a refresh, read the post or watch this video instead. The following discussion of HSS may be a little too technical or detailed but the hope is to clarify some of the confusions I have seen in a few internet discussions.
Shutter speed is a relatively easy concept to understand. It is the time the image sensor is exposed to the light coming in from the lens. However fully revealing the sensor takes time as the shutter travels at a limited speed. For most DSLR cameras, the sensor has two curtains, the front curtain opens first then the second curtain follows to block the sensor. The speeds they travel has to be exactly the same or the exposure will be uneven across the frame. Shutter speed of 1/250s means, exactly 1/250s after the first curtain starts to open, the second curtain starts to close. If the first curtain takes 1/250s to fully open, the second curtain has to close immediately after the first curtain is fully open. For a higher shutter speed, for example 1/2000s, the second curtain has to begin closing before the first curtain is fully open. That means the two shutter curtains will form a slit that travels across the sensor. At no time the entire sensor is fully revealed.
(Maximum) sync speed
The maximum flash sync speed is defined as the fastest shutter speed at which the entire image sensor can be illuminated by a single flash pulse. But how long is the flash pulse? There isn’t a standard and the flash duration also depends on the flash power. The flash duration is the longest for the maximum power output of a typical Xeon flash. Typically it is about 1/1000s. If a camera has a maximum flash sync speed of 1/250s, the shutter has to take no more than 1/250s – 1/1000s = 1/333s to open in order to leave the entire sensor area open for the 1/1000s flash pulse.
It also means, you can sync a little faster than 1/250s if the flash duration is very short.
The next available standard shutter speed is 1/320s. If the flash duration is shorter than 1/320s – 1/333s = 1/8000s, you should be able to sync at 1/320s. At least, that is the case with my Nikon D200 and a SB-800 set to 1/32 power and triggered via PC port. Actually the black border is barely visible at 1/400s.
With a eBay trigger, I can sync at 1/320s but 1/400s gives me a large black band. It is reasonable to assume there is some delay in the signal conversion and transmission process when a wireless trigger is used.
High speed sync
During high speed sync flash photography, the shutter still travels at the same speed but the sync speed is no longer limited to the shutter speed at which the entire image sensor can be illuminated by a single flash pulse. The flash provides a uniform illumination as the shutter slit travels across the sensor. Even though only part of the sensor receives the flash illumination reflected from the scene, on average, the entire scene appears to be illuminated by a uniform light source during the exposure.
A typical uncontrolled Xeon flash discharge does not give out even illumination. In order to provide the appearance of uniform light, the flash control circuits pulses the discharge at high frequency. The controlled discharge provides consistent light output during each pulse. The amount of light output is controlled by the pulse frequency and duty cycle (percentage of each period during which the pulse is on).
The following image (adapted from Nikon’s patent application US2002/0048457 A1) illustrates the aforementioned concept.
If you are curious about a real world example, here is a capture of SB-800 firing 1/32 power at 1/8000s. The period of the pulse is 20 microseconds. The pulse intensity does not actually have the square shape shown here. I used a logic analyzer that is a digital, not analog, capture device.
High speed sync hack
Not all flashes support high speed sync. Nikon SB-600, SB-800, and SB-900 support it. None of the Nikon DSLR camera’s popup flash supports it. Some third party flashes do not support it even through they may claim to be Nikon iTTL-compatible.
Some smart people have figured out a way to enable high speed sync for the flash that does not support it. Basically you need to mount a HSS-compatible flash on the hot shoe of the camera, then trigger your remote flashes from the PC sync port on the flash or the camera. When HSS is enabled, the trigger signal on the PC sync port is sent as soon as the shutter curtain starts to move, instead of after the first shutter curtain is fully open.
How could this work if the flash duration is only ~1/1000s while the shutter slit takes longer than that to travel?
The following is a discharge curve of a SB-600 at 1/1 full power. Clearly, the “1/900s” 1/1 flash duration in the SB-600 spec isn’t the time it takes to completely discharge the flash. It is typically the time it takes for the light intensity becomes 1/2 of the maximum light intensity on the right shoulder of the discharge curve. The actual flash duration between the start of the flash to the point flash light is completely gone is quite long: ~1/125s.
This hack works to certain extent but the obvious problem is uneven illumination because the remote flash is still making a single discrete discharge. Sometime this may not be very noticeable if the subject takes up only a small area of the frame or the flash is used for fill in bright light.
Keywords: Flash photography, High Speed Sync, HSS