Taking the nonsense out of Flash sync and High Speed Flash sync.
There are those that state that Mid Format camera such as the Phase One 645DF+ and others can achieve flash sync up to 1/16000 second, where as 35mm DSLR are limited around 1/250 second.
(Note that these values are just examples and can vary between vendors)
DSLR users state that they can achieve the same. Well reality is that they can and cannot. DSLR’s can use flash at higher speeds then the normal 1/250 second limit by using a special method called “Hyper Speed Sync or High Speed Flash”. Obviously that is not the same as standard flash synchronization.
What is flash sync?
It is very simple, it is the time that the sensor is fully exposed to the light, and hence the shutter is fully open. Meaning that the first and second curtain (in case of a focal plane shutter) are fully open. That time is determent by the shutter speed setting.
A shutter speed set at 1/1000 second, exposes the sensor in full for 1millisecond to the light. Within that time window the flash pulse must ignite and complete.
Flash pulses are powerful bursts of light with a very short duration and defined by a T0.1 and T0.5 value. For example a broncolor move pack produce a flash pulse of 763 microseconds at 300 Watt/s.
As long as the pulse fits in the window, a proper exposure will be achieved.
What is High-Speed Flash sync?
Different vendors use different names for this, some call it hyper speed or high-speed flash. It allows for higher shutter speeds then the standard physical limitation of the shutter mechanism. The shutter mechanism in many DSLR’s consists of two mechanical curtains that open and close. And because they are mechanical they have a finite speed to open and close.
Note that today some sensors are equipped with an electronic shutter instead of a mechanical version.
However the majority of DSLR’s is equipped with a mechanical focal shutter. The flash sync is limited to 1/64 to 1/250 sec. (variations do exist between the different brands and models).
To achieve higher shutter speeds, the flash pulse is modified. The Flash is no longer a single high power pulse but is now chopped into multiple lower flash pulses extending over time. The sensor is no longer exposed in full (both curtains fully open) as was the case with normal flash sync. Instead the two curtains move over the sensor with a small opening/gap between them. During this move the flash unit transmits multiple pulses of light so that each part of the sensor is equally lit.
It is not uncommon to find High-speed flash implementations on DSLR’s up to 1/1000s. However that method only works when a compatible flash unit is used e.g. Nikon SB series.
Is the result the same?
Near equal results can be achieved, that is to say both the high end mid format camera’s with flash sync up to 1/1600sec and the consumer/semi pro DSLR’s can acquire captures at high speed. However there are several other aspects to be looked at.
It is these aspects that make the difference between a Pro Mid format camera and a 35 mm DSLR.
- Maximum Power:
A flashback has a maximum power level by design, that is to say that a flash unit can only produce a certain amount of joules or Watt second. A studio flash pack will produce much more power (e.g. 1200 Watt second) then a standard speed light or the build-in flash in the camera.
In other words the flash unit is limited in power.
Cameras with flash sync at high shutter speeds use the full power of the flash pulse, since the flash only fires once within the capture time.
Cameras with High Speed Sync must level out the flash power over multiple mini flashes and time. There is no time to recharge the flash within a capture period. That means that the flash intensity is seriously reduced, and can become an issue in certain light conditions.
In short, Flash Sync can use the maximum power of a flash unit, High Speed Sync CAN NOT use the full power for the exposure.
- Battery Live and Flash power issues:
Lets assume the following. An object requires 300 Watt/s exposure with a shutter speed of 1/1000 second.
The flash sync method when triggered will ignite a single Flash pulse at 300Watts/s
The High Speed Flash sync method will ignite 4 flash pulses (assuming the physical limit of the shutter mechanism is 1/250s). Each pulse must produce 300Watt/s to equally illuminate the different areas of the sensor. That means that the flash unit must be charged at 1200Watt/second.
Hence , High Speed Sync reduces the battery live considerable and limits the Flash power capabilities.
- Studio packs:
Studio packs and monoblock flash units generally do not have the capability to generate High Speed Flash pulses. Instead they are engineered around a single short flash pulse with very stable light temperatures and defined T0.1 and T0.5 pulse wave shapes. High Speed Sync camera methods are generally not useable.
Secondly studio packs with battery are often used on location with powerful lights (>1200Watt/s). If a High Speed Sync method was applied, then the pack had to have huge batteries and would become to heavy and to restrictive.
Speedlight’s typically support High Speed Sync in combination with a branded DSLR. However seriously constrained in power and battery life
- Motion Capture:
Shooting pictures with flash will freeze the movement of the object independent from the shutter speed (assuming that the ambient light is neglectable. It is the duration of the Flash Pulse that will freeze the movement. It is not uncommon to find flash units with flash durations of 1/10.000 second or 100microseconds.
Flash Sync at 1/1600s (shutter is open 625 microseconds) will freeze the motion in one single instance. That is the moment in time when the flash ignites , locking down the motion for the duration of the flash. If the flash duration was 1/5000 sec then the motion is captured for 200 microseconds.
High Speed Flash Sync at 1/1600s with a DSLR with physical shutter limitation of 1/250 second will require (4000 micro seconds/625micro seconds) about 7 mini flashes spread over 4 millisecond (shutter physical speed limitation). That means that different areas of the sensor will be exposed on different times and hence the final picture can look awkward and jerky for fast moving objects.
- What is better?
A difficult answer, it all depends on how you apply photography. For some the High Speed Sync limitations will not be an issue, where as for others it has serious constrains.
Personally I use both, for my studio and location work that requires day by night shots with a shallow depth of field I will use the mid format camera capabilities to achieve high shutter speeds with standard flash synchronization. For other events such as real life events, I use my DSLR and speed lights in High Speed Sync mode.
One thing is fore sure, 35mm DSLR’s do not have the capability that Mid format camera’s have in terms of Flash sync at high shutter speeds. Instead they have a work around to simulate that capability. The results can be similar but the use is very constrained.
Thanks for reading,