Flashing beyond the sync speed
Modern digital single-lens reflex cameras (DSLRs) use a mechanical shutter to control the exposure time. This opens at the set exposure time to allow light to reach the image sensor and closes again light-tight after the exposure time has elapsed. The shutter of current cameras consists of two curtains that run one after the other. After the shutter is released, the first curtain starts, releases the sensor and the exposure time is started. To ensure that the selected exposure time is maintained, the second curtain starts moving shortly before the end of the exposure time to completely cover the sensor again. In the following video you can see very clearly how the curtain of the Nikon D3s shutter behaves at shutter speeds of 1/100 second and 1/1000 second:
As the mechanical shutters cannot be opened and closed as quickly as desired, the described principle quickly reaches its limits with very short exposure times. On the video you can clearly see that the curtains behave quite differently at an exposure time of 1/1000 of a second. At the beginning of the exposure, the first curtain opens and increasingly exposes the image sensor. Before the first curtain has completely released the sensor, the second curtain already begins to close. As the first curtain is not yet fully open, an open slit now moves across the image from top to bottom. As the exposure time becomes shorter, this slit naturally becomes narrower. This is why this type of shutter is also called a focal-plane shutter (FPS).
If an picture is to be illuminated with a flash, the flash must be fired at exactly the right moment due to the very short flash duration, i.e. when the first curtain is fully open, the sensor has been completely released and can be illuminated. Therefore, only exposure times at which the two curtains of the shutter do not work as a “ slot” can be used, as otherwise dark bars will be visible in the final photo. This shortest selectable exposure time is called flash sync speed, sync time or X-sync. Typical flash sync speeds for cameras with a focal plane shutter are between 1/160 second and 1/320 second. This time includes the time in which the curtains are fully open and the movement time of the curtains.
Below this specific (flash sync) time, everything works without any problems with system flashes on the camera or studio flash units. But what options are there if you want to expose faster than 1/160s, 1/250s or 1/320s, for example, because you want to shoot with a wide-open aperture for creative reasons and despite sufficient ambient light, but don't want to use a gray filter or don't have one to hand?
Solution HSS/FP
Most system flashes have a technique that solves the problem with a simple trick. Instead of a single flash, the flash is transformed into a kind of fast “flickering light” and flashes continuously while the gap between the two shutter curtains moves across the sensor. This mode is called HSS for Canon and FP for Nikon. The advantage is obvious: the picture is illuminated absolutely evenly. But that's where it ends. Because the flash has to light up very often within a very short time, the loss of performance is exorbitant. In addition, only expensive system flash units that are HSS/FP-capable offer this technology, the flash tubes are put under a lot of strain and, above all, considerably more information has to be exchanged between the camera and the system flash than a simple “ fire now!”.
Solution Supersync
Portable flash generators or studio flashes are not HSS/FP-capable. Nevertheless, it would of course be nice to be able to use them faster than the flash sync speed?
The Supersync technology offers the desired solution to precisely this problem. Originally, the whole thing was discovered by chance when a studio flash was fired via a photocell with the TTL clip-on flash. It turned out that it can happen that the studio flash fires exactly during the time of the actual focal plane shutter exposure. But this is the crux of the matter - it can happen, but it doesn't have to!
The theory behind Supersync is quite simple. However, you should know something about the required technology.
TTL radio triggers are one of the most important. These work slightly differently to normal radio triggers and normally first send out a “metering flash” to measure the flash exposure. With Supersync, however, this metering flash “accidentally” fires the studio flash, which fires after a certain period of time. The camera then also starts its exposure. Actually, after the TTL metering flash, the main flash would be fired first. In this process, however, the studio flash has already been triggered and fires at the exact moment when the first curtain of the shutter is fully open. The result is a correctly exposed sensor. It is therefore important to have very precise timing, which is influenced by various factors, such as the firing time and the characteristic firing characteristics of the studio flash, the speed at which the signals are transmitted and processed, the exposure time set on the camera, the size of the sensor and the speed of the shutter. This makes it clear that Supersync cannot always work and that you simply have to try it out. However, a flash that fires “slowly” (but far too fast for the human eye) is a good basis, as you can see from the following diagrams.
The graph represents the flash firing characteristic and the gray bar the time the camera needs for the exposure. The setting for the exposure time is fixed. As the curtain moves from bottom to top, the drop in exposure is also clearly visible.
A flash that fires at high speed always has an extreme drop in performance. The faster the flash tubes, the less suitable they are for the SuperSync principle, as the drop in exposure is far too great.
So the slower a flash fires, the better. Even with just a little less speed, the exposure fall-off is already enormously reduced.
A flash with a slow flash tube achieves exactly the desired effect. An almost uniform exposure. Elinchrom's Ranger Quadra models with standard head (S-Head) are just suitable for this. The burn-off time of the Action series is already far too fast. However, the whole thing apparently only works at full power, as this is apparently regulated by the lighting duration.
However, the Jinbei FL-II Porty flash set incl. standard flash head is perfectly suitable for Supersync. Apparently, the flash duration of the Jinbei Porty is so long that you can select almost arbitrarily short exposure times across all power levels. Neither a great loss of energy nor a really visible progression is noticeable. Simply perfect! Together with the Yongnuo YN-622C E-TTL II radio flash trigger, an absolute dream team.
Conclusion
Supersync is simply great and offers incredible potential in terms of lighting and image composition. In addition, low-speed flashes finally have a reason to exist again. If there are still very slight gradients with some time/power combinations, these can easily be corrected with a graduated filter in Lightroom.
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