What is HSS?
If you have ever attempted to use a DSLR’s built-in flash, you would have noticed that you quickly run into a problem. Once the flash is popped up, you can no longer take photographs at really fast shutterspeeds, like 1/4000sec. This limitation in shutterspeed shows up when you use flashguns, studio lights and most other types of artificial light sources. While the limit varies from camera to camera, with most, you are limited to a shutterspeed of 1/200 to 1/250sec when using a flash. (Mirrorless cameras can go upwards of 1/1000sec, and leaf-shutter Medium format cameras allow you to go up to 1/1600sec.)
To overcome this, manufacturers like Elinchrom make use of flash sync technologies like Hi-Sync (HS), HSS (High Speed Sync) and HyperSync.
Flash and the Shutterspeed Limit
To understand how HSS works, we need to understand how the shutter curtains work in a DSLR. The shutter of most modern DSLRs consists of two parts, called the first and second curtain.
Let’s assume that we have selected a shutterspeed setting is 1sec. When the shutter button is pressed, the first curtain opens, and light is allowed to enter the camera. After 1sec has passed, the second curtain closes thus, cutting of additional light to the sensor.
However, for faster shutterspeeds like 1/1000sec, the second curtain follows closely behind the first curtain. As a result, a narrow slit of light is created that simulates a fast shutterspeed. At no point is the entire frame exposed to light.
“The “Flash Sync” is how the camera synchronises with the flash so that the light emitted by the flash is recorded on the image sensor.” – Elinchrom.com
When you use an artificial light source, like a studio light or a flashgun, the burst of light emitted from the light source needs to illuminate the subject between the two curtains. The sync speed, therefore, is limited to the minimum time that the entire sensor is exposed, between the time of the first and second curtains.
At shutterspeeds faster than the limit offered by your camera, there is not enough time to illuminate the subject before the light is cut off. As a result, you can't traditionally use fast shutterspeeds like 1/8000sec.
When You Shoot Faster than the Sync Speed
Any attempt to shoot at shutterspeeds faster than the sync speed will result in a black bar forming in the bottom (for landscape framing) or right-hand side (for portrait framing) of the frame.
High Speed Sync (HSS) At Work
Lighting devices like the Elinchrom ELB 500TTL use HSS technology overcome the shutterspeed limitation. HSS fires stroboscopic light to pulse the flash continuously at incredibly high speeds. As a result, the light can illuminate the slit between the two curtains continuously as it moves down the sensor. Since HSS creates so many pulses of light, it gives the illusion of a single continuous light source.
However, because HSS fires so many pulses the drawback is that the power of the flash is quite low and consumes quite a lot of battery power. Therefore, when using HSS, keep the subject quite close to the frame. In this regard, HSS can be quite useful for wedding, product and portrait photography.
The Benefits of High Speed Sync
The simple benefit of HSS is that photographers can use a faster shutterspeed when using flash systems. However, that opens up a lot of new possibilities for photographers.
Here are a few scenarios you can experiment with to make use of HSS creatively.
Scenario 1: For Wedding and Portrait Photographers
Let’s say you are shooting outdoor portraits in bright sunlight or even against the sun. In this scenario, you would be forced to use a narrow aperture because the shutterspeed couldn't be reduced more than 1/200sec to 1/250 sec. As a result, traditionally, getting soft background blur and bokeh was out of the question. With HSS, however, photographers can use shutterspeeds as fast as 1/8000sec. Therefore, shooting with wide open apertures like f/1.4 is no longer a problem.
Scenario 2: For Action and Sports Photography
Photographing sports and action shots require faster shutterspeeds than 1/250 sec to freeze motion satisfactorily. With the addition of HSS, both flash and shutterspeeds as fast are 1/8000sec can be used simultaneously to illuminate the subject and ensure tack sharp images.
Scenario 3: Overpowering Ambient Light
Without flash sync technologies like HSS, photographers were forced to either use either narrow apertures or carry expensive ND filters if they wanted to reduce ambient light and get dark backgrounds. With HSS, it possible to do by controlling the shutterspeed thus giving you more versatility in the field.