As photographers, using a flash or a strobe to light our images can accentuate and lift them up to an entirely new level. One of the key elements to be considered when shooting with flash, and one that gives us several creative options, is the flash duration of the strobe used to create the image. Historically, the top flash sync shutter speeds of 35mm film cameras and modern DSLRs have been around 1/200th to 1/250th second. A shutter speed of 1/200th second is pretty slow if you are trying to stop any sort of moving subject, even a slow moving subject. Because of this limitation, for decades now when shooting with flashes, photographers have relied upon very fast flash durations, not the shutter speed, to stop motion.
Before we get into all of the options, let’s first define the term flash duration. A flash duration for any strobe or flash is technically the length of time that the flash tube emits light for a single burst. Most flash manufacturers quote what are known as the t0.5 or the t0.1 flash duration specifications. The t0.5 value is the period during which the flash intensity is above 50% of its maximum brightness. The t0.1 value is the period during which the flash intensity is above 10% of its maximum brightness. The graph below shows exactly how these different flash duration specifications relate to the entire burst of light emitted by the flash. For a more in-depth article on flash duration check out “Demystifying Flash Duration.”
Using a strobe with a fast flash duration is one way to stop the motion of a moving subject when using flash. Typically, a t0.5 value of 1/2,000th second or higher will be more than adequate for most photographers who need to freeze the motion of a moving subject. For those who are photographing extremely fast moving subjects, like dancers or exploding liquids, a much faster flash duration with a t0.5 value upwards of 1/10,000th second is usually required to freeze the subjects movement.
Many pro photographers have historically relied on medium format cameras, which incorporate leaf shutters in the lenses, to help them gain faster flash sync speeds. On some medium format cameras with leaf shutters the top flash sync shutter speeds are as high as 1/1,600th to 1/2,000th second. When shooting with leaf shutters ideally the flash duration of the strobe would be shorter than the shutter speed used. Hence, when using a shutter speed of 1/2,000th second a flash duration of 1/2,000th second or shorter would be required. Otherwise, the closing of the leaf shutter would clip the burst of light from the flash. The beauty of leaf shutters is that they make syncing with strobes at high shutter speeds very straightforward. A good example of this is shown in the image below, which was shot with a Hasselblad medium format camera and a strobe with a very fast flash duration.
Flash manufacturers usually denote in their marketing materials the flash duration of their offerings at the highest power setting and also the fastest flash duration (typically at the lowest power setting). Sadly, most flash manufacturers don’t tell you the flash duration for each power setting, though some have this information in the manual that comes with the flash. A few manufacturers display the actual flash duration for each power setting on the flashes LCD readout, which is very handy. But these numbers aren’t always accurate. Because of this, using a light meter like the Sekonic L-858D-U, which can very accurately measure the flash duration, is quite helpful. Right now, the Sekonic L-858D-U is the only light meter on the market that can measure a strobes flash duration.
As most photographers don’t own expensive medium format cameras, for the rest of this article I will concentrate on flash durations as they relate to DSLRs. With DSLRs, we have a few options when shooting with strobes. If we intend to stay at or under the top flash sync shutter speed (i.e. 1/250th second) then using a flash with a very fast flash duration is advantageous. Using a fast flash duration allows us to freeze the movement if we stay close to the top 1/250th second flash sync speed or alternatively, we can “drag the shutter” and shoot at a slow shutter speed to show some of the motion blur and then let the flash freeze the movement, as show in in the image below.
Even when shooting portraits of a stationary subject, the flash duration can make the difference between a sharp image and a blurry one. With a slow flash duration, like a t0.5 flash duration of 1/250th second, if the subject moves even slightly during the exposure this will result in motion blur and a blurry image. With a fast flash duration, for example a t0.5 flash duration of 1/2,000th second or higher, even if the subject moves the flash will freeze the subject. Of note, if the background is dark, then you could literally throw the camera up in the air and trigger it as it falls and still get a sharp image using a fast flash duration. To sum up, anytime I am syncing at or below the top flash sync speed of my DSLR (i.e. a shutter speed of 1/250th second or less) using a fast flash duration is critical.
Alternatively, with the advent of Hypersync and Hi-Sync (HS) flash technologies, which allow us to sync modern DSLRs with strobes at shutter speeds up to 1/8,000th second, using a flash with a slow flash duration is paramount for success. HS flash technology works by timing the flash to coincide with the narrow slit of the shutter moving across the sensor. Ideally, the timing allows for the maximum flash output to line up with the shutter release, which usually means the flash is triggered milliseconds before the image is taken. Understanding HS (and how it is different than High Speed Sync) is a complex topic and the best explanation that I know if is in an article that I wrote for Elinchrom entitled, “HS vs HSS: What is the Difference?”
Because syncing the flash and the camera using HS techniques is so difficult, a slower flash duration helps to get good results. Typically a flash duration of less than 1/800th second or less is required for the best results when shooting with HS technologies. Slower flash durations like 1/500th second are even better. What these HS techniques allow us to do is freeze the motion of a moving subject like never before. As in the image below, this inline skater is frozen mid-jump with a flash sync shutter speed of 1/8,000th second. Freezing a subjects motion with the shutter speed is much more effective than trying to freeze it with a fast flash duration.
One of the things you will notice if you measure the actual flash duration of any flash—be it a speedlight, plug-in studio strobe, monobloc or battery-powered strobe—is that the actual flash duration from flash to flash can vary, and sometimes it varies widely depending on the flash. By using a light meter like the Sekonic L-858D-U it is easy to make sure you are using a fast enough flash duration to stop the motion or vice-versa that you are using a slow enough flash duration so that HS techniques can work well.
As shown in the images in this blog post, understanding how the flash duration—as well as the chosen shutter speed—will affect the final image is paramount to your creative vision. With a sound understanding of flash duration we can adjust the lighting on our subject so that it matches what we are trying to create. In many situations, the actual, real world flash duration of your strobe or flash is often much more important than the power output of the flash. Flash duration is one of the key specifications to look at when purchasing a strobe or flash. It is also the key specification to keep track of when trying to freeze action. Hence, using a light meter like the Sekonic L-858D-U, which can extremely accurately measure the flash duration [using t0.5 or t0.1 specifications] will help to make sure you can very accurately control the flash duration.
For more demystifying flash duration, check out the Sekonic blog!