What are the most important things to look for in a flash? While many photographers think that the flash power is the most important feature, there are other important specifications, such as the flash duration. This article will explain exactly what flash duration means and how it works.

The basics of how a flash works

Let’s start with the basics of flashes. The flash’s primary function is to release large amounts of energy, recharge and then release it again. No matter where the power comes, every flash circuit contains capacitors which store energy and collect it. Capacitors are fantastic because they can deliver high power in short bursts. This is exactly what you need to get that “pop”.

Although a smaller flash can only be powered by a 1.5V battery, a larger flash unit can produce a lot more power with the help of capacitors and transformers.

There are a set of times when capacitors can charge or discharge. The process can be compared to storing water in an aquarium. Floodgates large enough to quickly drain the water are essential. This is also true for capacitors. They must discharge as quickly as possible.

Capacitors do not discharge linearly. They release less energy as time passes. When the initial release ignites gas in the flashtube, this is when the output is the most powerful. The flash duration is extended by the release of energy from the capacitor as it discharges less. However, that energy is still ionizing xenon gas and prolonging its life. The flash output will still be visible in the final image, even though it isn’t at its maximum power.

T0.5 vs. t0.1

The majority of strobe manufacturers will only provide one or two values for their flash: , t0.5 and HTML0.1. These two dictate different properties. T0.5 measures the time taken for flash output to drop half-way (50%), while t0.1 measures the time required for flash output to drop to 10%.

The obvious question is: At what point does the light output drop to a level that is acceptable and not detected by a camera? This question is not easy to answer. It’s variable, depending on the speed of the action, ambient light, and other factors.

The flash duration changes when the power is increased or decreased. Below are two images that demonstrate this. One is at lower power and one at higher.

Low flash power = short duration

Maximum power is not the best setting for short flash duration. It is usually the lowest level at which flash duration is longest. It is usually around t0.5 to 1/500 in most flashes. However, more expensive units can produce t/0.5 to 1/1000. But that is still too low. To freeze motion with flash you must go lower. The magic happens between 5 and 7. This is where the magic happens, but it is usually short.

Reflectors are a trick many photographers use to increase the flash power. Hard metal reflectors will capture all light and bounce it to the areas that are most important.

The Key to Freezing Motion

It is important to remember that flash freezes motion using flash duration and not shutter speed. Here is an example of an image that I took at 1/30th second. Note the sharp detail in the eye.

You can read more about freezing motion with light by clicking on “HSS Doesn’t Freeze Motion: Light Is the Key, not Shutter Speed”