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What is flash duration in photography?

What are the most important features you look for when buying a flash? Many photographers consider a unit’s power to be most important, but there are a few other specs that are just as important, including flash duration. In this article, I’ll break down exactly what flash duration is and how it works.

The basics of how a flash works

We should start by diving into the high-level basics of how a flash works. A flash has a role: to release a lot of energy at once, to recharge and to release again. Regardless of where the power comes from, every flash circuit has capacitors that collect energy by storing charges. Capacitors are great because they can release large amounts of power in short bursts: exactly what you need to get a “pop”.

A smaller flash can only be powered by a small 1.5V battery, but with the help of transformers and capacitors the flash is able to produce much more.

Capacitors have certain times when they charge and discharge. Think of the process as storing water in a tank. In order to drain the water as quickly as possible, you need to have large valves. The same goes for capacitors: they must discharge as quickly as possible.

Capacitors do not discharge linearly. Over time, they release less energy. The initial release ignites the gas in the flash tube which is when the output is strongest. As the capacitor discharges, it releases less and less energy, but this energy still ionizes the xenon gas, which extends the duration of the flash. Although the flash is not at its maximum output, the output it gives will still be visible in the final image.

t0.5 versus t0.1

Most companies that make strobes will provide one or two values ​​with their flash: t0.5 and t0.1. These two dictate different properties. t0.5 is a measurement of the time it takes for the flash output to drop halfway (50%), and t0.1 is a measurement of the time it takes for the flash to drop to only 10% of the exit.

A natural question would be: how much light output is reduced to an acceptable level not detected by the camera? There really is no definitive answer to this question. It is fully variable and depends on the speed of the action you are trying to capture, the level of ambient light present in the frame, and other additional factors.

The flash duration tends to change as you increase or decrease the power. To demonstrate this, here are two images, one taken at a lower power and the other at a higher power.

Low flash power = short duration
High flash power = long duration

The magic of the short flash duration won’t happen when using maximum power, as that’s usually the level where the flash duration is the longest. In most flashes it’s around t0.5 1/500, and more expensive units can push t/0.5 1/1000, but that’s still too low. To freeze the movement with the flash, you have to go down. On a scale of 1 to 10, magic will happen between 5 and 7.5. This is usually where there is enough power, but the duration is short.

A “trick” that many photographers use to increase the power of their flashes is to add reflectors. A hard metal reflector will collect all the light and bounce it where it is needed most.

A key to freeze motion

One important thing to keep in mind is that when flash is involved, you freeze motion with flash duration, not shutter speed. For example, here’s an image I took at 1/30th of a second, note the ultra-sharp detail in the eye:

Styling @hadishasovetova
Hair @karinajemelyjanova
Makeup @knollmakeup
Model @rekafedra / @facemodelmanagementhungary
First assistant @seloua_benkaid_kasbah
Retouching @retouchconcept

If you want to know more about freezing motion with light, be sure to read: “HSS doesn’t freeze motion: light is key, not shutter speed.”

Picture credits: Header photo from Depositphotos