Flash Duration t.1 vs t.5

Let’s start at the beginning. What is the “Shutter Effect”?

Your film and digital camera works exactly the same. Light needs to fall on the sensor (or film) to create an “exposure”. To block light away from the sensor, there is a shutter in front of it, preventing light reaching the sensor (or film) and only opens for a very short time, to let in a certain amount of light to properly expose the scene.

So it’s a dark sensor, that gets light, and then goes dark again.

The shutter effect is the same, except the shutter remains open for longer, but because there is not enough ambient light, the sensor is not exposed. By providing light for a time, and then removing it, we duplicate the effect of opening and closing the shutter.

No light -> light -> no light = Exposure

The faster we can “flash” the light on the scene, the faster is the shutter effect and if we can flash the light fast enough, we can stop motion. The actual shutter speed setting for the camera doesn’t really matter anymore, as long as the setting we choose is slow enough that the whole sensor is open when the flash occurs, but still fast enough that ambient light doesn’t have too much of an impact.

Our main problem remains though that we need to get enough light on the subject in a single flash, and that we can do through standard studio flash units (like the Profoto D1 kit) or off-camera flashes (like the Canon 600EX-RT or Nikon SB900, etc) – both of these are essentially a light bulb that turns on, and turns off. Reaching maximum power quickly and then slowly going dark. The speed that this happens at, is measured in t-values and with a bit of reading and research, you can generally find the t-values for your flash unit at a given power setting.

Please note, that the values differ from brand to brand, and model to model depending on technology and construction techniques used.

Now, generally, you will find that the flash duration is quoted in a t.5 value. That means the time from start of the flash until 50% of the power has dissipated. Unfortunately, that still leaves a lot of light from the flash-tube which can cause ghosting, so to get a more accurate, I would suggest that you work on a t.1 value, which is approximately 3 times longer than the t.5 value. The t.1 value is the time it takes for 90% of the power from the flash-tube to dissipate.

In the graph you will see more or less how the light/power dissipation runs, but essentially you will hit full power, then 50% light gone (t.5) and then 90% light gone (t.1) so if the value for t.5 is 1/3,000th of a second, the t.1 value will be approximately 1/1,000th of a second (1/3,000 times 3), and that is the value you need to calculate to know whether the flash duration is fast enough to stop the action completely.

Now different things move faster and slower and the speed required to stop the action therefore also differs. Some examples:

• Person walking briskly = 1/500th to 1/800th
• Dancer in large jump or movement = 1/1,000 to 1/2,500th
• Pouring water = 1/2,000th
• Splashing water = 1/6,000th to 1/8,000th

(Please Note: These are t.1 values)

So let’s put it into context:

• The Bowens Gemini 500R (500 watt) head has a Maximum t.5 Flash Duration of 1/900th of a second (or 1/300th t.1)
• The Profoto D1 (500watt) head has a Maximum t.5 Flash Duration of 1/3,000th of a second (or 1/1,000th t.1)
• The Profoto B4 (1,000watt) Battery generator and ProHead combination has a Maximum t.5 Flash Duration of 1/25,000th of a second (or 1/8,000th t.1)

Because of the way small flashes work (Isolated-gate bipolar transistor or IGBT power switching) the flash duration of the flash can be quite a lot faster than a standard studio type flash head or monoblock head. this means that speeds in excess of 1/25,000th of a second can be easily obtained, unfortunately at the cost of power output.

As an example, the SB800 will give you a flash duration of approximately 1/10,000th of a second at 1/16th power. Unfortunately, that means you need 4 flashes shooting simultaneously at 1/16th power to give you equal power of a single flash unit at 1/4 power.

They are great for small setups (ice falling into a glass of water) but not for full length portraits.