Flash Duration

Had a great time over the weekend at the Heard Museum in McKinney, Texas. The Heard Nature Photographers Group hosts a workshop each year when members can come by and try all kinds of new and different things. For the past few years, I’ve set up an aquarium and hosted a booth called Splash Photography.

This is one of those things that so easy to do and really a lot of fun. You can get quite elaborate in how you set it up, but for this event, it’s simply filling an aquarium with water, setting up a camera and a flash and dropping things into the water. We use a remote trigger to take the photo so we get lots of images of flying fruit or a tell tale stream of bubbles. But that’s part of the fun. It become a competition of sorts to see who can time it best.

In years past, I would look at the images and some would be a little blurry, even though I’m using a flash to “freeze” the subject. Yes, as a rule of thumb, a flash is used to freeze motion but you’ll find that different flashes vary quit a bit in how well they do this.

 Ooops, lucky I had both lens caps securly attached

Ooops, lucky I had both lens caps securly attached

For the typical compact camera flash, the flash duration is between 1/200 second at full power and 1/20,000 at the lowest output level. That sounds fast but when I add some diffusion to the flash - like a small soft box - I end up having to use the flash at, or very close to, full power. That just left some subjects still a little blurry.

This year I decided to use my Profoto B1. That’s a 500 watt-second battery powered mono-light that is part of the Profoto (OCF) Off Camera Flash family. In Freeze mode, the flash duration of the B1 runs from 1/1000 second at full power to 1/19,000 second at the lowest setting. Being a more powerful flash, I was able to set it at a little less than 1/2 power, giving me a flash duration that froze any motion in our little aquarium setup.

If flash duration is important to you, let me share with you one caveat when looking into the specifications for your equipment. Not all flash duration specifications are equal.

There are two different flash duration time specifications that are used. They are designated with a “t0.5” or a “t0.1”. The difference is how the time value is measured. To understand the difference take a look at the chart below. Let’s avoid all the deeply technical stuff and concentrate just on the concept. The flash's light is turned on and left on for a very short period of time. When the power is turned off, it takes a moment for the excited material to stop glowing so the light take a moment to trail off.

Flash-Duration-Graph.jpg

To measure flash duration, some systems use t0.5 which means that they begin measuring the light when the light intensity reaches 50%, and stop measuring when the downward curve crosses the 50% threshold again.

When using t0.01, measurement begins when the curve crosses the 10% line.

As you can see by the chart, t0.01 can be a much longer flash duration than t0.05. Not that either one is a better yardstick, but don’t read specification from one unit that is measured one way and compare it to another flash unit that is measured a different way.