Tips & Tutorials >> All Tips & Tutorials In Focus: The Basics of External Flash Photography- Part 9

Adjusting the Flash Duration to Capture Subjects

The light intensity of a Canon Speedlite flash is controlled by the flash duration, so we can also refer to a strong flash as a long flash, and a weak flash as a short one. In cases where a Speedlite unit is set to a small flash output, the flash duration can be as short as several hundred microseconds. In this article, I will introduce photography techniques that make use of the properties of a Speedlite flash. (Reported by: Koji Ueda)

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Capturing a Fleeting Moment of the Water's Movement Using the Speed of Light

Using the flash enables you to capture fleeting moments that are not visible to the naked eye. The flash output of a flash unit can be lowered to reduce the flash duration to as short as several hundred microseconds. In order to capture the shape of the subject clearly, it is necessary to ensure precise focusing by stopping down the aperture to achieve deep depth-of-field. However, reducing the flash output to shorten the flash duration may sometimes result in insufficient light. In this case, instead of raising the flash output, increase the ISO speed. Also, using a black background allows you feature your subject by creating a high colour contrast.

EOS 60D/ EF-S18-55mm f/3.5-5.6 IS II/ Manual exposure (200 sec., f/14)/ ISO 400/ WB: Auto/ Flash: SPEEDLITE 430EX II (Manual, 1/64)


  • Use a small flash output
  • Adjust the brightness with the ISO speed

Shooting Condition

I turned off the lights and opened the door slightly to capture a glimpse of the subject in the bathroom. The camera was secured on a tripod. I made use of an off-camera shoe cord, and positioned the flash directly above the subject to provide direct lighting.

Position of the subject, camera and Speedlite

A: Off-camera Shoe Cord

B: Approx. 0.6m

C: Approx. 0.4m

Steps for Flash Photography

1. Attach a cloth in the background

Attach a piece of plain-coloured cloth or paper in the background. A material that prevents reflection of the flash light would be ideal.

2. Set the focus in advance

Set the focus in advance using MF and fix it at this point.

3. Release the shutter at the right timing

Release the shutter according to the subject's timing.

Tips for Flash Photography

1. Use a small flash output

The flash duration is shorter when the flash output is small. To capture a fleeting moment of the subject's movement clearly, it is better to reduce the flash output as much as possible. Doing so allows you to shorten the flash duration to several hundred microseconds. This is even faster than the maximum shutter speed of 1/8000 second on a professional camera model.

2. Alter the ISO speed to adjust the brightness

You may not be able to obtain sufficient light if the flash output level is set to the minimum in order to shorten the flash duration. In this case, instead of raising the flash output, you can increase the ISO speed to produce the intended level of brightness.

3. Set the shutter speed to 1/200 sec.

To capture shots by making effective use of the flash duration, there is a need to eliminate the influence of ambient light as much as possible. That being said, it would be difficult to photograph in a pitch dark condition, so an amount of brightness enough for identifying the subject will be needed. To minimise the influence of ambient light, set the shutter speed to the highest flash sync speed of about 1/200 to 1/300 second.

Principles of Flash Photography

Unlike the sun or a stationary light, flash is an "instantaneous burst" of light. In an environment where outside light is completely shut out, therefore, the subject will not be exposed to light except for the duration when the flash is emitted, regardless of how long the shutter remains open. For example, if the shutter is left open for one hour in a pitch dark environment, exposure takes place only at the instant the flash is fired. In other words, a fleeting moment of the subject's movement can be captured by adjusting only the flash duration without regard for the shutter speed setting. For a Canon Speedlite flash unit, a larger flash output increases the flash duration (maximum of about 1/500 second), which in turns leads to a longer exposure time and more noticeable blur. Conversely, a small amount of flash output gives a short flash duration (minimum of about several hundred microseconds), and causes less blur.

Flash duration of a small flash output

A: Flash Duration (Exposure Time)

Flash duration of a large flash output

B: Flash Duration (Exposure Time)

When the same flash unit is used, the flash duration shortens when the flash output decreases, and becomes longer when the flash output increases. To shorten the exposure time, you can reduce the flash output to shorten the flash duration. Doing so allows you to capture a fleeting moment of the subject's movement more clearly.

Koji Ueda

Born in Hiroshima in 1982, Ueda started his career as an assistant for photographer Shinichi Hanawa. He later became a freelance photographer, and is now engaged in a wide range of work from magazines to commercials while shooting different cities and landscapes all around the world. He is also a writer and a lecturer at photography lectures and workshops.