D300 built-in flash, Part 1: Blinding speed

Started Dec 25, 2008 | Discussions thread
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Marianne Oelund Veteran Member • Posts: 6,416
D300 built-in flash, Part 1: Blinding speed

If you've ever wanted to photograph a speeding bullet, your camera's built-in flash may be just the ticket. As is the case with many electronic components, smaller -> faster, and the tiny flash tubes used for the built-ins can be very fast indeed.

This is the first of a multi-part posting which will look at many aspects of the D300 flash and speed, including operational details which affect shutter response, high shutter speed considerations, TTL exposure, commander mode, etc. Let's begin with a short study of the basic element of flash photography: The flash pulse.
Triggering and Power Control

Flashes work by discharging a storage capacitor into the flash tube. When full power is used, the discharge is allowed to run its full course. To obtain lower powers, the pulse is shut off mid-way. Thus whatever "power level" is used, the pulse profile follows the same curve, with lower-power pulses simply being shorter.

Once the storage capacitor is charged and ready, the process begins with the application of a high-voltage trigger pulse to a trigger electrode on the flash tube. After this, it takes about 10-12usec for the flash tube to start producing light. The initial build-up is exponential until about 15usec after trigger (light level is still very low here) and the light output then ramps up roughly linearly until about 30us, where the light level is at half peak brightness. After that, it rounds off slowly to its peak value at 120-140us, then begins an exponential decay as the storage capacitor loses voltage.

Here is a family of flash output curves as collected by my scope equipped with a fast photodiode. The long curve extending to the far right is full output. Also shown are 1/1.3 power (shutoff at 920us), 1/2 power (shutoff at 480us), 1/4 power (shutoff at 220us) and 1/8 power (shutoff at 120us).

Taking a Closer Look

Using a fast scope sweep rate, we can have a good look at the lowest power settings. Here, at 20us/div, are 1/8, 1/16, 1/32, 1/64 and 1/128 power pulses:

At the far left, you can see a burst of RF energy, which is radiated by the high-speed, high-voltage trigger circuitry and picked up by the scope probe. This is a useful "marker" to tell us when the camera is initiating the flash pulse. Note that the lowest power pulses cut off during the early, linear rise in brightness. This means that the shutoff time doesn't need to be as early for these low-power pulses, since the area under the curve varies approximately with t^2. [Contrast this to the higher-power pulses, where the pulse duration is roughly proportional to the flash power setting.]
Pulse Variability

Flash tube discharge is a somewhat random process, and the shape of each pulse is variable. There are factors such as battery type and charge state, and charge-up time for the storage capacitor, which result in even more variation. This family of curves, all at 1/16 power, show how the flash compensates for this variation; when the pulse amplitude is higher, the shutoff occurs a little earlier. In extreme cases, such as when the flash is fired quickly in succession, or when the batteries are barely able to charge the storage capacitor, the pulse durations can become 20-25% longer than nominal.

Obtaining Real Speed

As shown above, at the lowest manual setting (1/128 power), the total light pulse duration is around 15us. This is nowhere near the limit of the flash unit's speed, however. Faster, lower-power pulses are used routinely by the flash, such as in commander mode, and the very lowest outputs are achieved when using fill flash with TTL auto exposure.

By using high ISO settings, close subjects and sufficient ambient lighting, fill flash pulses can be as short as 3-4us. The light output for these fastest pulses is more than 4 stops below Manual 1/128 power, i.e., about 1/2000 power.

Now you know the "recipe" for freezing hummingbird wings and other small, fast subjects. Enjoy.

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