Saturday, October 05, 2013

High Speed Flash Photography.

SB-910. Shot at 1/32th power (approximately 1/17,000th of a second)
When we need to freeze action, the shutter speed of the camera has to be high, otherwise you will end up with a blurred image. Typically for sports, a shutter speeds between 1/500th of a second to 1/1000th of a second should give very sharp images. To get these kind of shutter speeds, one or more of the following should hold true:
    1. There is plenty of available light (e.g. daytime)
    2. You have a fast lens.
    3. Your camera can shoot at high ISO without too much degradation of image quality.

In broad daylight, you should be able to get those kind of high shutter speeds even with a normal lens, and normal ISO. But once the light starts fading, then your choices are limited to managing with #2 (fast lens) and/or #3 (setting a higher ISO). It is for this reason you see sports shooters with those huge professional lenses that are designed to gather as much light as possible, and big camera bodies that are designed to shoot at high ISO speed and still give excellent image quality.

But what happens when the light is so low that neither a fast lens or high ISO are sufficient to effectively use a high shutter speed? The answer would be obvious. You introduce light. This could be any type of light, but for the purpose of high-speed photography, we will assume it to be an external flash or the built-in flash (if you have one).

Nikon SB-900 flash mounted on a mini-tripod.
External flashes are great to create light, and allows you to use higher shutter speeds. But they have one limitation called the 'sync speed'. Due to the way they generate light, the highest shutter speed you can effectively set on your camera usually ranges from 1/200th of a second to 1/320th of a second. That is nearly not fast enough to freeze action.

To overcome this limitation, and to achieve very high shutter speeds (like the 1/17,000th of a second that I managed to pull off in the first photo), we need to look at the external flash from a different perspective.

A flash can generate different levels of power. These are normally referred to as "full-power", 1/2 power, 1/4th power, and so on. What this essentially does is that the flash stays on for a longer period, or shorter period depending on the power output. Here are the typical values of a normal external Nikon flash (SB-800).

Flash Output Level Duration of Flash
Full Output 1/1 1/1050th of a second
1/2 Output 1/1100th of a second
1/4 Output 1/2700th of a second
1/8 Output 1/5900th of a second
1/16 Output 1/10,900th of a second
1/32 Output 1/17,800th of a second
1/64 Output 1/32,300th of a second
1/128 Output 1/41,600th of a second

From the above table, we can observe that if we lower the output level of a flash, the duration of that flash becomes shorter. We can use this information to convert the flash duration as a virtual shutter speed. i.e. if we were to shoot in a near dark environment, where the only light source is the external flash, the output of the flash becomes the shutter speed. The actual shutter speed of the camera is not important!.

The following example illustrates how this knowledge can be translated into an useful exercise. I wanted to do some splash photography, by dropping random stuff into a tank of water, and capturing the action by freezing it at a very high speed. I chose 1/32 output, which gave me an effective 1/17,800th of a second shutter speed.

My Splash Photography setup.
Equipment used:
1. Nikon D800 on a tripod with the 105mm f2.8 Micro-Nikkor lens.
2. A fish tank filled with water.
3. SB-910 flash on the side of the tank (The plastic is used to protect it from water splashes)
4. An A4 white paper stuck to the other side of the tank to act as a reflector.
5. A colored backdrop.
6. Random fruits and vegetables used for the photography.
7. A remote to trigger the camera.

Camera settings:
1. Manual Mode.
2. Aperture f11 (gives nice depth of field)
3. Shutter speed (1/60th of a second). Remember, the shutter speed is irrelevant here - as long as it is lower than the sync speed of the camera.
4. Manual focus. I placed some fruits in the middle of the tank, and manually focused on that before switching off the lights.
5. Flash in manual mode at 1/32 power, controlled by the camera through wireless mode.

Once everything was setup, it was time to switch off all lights and work in the dark. The actual sequence of events went something like this:

1. Hold a bunch of fruits in the left hand.
2. Hold the remote in the right hand.
3. Drop the fruits in the water tank at approximately the same place where the camera is focused - and hit the remote at the same time.

Depending on how well you coordinated the 2 tasks (dropping the stuff in the water, and hitting the remote), you will get results like these:

Shot with Flash power at 1/32.

From here onward, it is a matter of patience, and timing.

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