High Speed photography – using the flash to control exposure time

There are other ways to achieve High Speed Photography but everything I’ve written here is based on using normal equipment and not ultra special hardware that can film at 500+ frames per second.

The idea is simple but requires the room/environment to be dark. It works like this:

The camera is set to manual exposure with theĀ apertureĀ chosen so that you get the depth of view desired. You can either experiment with this to get the result you want or use a tool like this to calculate it. ISO should be as low a possible while still getting the exposure you want to minimize noise. Focus should also be preset manually.

If the environment is dark enough then the time of the exposure will essentially be the burn time of the flash. Most flashes can be set to different power outputs. It is usually stated in parts of full power e.g. 1/4 means a quarter of full power. Experiment with different flash power, aperature and ISO to get the exposure you want. Remember that the farther you are from full power of the flash the shorter the burn time will be. The total time a normal flash is lit, or the energy discharged, is around 1/1000 of a second (1 ms). If you lower the flash output then this time is made shorter. That is how normal camera flashes work. This does not apply to studio flashes which lower the voltage to control the flash power.

A really good article on this can be found here here are some numbers from the article (they are for an old Canon Flash called 430 EZ):

  • At full (1/1) power the flash is lit for 1/1000, 1 ms or 1000 us.
  • At 1/4 power the flash is lit for 1/1064 or 940 us.
  • At 1/8 power the flash is lit for 1/2730 or 365 us.
  • At 1/16 power the flash is lit for 1/6450 or 155 us.
  • At 1/32 power the flash is lit for 1/11400 or 88 us.

This means that with a powerful flash and low power output it is possible to achieve exposure times much shorter than what the camera itself can accomplish with the shutter. If the environment is dark enough you can have the camera set to Bulb or 1 second and still capture really fast moving objects.

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