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 the aperture 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.

High Speed Photography – using a BB gun

As I wanted to capture events very close to when they happen (like the popping of a balloon) I decided to use a BB gun. This way the sound that trigger the flash will happen “long before” the bullet hits the target and I can alter the delay (or move the microphone) to get the picture at the time I want.

This was the first try and adding more delay would give a nicer result.

As a side note; shooting indoors, in the dark, is very dangerous and requires a lot of preparation. A modern BB gun is very powerful and goes through a water filled balloon with high velocity. It may also change trajectory. I was fortunate to set this up correctly because my older son was skeptic to my initial bullet capture. It turned out he was right and I am grateful.

High Speed Photography – the first shots

One of the things I wanted to test was the famous balloon shot. To take a picture exactly when the balloon raptures. This turned out to be a bit more complex than I had thought. One thing was simply the physics involved. Although I am an engineer at hart I had  never really read up on balloons and what made them pop. It turns out that the big sound bang does not come from the air rushing out of the balloon but rather from the latex accelerating and passing the speed of sound barrier. The problem for me was that using the sound from the popping balloon to trigger the flash leaves very little time.

High Speed photography – the beginning

After having seen so many great photos of what I later learned was high speed photography I wanted to make some myself. I had so many ideas but all of my first experiments failed due either the delay of the shutter release (the time it takes for the camera to actually take the picture) or the limits of the flash synch.

Once I had understood the basic concepts; the fact that the extremely short exposures are not made using the camera but rather the burn time of the flash I started to look for hardware solutions to solve my needs. There were a couple of nice products available but I thought it should be possible to make one cheaper myself and then I could also modify it to my needs.

Velleman is a company that develops small circuit boards for home assembly and they have a product called MK139 – it is a clap on/off switch that you can use to control lights etc. After assembly it turned out that the delay after a clap until the relay closed the circuit was not useful.

The project grew and I replaced the PIC processor that came with the kit with a PIC12F683 and wrote some code to make it work. Once this was done I wanted to be able to control the time between the “clap” and when the flash fired so I added a trim-resistor and modified the code for it. If you want to do this yourself I put the code on https://github.com/reveman/mk139_and_PIC12F683.

If you think the picture looks a bit complicated (messy?) it is because I added some extra connectors to be able to change power, flash connector and microphone/accelerator. These are not required.