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Image of the Month: March


This month's image comes from club member Alan Pounder who has developed an interest in high-speed photography where the author attempts to capture extremely short duration events such as lightning flashes or balloons bursting. Alan takes up the story behind this shot.

These events happen in such a short time that conventional camera shutters can't operate quickly enough and human reaction times are so slow comparatively that by the time you've triggered the camera, the event has already happened and you've missed it. As an example, lightning flashes typically last for about 120 milliseconds, a digital camera shutter has a latency of between 5 and 50 milliseconds and the best reaction time for a human is over 120 milliseconds.

If you try to press the shutter when you see a lightning flash, then, the flash has been over for about 50 milliseconds by the time the shutter actually opens. Some form of electronic trigger is required.

I built myself both light and sound sensors that could operate a camera shutter but had limited success with my equipment and the techniques I was using; this was in the days of 35mm film cameras and I moved on to other things. Going forward some years, I discovered people were using cheap micro-processors like the Arduino do high-speed photography successfully and I also discovered the weird and wonderful world of liquid-drop photography where you capture the result of two liquid drops colliding. By using liquids of different viscosities and surface tensions, you can modify the appearance of the collision and the liquids can be coloured for other effects.

FREEZES
The problems associated with capturing a droplet collision are primarily to do with getting a series of timings right and an Arduino can handle this easily. Then, of course, you've got to take an in-focus, macro image that freezes the droplet collision.

My controller box is a commercial device called a Camera Axe. It is Arduino-based and the software used is open source. As delivered, it works well but I've made some customisations to it which seem to work well for me. It has an output that is connected to the camera shutter release, another which connects to a wireless flash trigger (so I can trigger multiple flashguns), and a third connection to a solenoid valve to release the droplets.

The first problem is to freeze the action and deal with shutter latency. Simply, I don't worry about the camera shutter by opening it in bulb mode before everything happens and use an electronic flash on its lowest power setting, which gives a flash duration of about 1/32000th of a second, and is triggered at the appropriate moment by my controller box. All this is done in a completely dark studio so extraneous light doesn't affect the image.

OPEN THE VALVE
Then you have to get the droplets to collide. The controller box handles that in a sequence which is: open the camera shutter in bulb mode; open the valve for a number of milliseconds to create a particular size of droplet; wait for a number of milliseconds to let the first droplet start falling; open the valve a second time for a second droplet to form; wait a number of milliseconds to allow the first drop to create a splash column and the second drop to collide with the rising column; trigger the flash; close the camera shutter. You then repeat this until you get the result that you want. The delays and sizes of droplets are all determined in a calibration routine that is built in to the controller.

The image above required two Speedlite flashes pointing at the backdrop to reflect the light through the liquids, and they are positioned so there is no direct light on the droplets. I often use LP covers as a backdrop and these are placed upside down in case I manage to capture an in-focus view of the backdrop in a droplet, which acts as a micro-lens, inverting the background. In the case of this image I used Pink Floyd's Dark Side of the Moon and the spectrum and prism on the cover are responsible for the colour swirl in the splash bath.

The camera used was a Canon 7D mk2 with a 50mm f/1.8 lens and 35mm extension tube to give the necessary magnification. The aperture was set at f/13 to give the exposure I wanted and still retain the most depth of field. The camera is mounted on a focus rail which allows me to move it from side to side for framing and forward and back for precise focus on the splash. I pre-focus in a lighted room by standing a large wood screw in the liquid bath where the droplets will fall and focus the camera on the screw's point.

For this image, I probably took about 300 shots to get the right sort of collision. Luckily, the final image only required a small amount of work in Photoshop to remove some unwanted reflections and air bubbles in the liquids.

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