So how do you "Catch a Falling Star"?



Fun and games with High Speed Flash Photography

Firstly, you need to be using a Film or Digital SLR camera with, ideally, a separate flashgun(s). If you use a point & shoot camera it is impossible to achieve this type of image as you do not have enough control over the flash duration the camera gives you.

Secondly, the basic concept of the shutter speed, aperture setting and iso settings give the correct 'exposure' can be thrown in the bin.

Thirdly, you need to understand that this type of event lasts for less than 10 thousandths of a second (10/1000 sec) from the initial pop to the collapse of the water sphere and the time whilst the balloon is out of shot and the water starts to collapse is only a couple of thousandths of a second.



Yes I know cameras are capable of shutter durations as fast as 1/8000 second which, in practise would catch this event but the image would still be blurred due to the very high speeds involved. Bear with me and we'll get to the explanation

What I am going to try to give you is a method for consistantly repeating this image so that you can adjust your lighting and attempt the same shot again and again with some degree of repeatability. It is not impossible to get this type of image if you take enough images or if you are just lucky with one shot but it certainly isn't repeatable if you rely on your reactions to take the photograph because they just aren't good enough

The Setup

1. SLR camera on a tripod with a cable release or use the self timer.
2. Water filled balloon suspended from some strong elastic bands.
3. Flashgun (one or more) positioned to provide the light on the subject.
4. A bin to collect all of the water
5. A pin.
6 A triggering sensor, if you want repeatability.

The Techie stuff
Skip to the summary, if you are a geek like me

Every SLR camera has a built-in electromechanical delay caused by the mirror lifting out of the way and then the shutter curtain opening and then the flashgun firing. So everytime you press the shutter button you are actually taking an image that is there least 10 milliseconds after you press the button. Most of the more domestic digital SLRs have even longer delays upto 50 milliseconds. For day to day photography this built-in delay isn't noticable and can be completely ignored.



So in order to use the shutter trigger to take an image of the balloon as it bursts I would need to press the shutter trigger BEFORe I popped the balloon and hope that I guessed the right amount of delay. Not the most repeatable of techniques.

So how do we remove this built-in delay so that the image is taken exactly when we want it to happen? This is where the use of flashguns comes in. By manually triggering the flashgun while the shutter is open we can remove this delay and the subject is only exposed during the length of the flash. To avoid any secondary image that happens whilst the shutter is open you need to take the images in a darkened room.

Flash duration vs. Shutter speed
Using flash to freeze fast moving subject is a very old technique and is used everyday. What a lot of people dont realise is that a typical flash duration is only around 1/1000 second which is actually quite a long time.

The trick is to use the flashguns placed very close to the subject, which makes the flash duration much shorter, or, if your flashgun has the facility, adjust the power output of the flashgun which does the same thing.

Using my 580EX flashguns at 1/128 of full power the flash duration is now very short, typically 1/35,000 second, which is much shorter than any shutter speed on a normal camera and will remove the motion blur from fast moving subjects.



Triggering Sensors
An acoustic sensor is used to react to the popping of the balloon and then fire the flashgun for you. The most basic sensor is a small microphone that is wired to an electrical circuit which, in turn, is connected to your flashgun. If you know a tame electrical person they can easily knock this circuit up for you or you can buy a completed circuit on-line. There are numerous home-made sensors that react to light, sound, vibration or movement, a google search will find you plenty of links.

Some sensors can be plugged into magical black boxes like the Mumford Time Machine to allow you to delay the reaction time or to cause some other event to occur, this is what I use.

If you use a basic sensor without a built-in adjustable delay you can use the Speed of Sound to introduce a small delay for you. Every foot from the sound source adds approx 1 millisecond delay so with the microphone placed 2 feet from the subject you get a 2 milliseocnd delay before the flash fires.

Connecting the Flashgun to the Sensor
Some flashguns have a PC socket that can used to trigger the flashgun manually, you can modify some guns to fit a socket to them, like the 580Ex. An alternative method is to use a hot shoe flash adaptor that you can wire to the sensor circuit and sit your flashgun on top of.

To summarise all of the above
1. Set up your camera with either a cable release or self timer.

2. Position your subject ie the water filled balloon suspended by rubber bands over a bin. I use a black velvet cloth as a background.

3. Focus on the subject and then turn OFF the autofocus function. Later in a darkened room the camera will try to refocus and you don't want that to happen.

4. Exposure: I typically use ISO 200, 2 seconds and f16 as a starting point for exposure.

5. Position your flashgun close to the subject and set the power output then connect the gun to the sensor circuit.

6. Position your microphone and connect it to the sensor circuit.

7. With everything ready TURN OFF the room lights, this is my favorite mistake lol

8. Open the camera shutter and then pop the balloon before the shutter closes. See the logic in having a 2 second exposure now?

9. You should see FLASH then hear POP then feel WET Actually the pop happens first but if there is zero delay in the trigger circuit you will see the flash fire 'first'. Think about thunder and lightning for the explanation.

10. Turn on the room light and check the results. You will need to experiment but that is always the challenge.

11. Have fun.


If you are still confused just note me and I will try to help you


best regards
Kev Lewis
Gallery Director for
Animals, Plants and Nature