Activity 5. Photographing bursting balloons
If you're using a Polaroid camera, see Appendix C.
- Vivitar 283 flash unit
- 4 AA batteries or SB-4 AC adapter for flash power
- Flash-to-trigger cord
- Flash clamp1
- Piezoelectric sound trigger (requires 9-V battery)
- SLR camera and film
- Balloons, pin
- Background cloth or paper
Background: In this activity, you’ll learn the basics of taking photographs of high-speed events using a single flash unit, triggered by sound. Before you take a photograph, there are several decisions to be made.
- What camera, lens, and film will be used?
The answer to this question may depend on what you have available. There are only a few requirements. The camera should have a B shutter setting and the lens should have manually-adjustable aperture and focus. The choice of film is not critical. A medium speed film (ISO 400) is sufficient for most purposes. The only advantage of higher film speeds is to make it possible to use smaller lens apertures, thus increasing depth-of-field.2
- How far from the subject will the camera be?
Usually, one tries to place the camera as close as possible so that the image of the event to be photographed will fill the frame of film. If the subject is small, the minimum focusing distance of the camera may limit the size of the image.
- What background will be used and how far behind the subject will it be?
It is frequently desirable to have a dark, featureless background that will not distract attention from the main subject. A sheet of dark cloth or paper can be placed behind the subject. The surface should be rough to minimize glare. The farther behind the subject the background can be, the better. A black background, if placed close to the subject, can produce significant and distracting glare.
- Where will the flash unit be placed?
The flash unit is usually placed as close to the subject as possible, while still giving uniform coverage. This increases the concentration of light on the subject, as needed for achieving short flash duration. One should also consider the angle at which the light strikes the subject. By choosing the angle correctly, shadows can be produced that help to show structure and texture in the subject. If the flash unit is placed on the hot shoe of the camera, care must be taken that opening the shutter of the camera does not discharge the unit. This can be done simply by putting a piece of tape over the center contact on the flash foot.
- How far from the source of sound will the sound trigger be placed?
The greater the distance of the sound trigger from the source of sound, the longer the delay will be before the flash discharges. If one is not sure how much delay is needed, the trigger can initially be placed very close to the source and then moved farther away if necessary.
- What lens aperture will be used?
- What flash duration is needed?
If the flash unit is used on an automatic setting, the selection of the lens aperture is easy.3 Set the calculator dial located on the side of the flash unit to the speed of the film being used. Then read the aperture opposite the color of the automatic setting. This assumes that the flash unit and camera are the same distance from the subject. If this is not the case, the auto-thyristor module can be moved to the location of the camera using an auto-thyristor extension cord,4 if one is available. Otherwise, follow this guideline: If the flash unit is closer to the subject than the camera (the usual situation), open the camera aperture wider. If, for example the camera is twice as far from the subject as the flash, open the aperture by one stop. An explanation is provided in the Notes.5
The flash duration must be short in order to freeze high-speed events. In Activity 2, you learned how to achieve this. There are times, however, when a longer flash duration is desired so that there will be blur in the photograph. This can provide a sense of movement that a sharp, frozen image cannot. The choice is a matter of personal preference. In order to create blur, the flash duration may be increased by placing the auto-thyristor module in a mode other than yellow. Your tests in Activity 3 should help in making a decision here.
As examples of photographs that use blur to good effect, consider those below. Both were taken with the flash unit in manual mode to give maximum flash duration. For the hammer smashing a light bulb, note the streaks of particles on either side of the hammer head. These provide effective contrast to the sharply-defined cracks in the bulb. Likewise, the water spray emerging from the mass of water once held in place by the balloon provides the clue that something dramatic is about to happen to the seemingly static mass of water. (A needle sticking out from the bottom of a cork was dropped on the balloon to pop it.).
Hammer breaking a bulb
Burst of a water balloon
- Before taking photographs, prepare a table to record information about each photograph that you take. With this data and the corresponding set of prints, you’ll be able to decide how to make changes in your setup in order to improve the photographs for your next roll of film. In addition, if you happen to discover something interesting, you’ll have the record to corroborate your discovery. A ready-made table is provided for downloading.
- Set up to take a photograph of a balloon burst. Load your camera with film and mount the camera on a tripod. Slide the flash unit onto a flash clamp, which you then clip to a table or chair. Be careful, though, to place the flash so that you are not prone to bump into it as you move around.
- When the equipment is arranged, check to make sure that the lens aperture is correct, the camera shutter is placed on B, and the lens is focused on the subject. Then turn out the room lights, hold the shutter open, pop the balloon, close the shutter, and turn on the lights.
- Open the aperture by one stop and take another photo under otherwise identical conditions. One reason for this is that the flash unit's calculator dial frequently underestimates the size of the aperture needed for correct exposure. The dial is calibrated for small rooms where there is significant reflectivity from the walls. The latter situation is generally avoided in taking high-speed photographs. Another reason for opening the aperture is that the film may be less sensitive than normal at the short exposure times typical of high-speed photography.
|Tip: Whenever taking high-speed photographs under new conditions, experiment with the lens aperture to be assured of getting good exposures. In addition to the calculated exposure, make a second exposure where you open the aperture one stop more than the calculated amount. For added safety, make a third exposure with the aperture opened two stops more.|
- Take a series of photos like the ones above, moving the sound trigger to a different position each time to capture the burst in a different stage.
- Take some photos with long flash duration in order to test the effect of blur. When you increase duration, remember that brightness increases also. That means you need to make the lens aperture smaller. If you're using the auto-thyristor module, you can use the calculator scale on the side of the flash unit to determine what aperture to use.
- Develop and print your film. For future reference, decide what your optimum experimental conditions were and what aperture gave the best exposure. Record your conclusions with your data. The next time you take photos like this, you'll either know how to get the best results or you'll have a good idea of what to change to make improvements.
- For a variety of options for flash mounting accessories, go here.
- If you’re not already familiar with the subjects of film exposure and depth-of-field or if you need a review, see Appendix D.
- If you've substituted a variable resistor for the auto-thyristor module, you can determine what lens aperture to use by following these guidelines. Of course, if you have a flash meter, that's the easiest way.
- Appendix B provides more information on the auto-thyristor extension cord.
- The camera aperture determined from the flash unit's calculator dial is only valid when the camera and flash are at the same distance from the subject. As the flash unit is moved closer, the light intensity reflected to the flash unit's photocell increases. This causes the flash discharge to be quenched sooner. Thus, the light reaching the film is less than the amount needed for correct exposure. Opening the camera aperture corrects for this. The guideline of opening the aperture by one stop when the camera is twice as far from the subject as the flash is only an approximation. Bracketing exposures is always a good idea.