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Activity
5. Photographing bursting balloons
If you're using a Polaroid camera, see Appendix
C.
Equipment needed:
- 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
- Tripod
- 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?
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
- What flash duration is needed?
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.
Data table in rich-text
format (editable in a word processor)
Data table in pdf format
- 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.
Notes
- 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.
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