Activity 7. Using a contact trigger to capture a football kick
- Vivitar 283 flash unit
- 4 AA batteries or SB-4 AC adapter
- Flash-to-trigger cord
- Piezoelectric sound trigger (optional)
- SLR camera and film
- Mini-tripod (optional)
- Football and tee
- 1-2 m long hookup wires
- Aluminum foil
- Large cloth sheet or blanket
- Large pad or mattress
This activity uses a very simple trigger, a metallic switch, to photograph a football kick like the one above. Two wires are connected to the PC cord and are placed in front of the football so that they are not touching. When the football is kicked, the wires touch and the flash discharges.1
The arrangement of equipment is shown in the overhead view below. The football rests on a tee. A heavy blanket hanging from the ceiling serves as the backstop. The bottom of the blanket must not be secured tightly; otherwise, the ball will bounce from it. Secure the blanket just enough to prevent the ball from slipping underneath. It will help if the blanket drags on the floor. It's also a good idea to place a large foam pad, mattress, or other cushion below the blanket to keep the ball from bouncing off the floor.
The camera may rest on the floor, be raised with a stack of books, or be mounted on a mini-tripod. The flash is placed nearby. The wires from the PC cord should be taped in place in front of the ball (or just to the side if you don't want the wires in the photograph). Small strips of aluminum foil can be taped to the bare ends of the wires to provide greater contact area. The other ends of the wires can be taped to the cord from the flash unit.2 When the ball is kicked, it should force the wires into contact and set off the flash unit. You may want to experiment with the positioning of the wires in order to capture the football at different stages during the kick.
The photograph below shows the placement of football, camera and flash and the blanket which serves as the backstop. (In retrospect, the black cloth on the floor was not a good idea, because the kicker could slip on it easily.)
|Safety considerations: As for the previous activity, be sure that everyone stands away from the possible path of the ball. Letting some light leak into the room during the kick is very important. The kicker must be able to see the ball. Injury could result if the kicker missed the ball and ended up on her back on the floor.|
On photographing other collisions:
Football kicks and tennis ball smashes are relatively easy to capture. This is probably because the objects are soft and the collisions last several milliseconds. On the other hand, the collision of a bat with a ball or a club with a golf ball is trickier to photograph. (They're also more difficult to set up.) Trying to capture any crush in a golf ball is very difficult. Sound triggers are problematic to use in these situations. The unavoidable delay in the transmission of sound from the collision to the microphone is great enough that the collision is likely to be over when the flash discharges. The photograph to the right shows a typical result. A contact trigger like that used for the football may work better, but it would be difficult to hide the wires. Designing a trigger that would work well for such collisions would be a good challenge.
|Photographing soccer ball kicks, like football kicks, is relatively simple. However, the results are typically undramatic. This is probably because the area of contact of the foot with the ball is so great, that the force per unit area on the ball is too low to deform the ball very much. Moreover, the way the foot cradles the ball may mask any significant deformations. This makes the angle at which the photograph is taken important. The ball in the photograph to the right shows greater deformation than usual. Perhaps this is because the ball was kicked with the front of the foot rather than the side.|