Methods and Setup

Equipment

Camera

• Minolta Dimage Z1
• Tripod
• Protective Plexiglas shield

Flash

• Vivitar 283 Flash Unit (shorted w/ paperclip)
• Flash Mount
• SB-4 AC Power adapters

Trigger

• Piezoelectric Sound Trigger
• Delay unit
• Flash to trigger Cord
• Flash to delay unit cord

Pellet Gun

• RWS Diana Pellet Rifle
• BBs (RWS Meisterkugeln;.177 in.)
• Bag of sand to trap BBs

CDs

• Drill
• CDs
• Stand to hold CDs in vertical position

Other

• Stand to mount gun in vertical position
• Black backdrop

Overhead View of Setup

Side View of Setup

Experimental Setup

We secured one CD vertically in a stand, as shown above, so that the flat, round side of the CD faced the camera. The stand held the CD ~.4 meters off the floor. We placed a black backdrop ~.5 meters behind the CD, far enough away from the subject to avoid producing a glare from the flash unit.  The pellet gun was secured ~.5 meters above the CD, in a wooden harness that allowed the gun to aim directly downwards. A bag of sand was placed under the CD to trap the fired pellets. We lined the CD up in such a way that its top edge was directly below the gun. We set up our camera on its tripod 1.7 meters away from the CD. A Plexiglas shield was placed upright ~.3 meters in front of the camera, secured by a brick on either side, to insure that no CD shards hit the camera. We placed the sound trigger 1.5 meters above the subject on a shelf. The sound trigger was connected to a delay unit which was then connected to the flash unit. The flash unit was secured to the stand that held the gun, and was placed .5 meter from the subject.

Triggering, Timing, and Imaging Methods

For our experiment, we used the sound of the rifle being fired to set off the flash using a Piezoelectric sound trigger and illuminate the shattering CD. Our other alternative was to use a photogate to detect the pellet passing by, however we decided to use the sound trigger.

Achieving the correct timing was a bit challenging. We wanted to capture an image of a shattering CD with the bullet still in the picture frame or just after the bullet had left the frame to maximize the degree of shattering. We placed our sound trigger away from the gun (see description in “Experimental Setup”) and used a delay unit to control the timing of the flash. To find the correct settings for when the bullet would be in the picture, we fired the gun and took pictures of the bullet without the CD set up. By adjusting the delay, we were able to find pictures in which we could see the bullet.

Since the collision between the pellet and the CD happens so fast, we needed to take a picture that had been exposed for a very short duration of time to avoid blurring and maximize the clarity of the picture. To do this, in a completely dark room, we illuminated the subject extremely briefly using a flash and, at the same time, opened the shutter of our camera.

We used the open-shutter technique to take our pictures. The sound of the rifle firing set off the trigger, causing the flash to go off. When using digital cameras, leaving the shutter open for longer than one or two seconds induces thermal noise in the picture in the form of white specks. To avoid this, we used a shutter speed of 1 second. Our f-stop was set to f-8 and we used a film speed of ISO 400. We used manual focus, as automatic focus would have resulted in the camera attempting to focus in the dark with the change in lighting. We also used enough zoom to fill the frame with the image of the CD.

Experimental Method

            The first thing we wanted to do was obtain a rough idea of how the CDs would respond to being shot by a pellet. After some testing, we determined that shooting the CD from the side (in the vertical position as described in “Experimental Setup”) would produce the best pictures.  Using a lead bob, we centered the CD under the rifle to make sure that we would get a clean hit.

First, we experimented with shooting a CD, completely intact. We found it difficult to shatter, the pellet only took chunks out of the CD. As we were looking for a shattering effect, we experimented with drilling holes into the CD. First, we tried drilling holes in a circle around the middle of the CD. This still did not produce the dramatic effect we were looking for. We discovered that the “optimal CD” had many holes drilled close together in the top half (see fig. 1). This allowed the pellet to penetrate the CD and produce a shattering effect.

      Figure 1. Ideal CD with holes drilled in it prior to being shot

            While we had aligned the CD using a lead bob, the “kick” of the gun forced us to make some additional adjustments. To align our CD stand correctly (so that the bullet penetrated the CD), we placed a piece of paper on the bag of sand that sat on the floor to catch the bullets. We then fired the gun (we always pumped it once and then took the safety off prior to firing), and looked at where the bullet hit the paper. After several trials with the CD in place, we finally hit the CD with the pellet. 

            Once we had our CD stand aligned correctly, we set the correct aperture, shutter speed, and focus on our camera.  We then turned out the lights, pumped the gun, removed the safety on the gun, released the shutter, and then fired the gun. The sound of the gun firing triggered the flash which allowed the high-speed image to be captured by our camera. After each picture was taken, we turned on the lights, recorded our picture, and examined our picture to decide if we needed to change anything for the next picture.