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Photographing Collisions involving High-Speed Projectiles


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We're frequently asked how to capture gunshots using a sound trigger. This is fairly easy to do. A sound trigger is sufficient for the trigger, but you may want to use a sound trigger with delay in order to be able to use the timeout (reset delay) function of the delay unit to suppress multiple flashes. Whether you use a delay unit or not, though, it's easy to set delays by adjusting the distance of the microphone from the source of the sound. (See the timing section.) Use the open-shutter technique for photography. For this method, connect your flash unit directly to the output of the sound trigger or delay unit. The camera will be independent of the trigger and flash. The method of taking a picture is to turn off the room lights, open the shutter on bulb or a long exposure time (1-2 seconds), fire the gun, close the shutter, turn on the lights.



Subsonic projectiles

Consider the set up shown to the right. This is a typical setup for subsonic projectiles. In order to determine a starting position for testing, use the formula d13 = d12/(Mach number). The Mach number is simply the speed of the projectile divided by the speed of sound (340 m/s or 1100 f/s in air). You'll need to have an estimate of the speed of the projectile in order to determine the Mach number.


Here's an example. Suppose the projectile speed is half the speed of sound. Then the Mach number is 0.5. That means that your starting position for the trigger would be twice as far from the gun as from the target. Of course, this is an estimate that you'll need to fine tune. You don't need to have the target in place in order to do your testing. The projectile can typically be seen with the naked eye against a dark background when the flash discharges in a dark room. If, however, you have trouble seeing the projectile, try putting a piece of paper or cardboard at the position of the target. Position your camera to view either the front of back side of the cardboard. If the flash discharges after the projectile passes through the cardboard, a photo will show a hole in the cardboard. Otherwise, there will be no hole. In either case, adjust the position of the trigger accordingly.

Subsonic setup
Figure 1. Set up for subsonic projectiles

Supersonic projectiles

For projectiles that travel faster than sound, the trigger is placed closer to the gun than the target is. The reason for this is that the projectile produces a conical shockwave as indicated in the diagram to the right. The trigger is actuated by the passage of the shock wave. For the situation shown, the trigger could be placed anywhere along the shock front, and the flash would discharge as the projectile reached the target. Of course, you can't see the shock wave, so you have to estimate what angle A it will make with the path of the projectile. The mathematical relationship is sinA = 1/(Mach number). Here's a table with some values.


Mach # A in degrees
1 90
1.5 42
2 30
2.5 24
3 19
Supersonic setup
Figure 2. Set up for supersonic projectiles




In order to minimize blur, use the minimum power setting on your flash unit. This will give the shortest duration of the flash discharge. Let's suppose the shortest duration is t = 30 microseconds (1 microsecond = 0.000001 second). This is about a minimum for a typical photographer's flash unit. The distance traveled by a projectile moving at speed v in time t is d = vt. If v is the speed of sound, then d = (340 m/s)(0.00003 s) = 0.01 m. That's 1 centimeter or 0.4 inches. That's a significant distance compared to the length of most pellets or BBs. Therefore, with a typical flash unit, you can't expect to freeze the motion of the projectile, even at half the speed of sound. The worst possible situation for blur is when the projectile is perpendicular to the axis of the camera lens. That gives the greatest blur. If you position the camera so that the axis of the lens makes an angle less than 90° with the path of the projectile, blur will be reduced. For most shots, though, you can't expect the projectile to show up as anything other than a bright, blurred spot. That's ok as long as the projectile isn't the primary subject. Usually, whatever you're shooting with the projectile is primary and that, of course, will be much more visually interesting than a blurred spot.


Figures 3 and 4 below show example photos of shooting a water balloon with an air rifle BB traveling about Mach 0.5. (Click on the images for close ups.) They were taken with a Vivitar 283 flash unit. The flash duration was adjusted for the minimum value using the method described here. Note that with such short exposure times, it's necessary to place the flash close to the subject in order to use apertures of f/8 to f/11 at ISO 400. In Figure 3, for which the BB traveled perpendicular to the axis of the camera lens, the BB appears as a bright, featureless spot. In Figure 4, for which the lens axis made a significant smaller angle with the path of the BB, the shape of the BB can be seen but some blur is still present.


Water balloon shot with BB (transverse view) Water balloon shot with BB (exiting view)
Figure 3. The BB traveled perpendicular to the axis of the camera lens. Figure 4. The axis of the camera lens was about 45° from the path of the BB.



Figures 5 provides another example. A 0.22-calibre bullet traveling at about 9/10ths the speed of sound was fired in front of a plastic figure. A Nikon SB800 flash unit set at M 1/128 was used. The bullet is completely indistinct. Figure 6 was taken under similar conditions. The double image resulted when the flash discharged twice in quick succession. This may have been due to i), the echo of the gunshot in the room where the photo was taken or ii) the secondary sound of the bullet striking the trap. Methods of preventing the sound trigger from discharging twice in quick succession in order to suppress multiple exposures are described here.


Bullet at Mach 0.9 with plastic figure Bullet at Mach 0.9 (double exposure)
Figure 5. The bullet shows significant blur. Figure 6. The flash discharged twice, producing a double exposure.


If the goal is to capture a distinct photo of the bullet as it comes out of the gun barrel, you won't be able to do that with a typical photographer's flash unit. The photos below show a 22 rifle being fired. In Figure 7 the bullet is an indistinct blur against a white background. In Figure 8, taken against a black backdrop, the bullet is shrouded by the smoke from the gun.


Bullet on exit from rifle (white background) Bullet on exit from rifle (black background)
Figure 7. The bullet is a ghostly blur against a light background. Figure 8. The bullet is lost amid smoke from the rifle.


With a specialty light source such as the Spot, which has a duration of half a microsecond, the motion of a bullet traveling near or greater than the speed of sound can be frozen. See Figures 9 and 10 for examples. In Figure 9, the rifling marks on the surface of the bullet (in front of the hand of the toy figure) can be seen. In Figure 10, the bullet is smaller in relation to the subject but is still distinct as can be seen by clicking on the photo for an enlargement.


Bullet frozen with sub-microsecond spark source Bullet frozen with sub-microsecond spark source
Figure 9. The light source shows the detail on the surface of the bullet. Figure 10. A balloon is burst with a bullet traveling near the speed of sound.


A light source like the Spot isn't likely to be in the toolkit of most DIYers. While there are plans on the web for building such units, most people wouldn't want to deal with the potentially deadly voltages involved.


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