Friday, March 23, 2007

How to take a photo of a bullet

In this photo of a paintball passing through a light bulb, the paintball entered from the right, and was at the filament as the photo was taken.

The shell of the paintball is already shattered. We see the blob of paint through the frosted surface of the bulb.

Daniel Pressl, a graduate student at MIT, and I took this photo.

The band of yellow material at the base is modeling clay.

In the comments to Howdy! I gave the following methodology for how we take a bullet photo:

1) We set up the card and line up the rifle (by bore sighting) so that the bullet will hit the card.

2) Then we turn out the lights ...

3) ... and open the shutter of the camera.
(Since the room is dark the film/imager is not exposed.)

4) We fire the rifle.

5) A few milliseconds later, as the bullet is cutting the card, the strobe fires, lighting the target for a half microsecond, after which the room is dark again.

6) We close the shutter and turn on the room lights to set up for the next shot.

Here is a question for you.

In step 5), how do we get the strobe to go off just when the bullet is in the right place? (Update: Answer here.)



Blogger Fargus... said...

I'm guessing you use the known muzzle velocity and drag equations, coupled with the knowledge of the distance between the rifle and the target, to determine the precise amount of time it will take for the bullet to travel the given distance.

7:16 AM  
Blogger Bryan said...

I would do it with a sound switch, like the "Clapper." You can probably trip a transistor with the output from a microphone.

you could also set up a an infra red detection circuit just in front of the muzzle to note the bullet passing, but the sound circuit would probably be more reliable.

7:42 AM  
Blogger Jim said...

I'll wait until Tery's return to post the answer.

However, let me note that there is a difference (perhaps subtle) between how we trigger the strobe for a supersonic round (e.g., the .22-long rifle slug of the bullet through the card) and for a sub-sonic round (like the paintball).

8:38 AM  
Anonymous El Tiburon said...

String attached to trigger of gun is also attached to 'on' switch of strobe light?

What do I win?

11:52 AM  
Blogger J. said...

By asking politely?

5:47 PM  

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