Introduction to Digital Stroboscopic Motion photography

Andrew Davidhazy
School of Photographic Arts and Sciences
Rochester Institute of Technology


Stroboscopic photography for depicting the changing features of subjects in motion is a technique that was enabled and popularized by one of the "fathers" of high speed photography, Harold "Doc" Edgerton. He applied this technique to numerous situations where a still camera, in a single shot, did not capture enough information about a subject in motion and where a motion picture camera record was unsuitable for being reproduced on the printed page.

Photographers use two types of stroboscopes. The first, and most popular, is nothing more than a flashing light source. This is the kind that Doc Edgerton popularized.

 
     

 
There is also a cheaper, simpler, mechanical alternative. This consists of nothing more complicated than a rotating disk with a slot cut into it. With every rotation of the disc we are able to glimpse the position of the subject at the time the slot passes in front of our eyes. In either case, a stroboscope allows us to view a subject on a periodic basis.

Photographers have generally used a flashing light, a stroboscopic light source, to illuminate a moving subject in order to track the subject position over time. This is accomplished by setting up an action situation if front of the camera generally firmly attached to a tripod, opening the shutter of the camera while the subject is moving and the stroboscope flashing and after a while closing the shutter and terminating the exposure.

 During the time the shutter is open the moving subject is illuminated by several flashes of light. These leave a superimposed sequence of images of the subject and it is often possible to gain much valuable subject motion evaluation information from such a record or to simply connect with and understand the graceful (or not so graceful) flow of action  of a given subject.

When engaged in stroboscopic photography generally one is interested in photographing relatively long duration events (even though one may only be talking about a second or two!) because it is really pointless to make a stroboscopic record while only recording the subject in only one, two or three positions over time!

For example, to make a record of a golf swing or something similar, we would be looking to make a record over a time period of a second or so. During that time we might want to record our subject in maybe 20 to 100 different positions. This, of course would require a strobe flashing at a frequency of 20 to 100 flashes per second if we kept the shutter open for a second.

Historically, an interesting application for stroboscopes has been their use to track the motion of rebounding objects such as tennis or ping-pong balls. My rotating disk stroboscope can easily deal with this subject. The ball is simply thrown across the field of view of the camera while the disk rotates in front of the lens. Using an exposure time of a couple of seconds or so the periodic glimpses of the flight path of the ball seen by the lens with each passage of the slot in front of it capture an informative photograph of the flight-path of the ball.

 

 
I also attached my home-made mechanical, rotating disk type, stroboscope to a Nikon 990 Coolpix camera and used an exposure time of one second to obtain the photograph shown below. I stood against a large, black, velvet background. The lighting level was adjusted so that the results obtained were of an acceptable quality by making a few preliminary tests and judging the quality of the images on the LCD display screen of the camera. Very convenient and effective!

By the way, from this example made over a period of 1 second, the camera recorded about 15 separate images of my action. From this it can be determined that the mechanical stroboscope's disc was turning at about 15 revolutions per second.

 

 Note that, as with traditional cameras, those parts of my body that remained essentially in the same position appear somewhat overexposed because they reflected light to the same location on the CCD. On the other hand my moving arms were exposed in different positions on the CCD with every pass of the stroboscope disc. The arms, therefore, are exposed only once on any given area on the CCD and reproduced with less exposure than the stationary parts of my body. 

If one leaves the shutter for too long the sequential images will start to overlap until a point is reached that they all blend into a uniform "blur" and the specific position of the subject at any given time  can no longer be perceived. If one is interested in motion detail then, one can move the image across the CCD by panning the camera while the mechanical stroboscopic disk rapidly rotates in front of the camera's lens and the subject performs in front of the camera.

A major drawback of this technique is that the field of view of the camera (over time) is now very wide and the surrounding  studio equipment may be seen within the angle of coverage of the camera as it is panned from side to side and this will make these items appear to "bleed through" the subject's image.

To deal with this difficulty I would suggest that the camera be simply surrounded by a curtain of black velvet set up in such a manner that there is a small open slot located between the camera, rotating on a tripod, and the subject. This way the camera will be prevented from seeing and recording anything but that which is available to it through the gap or slot in the curtain. This provides a "ghost" free final record although superimposition of certain parts of the moving subject is still a possibility. This often just adds to the fluidity of the final motion record.

Since one is limited in the size of the camera's sensor the total number of images that one can clearly record depends on how small one is prepared to allow the subject to be within the frame of the camera. The smaller the image size one can tolerate the larger the time over which one can "track" the subject or the number separate images of the subject that can be placed from one side of the frame to the other.

The stroboscope that was used for the photographs shown in this article was a self-built piece of equipment. It had the capability of “flashing” (actually turning) up to 20 times a second. For image capture the camera that was used was a Nikon CoolPix 990, with the set camera on manual, the lens set to the widest aperture, the exposure time set to 4 seconds and it was rotated by hand. The illustrations in this article were downloaded from the camera and adjusted in Photoshop 6.0 for final presentation.

 


Mechanical Stroboscope Construction Instructions 

To make a mechanical stroboscope as described and applied in this article you will need:

             A small electric motor such as Radio Shack number 273-223 (cost about $3)  1.5-3v DC                                       
            One plastic gear or other item that attaches to motor shaft
            Sheet of black construction paper
            Battery connectors
            About 6x9 inches of 1/2 or 3/4 inch  plywood or pine
            One 1/4 by 20 threaded “T” nut (3/8 or 7/8 depending on wood)
            One  1/4 x 20 threaded bolt to reach through wood
            One piece of stiff board

 Putting it all together:

 1.     Cut a circle 8 1/2 inches (or bigger) in diameter from construction paper.

2.     Cut a slot near its edge about the length of diameter of the lens you will be using on your camera and about 10 degrees in width.

3.     Line up camera next to one edge of wooden board and noting where the tripod thread on camera body is mark and then drill a 1/4 inch diameter hole in the wood so camera mounts with through-bolt from bottom of board. The camera lens front should approximately line up with the other side of the board.

4.     Mark and then drill another hole in wood (5/8 in diameter) so T-nut can be pounded in and fastened to the board from the camera side. This allows fastening the wooden camera platform to a tripod securely.

5.     Prepare a small piece of stiff cardboard  by cutting a hole in it so it fits the body of the electric motor and glue the motor with epoxy glue to the board as shown in illustration.

6.     Attach the paper disk to whatever device or item you  purchased that fit snugly onto the motor’s shaft and which had a large flange on one end. Such as a plastic gear.

7.     With camera attached to the supporting wooden platform align the motor carrying cardboard along the front edge of the board so that the slot in the disk passes in front of the camera’s lens as it makes a revolution and fasten it to the wood using flat headed thumbtacks.

8.     Attach the battery connector to the motor.



There is a more recent article that describes the making of this disc stroboscope from another angle.

You can find it here.