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Tools - Video Cameras

 

Filming

 

Equipment and techniques
Examples
Using low-end video cameras

 

Note:  Part of the discussion below assumes the use of NTSC video cameras, which record at 30 frames per second. For PAL cameras, which record at 25 frames per second, values should be changed accordingly.

 

Equipment and techniques

 

Open the video clip below in whichever format you prefer. The clip should download quickly, as it is only 5 frames long. 

VFW version (avi)                     QuickTime version (mov)

The clip shows a balloon being popped by a needle. You can drag the slider to view individual frames. Note that the needle touches the balloon in the 3rd frame, but in the next frame the balloon is already completely burst. An ordinary VHS video camera was used to take this clip under floodlight illumination. While remains of the balloon can be seen, the actual ripping of the balloon was not captured. Moreover, the fragments are blurred. In order to capture sharp images of high-speed events, the camera must have certain special features. In addition, an electronic flash unit must be used with the camera. These requirements are discussed next.

 

We will not cover the use of specialized high-speed motion picture cameras. Such cameras are expensive and beyond the reach of most students and hobbyists. Instead, we assume the use of consumer-grade video cameras. Since some specialized features are required, these cameras tend to be at the higher end of the consumer-grade spectrum (see below for an alternative). They fall in the $1000 - 2000 range. The specialized features include manual controls for focusing and exposure. (Cheaper cameras typically forego one or both of these features in favor of providing totally automatic operation.) Another consideration is getting the highest possible image quality. Formats such as Hi8 and miniDV provide higher resolution than VHS and VHS-C. However, the higher the resolution is, the higher the price will be.

 

The method of taking high-speed photos with a video camera is much like that for taking such photos with a still camera. For example, when using a still camera to photograph a balloon burst, the following steps are typical.

  1. The lens of the camera is focused manually on the subject.
  2. The aperture is set manually for the expected flash exposure.
  3. The shutter dial is set to bulb. 
  4. The flash controls are adjusted for short duration (generally by using the flash in automatic mode). 
  5. The flash is positioned off camera and is connected to a sound trigger. 
  6. The lights are then turned off, the shutter opened, and the balloon popped. 
  7. The sound of the balloon triggers the flash discharge and captures the image on film.  
  8. The shutter is then closed and the room lights turned on.

When using a video camera for the same purpose, these would be the steps.

  1. The lens of the camera is focused manually on the subject.
  2. The aperture is set manually for the expected flash exposure.
  3. The shutter is set to 1/60 second. (This is typically the default exposure time and is set automatically.)
  4. The flash controls are adjusted for short duration. (Go here for information on adjusting flash duration.) 
  5. The flash is positioned off camera and is connected to a sound trigger. 
  6. The lights are then turned off, the record button turned on, and the balloon popped.
  7. The sound of the balloon triggers the flash discharge and captures the image on film.
  8. The record button is turned off and the room lights turned on. (Stopping the recording isn't essential in the case of a video camera, since new film is always being exposed.)

While the technique for using a video camera is much like that for a still camera, there are some differences.  Here are the important ones:

 

Relating to shutter speed (or exposure time):  Video cameras have no bulb setting. They typically record 60 fields per second (or 30 frames per second, there being 2 interlaced fields per frame) and hence have an exposure time of 1/60 s per field. When the balloon bursts, the image will be captured in the active field.  (There's a small probability that the image will be missed, because there's a short period of dead time between fields when the CCD sensor is dumping its data for processing.)  If the camera has a high-speed shutter, it's possible to set exposure times shorter than 1/60 s. However, it's important not to use these shorter times.  What this does is increase the dead time between fields. At short exposures, the camera still captures 60 fields per second. However, the sensor is only active during each 1/60 s interval for a period of time equal to the exposure. For example, if the exposure time is 1/120 s, the sensor will capture an image in that 1/120 s time period and then be inactive for 1/120 s. Thus, the probability of capturing the balloon burst has been reduced from about 100% to 50%. The shorter the exposure time, the smaller this probability will be.

 

Remember, it's not the shutter speed that stops the high-speed action. It's the brief flash of light. That typically occurs in less than a ten-thousandth of a second.

 

Relating to aperture selection:  Top of the line consumer cameras will generally have a dial or other means to adjust the aperture. The ability to close down the aperture is important. That's because the photo will be taken in darkness. If the camera selects the aperture automatically, it will naturally select a very wide aperture due to the darkness. When the flash discharges, the camera doesn't have time to close down the aperture. Hence, the image will be greatly overexposed. Thus, it's important to be able to set the aperture before the photo is taken.

 

Lower end consumer cameras usually don't have exposure control dials. However, they generally do have programmed exposure modes. The sports mode can often be used to provide the necessary exposure reduction.

 

Back to top

 

Examples

 

Here are some additional examples of video clips of high-speed events.  All were taken with a Hi8 camera.

 

VFW clip      QT clip VFW clip     QT clip VFW clip     QT clip
Balloon popped with a needle Hollow, plastic ball colliding with the floor Racquetball colliding with the floor. A dimple forms on the top.

 

Below is a clip from a miniDV camera.

VFW version (avi)                     QuickTime version (mov)

The clip was taken using the techniques described above for video. A Canon Optura digital video camera and Vivitar 283 flash unit were used. Note that the frames leading up to the flash of light and following it are completely dark, since the room lights were out. The bursting balloon appears in a single frame and is quite sharp. The object seen to the left is the sound trigger.

 

This clip was reduced to 25% of its original size so that it would download quickly.   In order to see the full-size frame in which the balloon's image was captured, click below. Note that the resolution is very good for video, but not nearly as good as that obtained from silver-based film. An improvement in resolution can be made using cameras that film in progressive mode. This will be covered in Capturing. The method of extracting single frames such as the one shown below will also be discussed.

View full-frame image

Using low-end video cameras

tom.jpg (6184 bytes)

We weren't telling the whole story when we said you needed a top-of-the-line consumer-grade video camera for high-speed photography. True, that's the way to get full manual control and the best image quality. However, it is possible to capture high-speed images using an inexpensive video camera. For example, we've tested one low-end model that has a fixed-focus lens (no need to worry about autofocusing in the dark!). The camera has no exposure control and no programmed exposure modes, it captured black-and-white images only, and it didn't accept tapes. The camera had to be attached to a VCR for recording (or a computer for frame capture). The way we dealt with the lack of exposure control was to place a neutral-density filter over the flash in order to reduce the light intensity. This camera has been running continuously at the Edgerton Explorit Center (Aurora, NE) for the past 6 years in an interactive exhibit in which people capture and print photographs of themselves popping balloons.  The photo above is an example.

 


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