How High-speed Photography Works

To understand the fundamentals of high-speed photography, it's important to first go over the basics of photography and what makes a camera work. More specifically, it helps to understand how manual cameras work rather than newer automatic and digital cameras. Although it's possible to take photographs of high-speed objects with automatic and digital cameras, when it comes to high-speed photography, the more manual a camera is, the more successful the photos will be. We'll therefore focus on manual single-lens-reflex (SLR) cameras and the basic principles of photography.

One of the most important parts of a camera is, of course, the lens. A lens is a simple, curved piece of glass that bends and redirects incoming light to form a real image, or an exact reproduction of the scene in front of the camera. Light bent through the lens is essentially "painted" onto the film inside the camera; in fact, it's no surprise that the root of the word "photography" is Greek, meaning "drawing with light." 

You can't just point a lens at an object and hope your picture will turn out. There are two very important factors that determine how film is exposed to light:

The first factor, the amount of light coming onto the film, is controlled by the camera's aperture, which is a circular opening that can expand or shrink in size. The aperture works just like the iris of your eye -- when you need to let in more light, the circle becomes bigger; when you need to block out some light, the circle becomes smaller.  A set of numbers on the aperture control of a camera called f-stops describe the size of the circle. The f-stop is inversely related to the size of the aperture: The higher the f-stop number, the smaller the opening. An f-stop of f/11, for example, will be smaller and let in less light than an f-stop of f/8.

The device that works with the aperture and determines the second factor, how long the film is exposed to light, is called the shutter. The shutter is basically a curtain that opens and closes to expose the film to light. More specifically, it's the shutter speed -- the rate at which the shutter opens and closes -- that really affects the film's exposure. Shutter speeds are usually measured in fractions of a second and typically range from one full second to 1/1000th of a second. The longer the shutter remains open, the more light is allowed onto the film.

 So how do these factors affect high-speed photography? To learn how high-speed photographers capture such fast action on film, read the next page.

To understand high-speed photography, we must also look at its opposite: long or extended exposure photography. This technique involves exposing the film in a camera for longer periods of time -- anywhere from an eighth of a second to several minutes.

Busy traffic scenes in cities during dusk, dawn and nighttime are the types of photos you'll usually see in an extended exposure photograph -- ones in whichthe headlights of cars aren't dots of light but rather long streaks of light that cross the picture, blending into each other. This occurs because the light from a car's headlight actually paints across the frame of the picture and is exposed onto the film for a longer duration. 

If you were to take a picture of something like a bullet using the same method of long exposure, you'd barely see a thin streak across the frame of the picture. Because the image of a bullet would cross the camera lens within a fraction of a second, too much light would capture the bullet's entire path from one side of the picture to the other.

The success of high-speed photography largely depends upon how quickly the film is exposed to light. Therefore, high-speed photographers rely heavily on flash units to take pictures, using extremely short flash durations -- the shorter the burst of light, the better. Because of this, many high-speed objects are photographed in complete darkness. In this case, the camera's shutter is simply left open while the shot takes place; if there's no light in the room, the film won't expose. Once the object passes through the frame, the flash unit lets out a burst of light, and that moment is the only thing that gets painted onto the film. Sufficient flash durations can be as short as 30 microseconds, or 0.00003 seconds.

Photographers shooting nature scenes or sporting events that take place outside obviously can't keep everything in complete darkness. In this case, photographers rely on extremely fast shutter speeds. While regular photography taken in sunlight might work with shutter speeds that are 1/125th of a second, shutter speeds for high-speed photography are much faster -- as fast as 1/8000th of a second.

How does a photographer get all of these factors to line up perfectly and result in such clear shots? To learn about detection, synchronization and other important aspects of high-speed photography, read the next page.

To get that perfect, crystal clear image of a moment frozen in time, three important factors need to work together to produce a high-speed photograph:

Because the actions being recorded take place too quickly for the eye to see, fast-moving and sometimes unpredictable objects need to be detected remotely. This is essentially the subject of the picture letting the camera know when to shoot. By hooking a variety of triggers -- including sound, vibration, contact or light interruption -- electronically to a flash unit, the high-speed object can "tell" the flash almost instantly when to let out a burst of light.

The sound of a gun firing the bullet in this photo actually triggered the flash, not the photographer.

Sound triggers are commonly used in high-speed photography, mainly because they're easy to make and use. There are essentially three parts to a sound trigger: the microphone, the amplifier and a silicon-controlled rectifier (SCR). Instead of using a regular microphone like the kind you see on a theater stage, photographers can use piezoelectric film, a pressure-sensitive film that reacts to sharp sounds like a balloon pop, broken glass or a hand clap. Sound picked up by the piezoelectric film is boosted by the amplifier, which sends an electric current to the SRC. The SRC, connected to the flash unit by a cathode and anode, acts as a switch for the burst of light by shorting the flash. If the object is photographed in a completely dark room, the shutter of the camera can remain open without exposing the film.

If a photographer is shooting outside and relying on fast shutter speed, the timing of the shot is also very important, of course. The camera must be synchronized to shoot a photograph the very moment an object crosses the frame. This involves knowing the delay time between the object's movement and the amount of time your camera requires to take a photograph.

Imaging is simply the process of light painting an image onto the filmstock. Sometimes the type of film a photographer uses can affect the outcome of a high-speed picture. High-speed photography takes into account film speed, or the film's sensitivity to light. Film speed measurements are commonly referred to as an ISO (named so for the International Organization for Standardization), and the lower the ISO, the longer it takes for light to expose on the film. A film with a speed of ISO 800, for example, is faster and more sensitive than one with a speed of ISO 100. Because high-speed photographers use low light levels, they typically use faster speeds in order to make up for the short bursts of light used.

Even with very precise preparation, high-speed photographers rely on luck as much as tight organization to get that great shot. Catching a perfectly shaped drop of water can take more than a hundred shots over several painstaking hours. To some, however, the possibility of stopping time and seeing something no one's ever seen before outweighs the time invested.

For more on the science of photography, see the next page.