In Decent Exposure

In a video camera there is no physical shutter, but, because there is a body of understanding about how film cameras work, we still call the electrical signal “shutter speed.”

Early last November one of the nearby fraternities scored a large shipment of pumpkins at a low price. They kept them around for a
while, but by the middle of the month, they'd started to go sour and the only sensible thing to do was to fling them from the third floor balcony into the street below while one of the brothers captured the event on video for future reference. After a few pumpkins, it was obvious to them that the video was blurry, and slow-motion replay wasn't giving them the sharp, smooth images they'd hoped to see, so they knocked on my door and offered me a case of beer and some slightly wrinkled Pam Anderson posters if I could figure out what was wrong. It didn't take long to diagnose their problem.

"You've left your camera set in 'program' mode. You have some spectacular pumpkin explosions which would be best served by a very fast shutter speed."

"Really?!"

"Yes, let me explain…."

What is a Shutter?

When a motion picture film camera takes an image, a spinning disc with a section cut out spins, covering the light sensitive film frame for a fraction of time and exposing it for a fraction of time. The slower the disc moves, the more light lands on the film.

In a camcorder, whether digital or analog, an electrical impulse "wakes up" the photo-sensitive CCD and tells it to start gathering light, then tells it to stop. The duration of this signal is equivalent to the amount of time an old film shutter would be open, so we still call this "shutter speed" despite the absence of an actual shutter.

Controlling the Amount of Light

There are typically three things which control the amount of light that is recorded by your camera.

1) Shutter Speed–how long the CCD collects light.

2) Aperture–variable sized hole by which light passes after going through the lens and before hitting the CCD. A "wide" aperture lets more light in than a "narrow" aperture.

3) CCD Sensitivity–a variable setting, known as gain, which tells the CCD how to rate the light which is coming into the lens. This is roughly equivalent to "film speed."

Mixing and matching these three things can give you dozens of acceptable variations for a range of acceptable lighting. A fast shutter speed and a wide aperture can allow the same amount of light to be recorded as a slow shutter speed and a small aperture. Raising the gain will allow you greater exposure. This process is an electronic one, introducing more grain as you raise the gain. +3 and +6 are usually safe, anything more will show noticeable grain. Choosing the right combinations of these three variables will improve your video over a range of situations.

The Function of Different Shutter Speeds

Your camera records 30 frames per second, each frame made of two "fields," for a total of 60 images per second. A shutter speed of 1/60 is considered optimal for playback; however, certain situations (such as pumpkin flinging) call for variation.

A "fast" shutter speed (over 1/125 of a second) will freeze action. Use it for sports, children playing, dogs catching Frisbees, etc. A "slow" shutter speed (under 1/30th of a second) will blur fast action but allow recording in lower light. Use for dark interiors, night-time, birthday cakes, etc.

Beware the Strobing Effect

One of the hazards of using a shutter speed other than 1/60th of a second is that the resulting image may look unnatural. Your television is playing back 60 images a second. If the camera's shutter speed is set to 1/15th of a second, that means it's only seeing 1/4 of the action it's expecting; one exposure is covering four frames of time. A quickly moving object will appear as a series of strobed, blurred frames. At the opposite end of the spectrum, a series of super-fast frames (say at 1/10,000 of a second) is still captured at 1/60th intervals, so there may be large jumps between sharp images in something like a golfer's swing, where a small amount of blur would actually make it look more natural to the eye. Experiment with these two extremes to develop an idea of how different shutter speeds look in different situations.

Beware Fluorescent Lights

Because fluorescent lights themselves strobe 60 times a second (this magical number comes from the Alternating Current coming out of the wall), photographing something under fluorescent lights with a shutter speed of greater than 1/60th of a second will cause weird strobing effects that you don't want. Try to shoot all scenes lit by fluorescents at 1/60th or below.

Conclusion

Combine shutter speed and aperture to allow the proper exposure. Use fast shutter speeds for quick motion (pumpkin smashing and football). Use slow shutter speeds for low light, night shoots, indoors, etc. As always, practice and experiment.

Contributing Editor Kyle Cassidy is a video artist, network engineer and co-author of Enterprise Internetworking and Security.

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