I don’t understand how using a flash can freeze motion better than ambient light. In the end, isn’t it all just light hitting the sensor?
I don’t understand why it would do this. I mean it’s not necessarily like the camera knows and is like “Oh they’re using flash, I’ll be nice and give them a sharper picture’. I mean obviously they don’t think like that, but all in all it’s light luminating the subject and hitting the sensor, why does something like the source of the light affect how much motion can appear in your image?
In: Technology
A camera sensor is sensitive to light, and it needs a certain amount of light to make an image. Too little light and the image will be too dark. Too much light, and the sensor will be overloaded (“saturated”).
More advanced cameras will have certain settings that let you change how sensitive the sensor is, but for this explanation let’s assume a sensor needs a fixed amount of light to produce a good image. Similarly, in older cameras you could change film to one that’s more or less sensitive to light, but let’s say you’re keeping the same film.
For a particular camera, sensor, and lens, there’s to things you can adjust to control the amount of light the sensor sees:
– The time the shutter is open and letting light strike the sensor.
– The amount of light entering the camera from the object you’re photographing.
Opening the shutter longer works great in the thing you’re photographing isn’t moving, or is moving slowly enough not to matter. But a sports photographer, for example, wouldn’t take a long-exposure photo because the players running would be all blurry.
A flash allows a photographer to project more light onto the object they’re photographing, which in turn means more light is entering the camera. Since there’s more light, they can use a shorter shutter time and “freeze” moving objects.
As an extreme example, I’m a scientist who conducts experiments that are driven by high explosives. My entire experiment is done in about 30 microseconds. I want to photograph the movement of objects as they interact with the explosives, but in order to “freeze” them and not have too much motion blur the electronic camera “shutter” can only be “open” for 20 nanoseconds, and take several such photos a few microseconds apart to see how things change over time. (A typical camera shutter is open for about 1/100 of a second, or 500,000 times longer.) I simply can’t leave the shutter open longer since things will get all blurred, so I need more light.
Thus, my lab has an extraordinarily powerful flash system. Rather than a tiny flashbulb in a camera, we have ones the size of dinner plates. (They’re actually strobe lights used at airports to help pilots in flying aircraft to find the runway.) We have a refrigerator-sized high voltage bank and beefy cables to cause the bulbs to light. This makes them a lot brighter than usual but is way out of spec for the bulbs, so while they may make millions of flashes in a normal situation, we’re lucky to get five or six from this system, plus when we actually fire the explosives the flashbulbs are destroyed.
Even with these powerful bulbs only a few centimeters from the experiment, the amount of light is still barely enough so I have someone do some photo processing after the fact to bring out the details I want. Without the flash providing that about light the photos would simply be black.
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