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
Some great explanations in here. I’ll add that a flash isn’t an instant in time. As quick as it may be, it has a beginning, a duration, and an end. It’s just really fast.
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.
??? can you give an example? I assume you are aware you know what exposure time is and how it is affected by the amount of light and affects motion blur?
With no flash the image is exposed by the duration of the shutter’s speed, typically in fractions of a second. With flash the exposure is by the duration of the flash. With a Xenon flash tube the duration is typically fifty to a hundred microseconds, an old school flash bulb shines for milliseconds.
Think of a camera sensor as a bunch of buckets, each pixel one bucket. And they collect photons, how bright a pixel is depends on how many it collects. If the incoming light is brighter, you get enough photons faster, which means there is less time for the subject to move while exposure is going on and photons are being collected. Flash, the bulb type, not the phone LED, is very bright for a very brief period of time. There just isn’t enough time for things to move much in the scene, and that’s why you get less motion blur than by doing a longer exposure.
It’s about duration of the light. Taking an image can take time. If the subject moves while the image has a forming you get a blur.
So you want to have the film/sensor exposed to light for a very short period of time. But this introduced a new problem, you get less light with the shutter open for only an instant, and the image will be very dim.
So you need a lot of light. Taking
It in daylight is usually enough light. But otherwise you need a very well lit studio.
Enter the third option, the flash. The flash is very, very brief, and so only need to be high power and bright for a moment. So you don’t need a lot of energy you just need a little bit really fast. This means we can have small portable flashes.
Originally they were chemical reactions, small bright explosive reactions. Then they were bulbs that would actually burn out in each use. Now we have durable LEDs that last for a while.
The other detail is you have to time this brief flash for when your shutter is open.
i tottally agrree with you on tthatt. it”it’s so frustrating when ccompannnies prioritize proffit over tthhe wwell-being of theeir eemployyees. i wishh more companies woulld take a stand and prioriitize their workerers.
Latest Answers