What is a Shutter? Shutter is a device, too... ...that's all? Lame joke... ...Let's get serious :)
Shutter is a gate that block light goes in the camera, so your Sensor (for digital camera) or film (for film camera) will not get expose. Remember the Black Box mentioned in "How does Camera work" ? Shutter is actually the "cover" mentioned in the chapter and it decide whether the picture is clear or blur. Why?
Imagine the Shutter is you eyelid, and it is close by now. You see nothing...of course ^^. Now open your eye and close it quickly. What did you see? Well, depends on how fast your eyelid can open and close. So, if you open for just 0.1 second, you only able to capture the image for 0.1 second as well.
Therefore, faster Shutter speed can Freezes the motion and you can capture a clearer picture.
And for a slower Shutter speed, you will capture something blur.
However, due to the faster Shutter speed, your camera does not have enough time to capture enough light, which we called "Exposure". You will get a darker picture as well.
Most cameras have 1 sec, 1/2 sec, 1/4 sec, 1/8 sec, 1/15 sec, 1/30 sec and 1/60 sec Shutter speeds. For higher entry level SLR, 1/8000 sec Shutter speeds is available.
This is a flame capture in 1/8000 sec, no PhotoShop or relevant software modified. I swear :)
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