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Author Topic: Why do we see a trail of someone moving in a photograph?  (Read 2624 times)

Shruti

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Shruti asked the Naked Scientists:
   
Why do we see a trail of someone moving in a photograph? Isnt the camera supposed to capture only one instant?

What do you think?
« Last Edit: 23/11/2010 22:30:03 by _system »


 

Offline JP

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Why do we see a trail of someone moving in a photograph?
« Reply #1 on: 24/11/2010 02:58:09 »
When you take a picture, a camera opens a shutter, which captures light coming into the camera.  No shutter is instantaneous, so it's open for some short amount of time, capturing light.  This is especially important in lower light settings, since you need to keep the shutter open longer to capture enough light for the film to develop.  Most good quality cameras will have an option on them that lets you set how long the shutter is open, although a lot of them can also do it automatically.

How does this effect a picture?  If you're taking a picture of a person who moves a lot during the time the shutter's open, you get a motion blur in the picture.  Also, if you're not using a tripod to keep the camera steady, you can get a blurry picture due to your hand shaking slightly while you take the picture.
 

Offline RD

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Why do we see a trail of someone moving in a photograph?
« Reply #2 on: 24/11/2010 03:20:08 »
...  you can get a blurry picture due to your hand shaking slightly while you take the picture.

Or if you toss the camera in the air ... http://www.flickr.com/groups/cameratoss/
 

Offline JP

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Why do we see a trail of someone moving in a photograph?
« Reply #3 on: 24/11/2010 04:07:23 »
...  you can get a blurry picture due to your hand shaking slightly while you take the picture.

Or if you toss the camera in the air ... http://www.flickr.com/groups/cameratoss/

:)
 

Offline Don_1

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Why do we see a trail of someone moving in a photograph?
« Reply #4 on: 24/11/2010 10:11:51 »
The degree of blur depends on the speed of the subject, the distance from the camera to the subject and the speed of the shutter.

The reduction of blur can be achieved in a number of ways. Put more distance between the camera and subject, pan the camera to match the subject's speed or use a very fast shutter speed.

If your camera's shutter runs at say 1/125 sec it will produce more blur than a shutter run at 1/2000 sec.
 

Offline BenV

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Why do we see a trail of someone moving in a photograph?
« Reply #5 on: 24/11/2010 11:42:13 »
And, of course, you can cut the amount of time needed for the shutter to be open by opening the aperture (the opening in the lens).  If the aperture is wide open, the shutter speed can be faster and still let the right amount of light hit the sensor (or film).
 

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Why do we see a trail of someone moving in a photograph?
« Reply #6 on: 25/11/2010 09:13:50 »
Quite so Ben. The wider the aperture, the faster the shutter can run, but this reduces the depth of field. A higher shutter speed can also be achieved by using faster film or increasing the sensitivity of the sensor, in the case of digital cameras. An ISO of 200 would be fine for most circumstances, but for freezing action an ISO of 3200 or 6400 would allow for a faster shutter run.
 

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Why do we see a trail of someone moving in a photograph?
« Reply #6 on: 25/11/2010 09:13:50 »

 

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