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Flash in cameras

  1. Oct 18, 2011 #1
    1. The problem statement, all variables and given/known data
    how is flash created?

    3. The attempt at a solution
    flash makes images caught on a camera brighter and the background dark. why does this happen? what exactly is flash made up of? how do you explain the effects of flash?
     
  2. jcsd
  3. Oct 18, 2011 #2

    PeterO

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    Am I to assume you are referring to the flash on a camera?
     
  4. Oct 18, 2011 #3
    yes.
     
  5. Oct 18, 2011 #4

    PeterO

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    What does it look like comes out of a flash?
     
  6. Oct 18, 2011 #5
    white light rays?
     
  7. Oct 18, 2011 #6

    PeterO

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    Good.

    Tell me about the intensity of light, and its variation over distance from the source.
     
  8. Oct 18, 2011 #7

    PeterO

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    Perhaps poor wording last time...
    How does light intensity vary with distance?
     
  9. Oct 18, 2011 #8
    the greater the distance, the weaker the light intensity?
     
  10. Oct 18, 2011 #9

    PeterO

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    Yes it is weaker - and weaker in a definite way.

    If you stood in front of a wall in sunlight - and could ignore all the extra light bouncing of the environment - you would be 150000 km from the sun, and the wall might be 150000.01 km fro the sun.

    With a flash camera, you might be 3m from the flash, while the same wall would be 13m from the flash.

    What would that mean?


    EDIT: you better throw 3 more zeros in those "distance to the sun" figures!! ie 150000000 km
     
    Last edited: Oct 18, 2011
  11. Oct 19, 2011 #10
    hi, can you explain it in simpler terms, i get the first part, but not the last part?
     
  12. Oct 19, 2011 #11
  13. Oct 19, 2011 #12

    PeterO

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    If the background is twice as far away as the subject, it will receive only 1/4 the light intensity.
    You camera, which tries to automatically adjust for the light, will have the subject showing brightly, while the background is dark.

    If you have a digital camera with flash, try it.
    Have someone stand 2m from the camera in the following situations.
    (a) against a wall.
    (b) 2m from the wall.
    (c) 4m from the wall.


    if the last part was the "add three more zeros" it as because I understated the distance to the sun.
     
  14. Oct 19, 2011 #13
    ah, ok i get it now!thank you!
     
  15. Oct 19, 2011 #14

    DaveC426913

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    A secondary effect:

    Because the subject is so brightly lit, the camera gets plenty of light for a properly exposed shot, and can stop down the exposure (higher f-stop, faster shutter). This mean the background, which might otherwise have been adequately lit, is now going to be very underexposed, i.e. black.

    Much to the chagrin of many Moon hoaxers... :biggrin:
     
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