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Homework Help: How far from camera person should stand?

  1. Jan 20, 2010 #1
    1. The problem statement, all variables and given/known data

    A 35 mm camera has a lens with a 50 mm focal length. It is used to
    photograph an object 175 cm in height, such that the image is 30 mm high.
    (a) How far from the camera should the person stand?
    Answer is
    The person should stand 297 cm away from the camera.

    2. Relevant equations
    1/f=1/o+1/i
    m=i/o


    3. The attempt at a solution
    1/50=1/o+1/i
    m=30/175=i/o
    I really do not know how to relate hight of the person to distance to the camera .please help someone!How to get this answer?297cm?
     
  2. jcsd
  3. Jan 20, 2010 #2

    tiny-tim

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    Hi agentas! :wink:
    nooo :redface: … 3/175 … look at the units!

    Try again! :smile:
     
  4. Jan 20, 2010 #3
    you look yourself. Iam using cm instead of meters.What does not change anything.
     
  5. Jan 20, 2010 #4

    tiny-tim

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    Yes, 3cm/175cm. :smile:
     
  6. Jan 20, 2010 #5

    rl.bhat

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    If u is the object distance and v is the image distance, then
    m = v/u = i/o.
    Now 1/f = 1/u + 1/ v
    Multiply by u to both side. you get
    u/f = 1 + u/v = 1 + o/i.
    Now solve for u.
     
  7. Jan 20, 2010 #6

    DaveC426913

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    Gold Member

    Uh guys? Zero.

    The person should stand close enough to the camera to look through the viewfinder!

    Of course, that doesn't say anything about how close the object should be...

    :wink:
     
  8. Jan 20, 2010 #7
    thanks I got the right answer :)
     
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