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With a basic box camera why the film blurries instead of growing the size of the imag

  1. Apr 19, 2005 #1
    this is what i would expect from a conical perspective on a flat plane that is how a basic camera works, the farther the film from the hole the bigger the projection of the image but insted the image gets blurry, why?

    in a close image the center of the picture is closer to the viewer than the corners how comes that if the center is foucused the corners are also focus when they are farther than the center

    could someone explain me or point me to a place where is explained focus?
  2. jcsd
  3. Apr 19, 2005 #2


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    For a thin lens, as in you simple box camera image formation is modeled by the thin lens equation.

    [tex] \frac 1 f = \frac 1 o + \frac 1 i [/tex]

    where f is the focal length of the lens
    i is the image distance and o is the object distance.

    In a simple box camera the film is placed at the focal plane of the lens so any image which is near infinity will be focused on the film. The usual approximation for lens is that anything greater then 10xf is at infinity. So a camera with a 2cm focal length lens and the film at the focal length would be reasonably focused for any object greater then about .2m in front of the camera.
  4. Apr 20, 2005 #3
    thanks a lot integral

    but still seems to me that optics is not a conic projection since in conic projection theres no focusing so what kind of projection follows the view of objects?

    i would say that in the interior of the eye or camera all in front of you is represented on 3d in your eye and depending how you move the film or retine you caught well whats closer or farther, like a 3d inverted hologram with the peculiarity that infinitum size is represented in the hologram finite by shrinking in half with double distance

    anybody knows a web about optics where i can learn on the subject?
  5. Apr 20, 2005 #4


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    I am sorry but your last paragraph does not make any sense.

    Do a web search on "geometric optics" you should be able to find some good information.
  6. Apr 21, 2005 #5


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    from what i know the "3d ness" that we see is really only there because of shadows, and the fact that we have two eyes. as far as I know its not connected to how the individual eye works, as in when something is close to you and it gets blurry, it would be blurry if there were no shadows, or if you had one eye, etc.

    and i missed what you said about conic projection, but all 'focusing' really means is if you think of a thin lens (converging lens), and light going through it, then light coming in parrallel 'focuses' on one point. if you put a piece of paper there or something you see a good image. if you move the paper too close or too far you are cutting the cone shaped light not at the tip, and you see it as blurry.
  7. Apr 21, 2005 #6


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    The next time you get your hands on a magnify glass, instead of looking through it, Hold it above the table top beneath a ceiling light and watch the spot of light formed by the lens. Move the lens up and down watching the spot of light. You should be able find an image of the light bulb (or source) This will work for any relatively bright source of light.
  8. Apr 21, 2005 #7
    To Aviator
    as Integral said this is geometric optics ( that means straight line ray tracing )
    To understand this you need to understand what a simple lens does .
    Take a point object emmitting light towards a simple lens , on the far side those rays will come to an almost point also but at a given distance . But only at ONE distance .
    That is the object to lens distance changes the point at which the rays focus .

    Attached Files:

  9. Apr 25, 2005 #8
    it think i know the answers to my questions now:

    in a box camera with no lenses theres no focusing and by moving back the film you zoom and by moving it forward you increase the angular

    with a magnifying glass you can focus light in a spot but if you take the glass away of it theres no way you can concentrate light therefore theres no focusing when a hole

    besides in the formula 1/f = 1/o + 1/i if the i and the f has to be the same so the image estays focused would imply 1/o = 0 which is imposible
  10. Apr 25, 2005 #9


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    Could you rephrase this in english?
  11. Apr 26, 2005 #10
    my point is that a box camera works as a perfect conic perspective with no focus.

    theres no focus because a hole cant refract light as a lens does.

    the image distance to the lens and the focal length has to be the same in order for it to be focused but that implys that if you use the formula for thin lenses then 1/(objects distance) must be equal to 0 which is imposible.

    is it because im wrong and the I and the F dont need to have the same value for the object ot stay focused?

    i hope to be clearer now but english is my second language
  12. Apr 26, 2005 #11


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    By "box camera" you mean a pin hole camera, you are correct. A pin hole creates an image without a lens, therefore is in focus everywhere. The further from the pin hole the larger the image, but it also gets dimmer with distance.
    Not quite, for every object distance greater then the focal length there is an image distance where an image if formed. So if you have a magnifying glass (convex lens) if you move the lens closer to a source you will find a focused image at the distance predicted by the thin lens formula, the closer the object is to the focal length the further back the image will be. If the object is at the focal length the image will be formed at infinity. On the other hand if you move the object away from the lens the image will move toward the focal length, so when the object is at infinity [itex] \frac 1 o = 0 [/itex] as I said in my original post any distance greater then 10 focal lengths is approximately infinity as far as the thin lens equation is concerned.
    OK, now I understand, sorry for the last post.

    I am sure your English is orders of magnitude better then my abilities in your native language.
  13. Apr 26, 2005 #12
    thanks a lot for your time integral i apreciate a lot the freedom of thought and expresion that there is in this forum which makes it the best i know of
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