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'Zoom in' in a camera

  1. Dec 24, 2006 #1
    From observation, whenever I want to 'zoom in' in a camera, the lens expands outward. What is the reason behind this?

    My guess, 'zoom in' => greater magnification => object closer to the focal point of the lens (although greater than the focal lengths since otherwise the image would be virtual). The lens expand so that the object under investigation can be closer to the focal point of the lens in the camera so a greater magnification can be obtained.
    It links with the magnification formula M=-i/o where i is image distance and o is object distance. The lens being closer to the object => o is decreased and i might be increase => increase in M.

    The other things is that the lens distance is increased so that could increase magnification. i.e. the magnification for the compound microscope has a L term in it which is the distance between the two lens. which if increased will increase the magnification.
     
    Last edited: Dec 24, 2006
  2. jcsd
  3. Dec 24, 2006 #2

    russ_watters

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    When you zoom, you change the focal length of the camera. It says that on the lens...
     
  4. Dec 24, 2006 #3
    From the thin lens formula it means increase in focal length => increase in magnification. So we when zoom in we increase the focal length of the camera?

    But its important the object distance is not within the focal length because otherwise a virtual image will be photographed which means no photograph at all because the film will only record a real image.
     
  5. Dec 24, 2006 #4
  6. Dec 24, 2006 #5

    DaveC426913

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    No. The lens is really several (sometimes many) lenses components all working together. When focussing, only some lenses (the outer ones) move, others (the inner ones) stay put.

    So when all the lenses are very close together (and the camera is flat), the light rays don't have much "room" to converge from one lens to the next in line. But when the lens structure expands (and the camera sticks out) the lenses now have "room" between them where the light rays can do their thing.
     

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    Last edited by a moderator: Dec 27, 2006
  7. Dec 24, 2006 #6

    okay. But when zooming, the chief aim is to increase the focal point so that the object under investigation can be closer to the focal point which will mean greater magnification.

    However, the object must not be inside the focal point otherwise the image won't be real. So is that way the focal point is usually very short.
     
  8. Dec 24, 2006 #7

    DaveC426913

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    The magnification comes from the distance between lens elements. If the lens elements are nearly touching, the image won't diverge much. You move the lens elements farther apart so that the image diverges greatly (thus leaving only a small portion of the object in view, i.e. magnified).

    The distance from the front of the lens to the object is not supposed to change. (Though, in fact, it DOES change, but that is an UNDESIRABLE side effect. If a technology existed where it didn't have to change, beleive me they'd embrace it. You could have micro-sized cameras whose lenses DON'T have to pop out. That would be a HUGE advance in technology.)
     
  9. Dec 24, 2006 #8
    So the magnification system in a camera is rather like a compound microscope? The magnification formula has 'distance between two lenses' as a variable and has a positive correlation with the magnification.

    Why would it be so good if the distance from the front of the lens to the object did not change? Would it make calculations of the magnfication more accurate so the photographer have more precise information? With current technology, they could factor this side effect in couldn't they?

    Also if the object distance is closer than the focal point of the camera than no real image can be formed on the photograph but light would still reach it so the photograph would just look blurred (not a blurred image but a blurred nothing of light or darkness depending if the flash was used).
     
    Last edited: Dec 24, 2006
  10. Dec 24, 2006 #9

    russ_watters

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    Since the film (or ccd) is only milimeters or centimeters from the front of the primary lens, that is rarely an issue.
    I don't understand what you are saying. Why would magnification change be anything other than the ratio of focal lengths? Try drawing a diagram of how a lens works - it may make more sense if you diagram it.
     
    Last edited: Dec 24, 2006
  11. Dec 24, 2006 #10
    My original qustion was in response to DaveC426913 comments in the post above my last post.

    Maybe my comments wasn't accurate because I was thinking the magnification for a single simple lens M=-i/o where i is image distance and o is object distance.

    The ratio of focal lengths is mainly used for telescopes? For one, the compound microsope uses a different formula.
     
  12. Dec 25, 2006 #11

    DaveC426913

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    Yes.

    Yes, but it is undesirable.

    It would be blurry, just like out of focus.


    Note that VERY blurry is what you described. Further, if you place an object directly on the lens, you won't see a blurry object, it will simply act exactly like raising the f-stop.
     
  13. Dec 25, 2006 #12
    Dave, the question still remains 'Why would it be so good if the distance from the front of the lens to the object did not change?'

    How do you explain 'if you place an object directly on the lens, you won't see a blurry object, it will simply act exactly like raising the f-stop.'

    I have tried putting the object directly in front of the lens, the problem was it was completely dark for a non illuminating object and completely bright for an illuminating object.
     
  14. Dec 25, 2006 #13

    DaveC426913

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    I'm trying to point out that error in argument that a longer lens is closer to an object. That's not why lenses are longer.

    A lens that could magnify and focus without changing its length would:
    - be simpler in mechanical design, more robust, less prone to breakage
    - fit in a single-sized case without having to take it off to put it away
    - make the design of compact cameras more compact and much faster**
    - etc., etc

    **a compact camera has to take several seconds to extend its lens to focusing range. Considering speed is a huge liability in compact cameras, a slow reaction time is a huge hit.




    If the viewing area of your lens is, say 2 inches in diameter (3.14sq.in), and you place a 1/2 inch dime on the lens (1.7sq.in), all you are doing is reducing the amount of light that passes into the lens by half. Because the dime is nowhere near the focal range and very near the f-stop iris, it has the same effect as the iris.


    How big is your lens in diameter? How big is the object in diameter?

    You should be seeing your image as normal (your living room, your backward, etc.) except it should be VERY dark (as of you had the exposure stopped up to f32 or more). You should not see the dime.

    See attached.

    BTW, this will be harder to do on a point & shoot camera with their teensy-tiny lenses.
     

    Attached Files:

    Last edited: Dec 25, 2006
  15. Dec 26, 2006 #14

    The object was much greater than the lens. i.e. the object was the computer and a normal camera. I put the camera right in front of the computer screen.
     
  16. Dec 26, 2006 #15

    The object was much greater than the lens. i.e. the object was the computer and a normal camera. I put the camera right in front of the computer screen.

    What happens if the dime in your picture was illuminating? Would you now be less able to see the tree?
     
  17. Dec 26, 2006 #16

    The object was much greater than the lens. i.e. the object was the computer and a normal camera. I put the camera right in front of the computer screen.

    What happens if the dime in your picture was illuminating? Would you now be less able to see the tree than when it wasn't illuminating?
     
  18. Dec 27, 2006 #17

    DaveC426913

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    I think I understand why you're not seeing anything...:uhh:

    It sounds like you're not trying to image something behind the obscuring object, it sounds like you're trying to image the obscuring object itself.

    What are you trying to do?

    Well, yes, in the same sense that it is much harder to see outside at night through a window when the light is on inside the house you're in. And in the sense that its much harder to feel a pea under your back if there are several mattresses between you and the pea.

    The additional light from your dime "swamps" the darker image from behind. Theoretically, that darker image is still there, and is picked up by the eye (and/or by the camera's imaging mechanism) but it's too faint, too subtle, to see.
     
    Last edited: Dec 27, 2006
  19. Dec 27, 2006 #18
    I am trying to photograph an object when placed inside the focal length of the lens so that theoretically a virtual image should be formed in the film of the camera so the film would need to register a virtual image (but would you call it a virtual image if there is no viewer?). So it may be the case of photographing just the dime without the background of the tree. I want to see what it looks like. Since the focal length is very short, I have to put it very close to the camera. The disadvantage is that not much light gets into the camera from the object because the camera is blocking off a lot of the light so an illuminating object would be good.

    If we assume the camera is just like the human eye. I could just look at an object very closely at a distance within my focal length but when I do that, I just see a very blurred image of the object. If there was another pair of eyes living in my retina, they should see a virtual image of that object shoudn't they?
     
    Last edited: Dec 27, 2006
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