Focal length confusion

Discussion in 'Photography Beginners' Forum' started by GaryPolter, Sep 2, 2010.

  1. GaryPolter

    GaryPolter TPF Noob!

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    I'm a little confused by what is meant by focal length. Wikipedia says it's both how strong the light converges and the distance from the lens to the image plane. They also give an example that is not quite clear to me:

    Assume the camera is set to a focal length of 50mm, and now I want to focus on an object 1000mm away. Wikipedia says "the lens must be moved 2.6mm further away from the image plane". Does this mean I change my focal length to 52.6?

    The thin lens equation says 1/u + 1/v = 1/f. u is the distance from the lens to the object. v is the distance from the lens to where the image is formed. f is the focal length. So does the image sensor go on v or f?


    Focal length - Wikipedia, the free encyclopedia
     
  2. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    I'm no lens engineer, but you need to consider the 'focal point' of the lens. Which is the point where the light converges. This may change as the lens is focused, zoomed etc.
    So it's not the whole lens that has to physically move, just certain elements, that will adjust the focal point (or not). :scratch:
     
  3. GaryPolter

    GaryPolter TPF Noob!

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  4. Polyphony

    Polyphony TPF Noob!

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  5. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    In practical terms, I think you may be way over-thinking this.

    This isn't the type of thing you need to be thinking about when considering which lens to buy. This is the stuff you think about when you are really bored and need to satisfy a geeky curiosity. ;)
     
  6. Polyphony

    Polyphony TPF Noob!

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    Or took years and years of physics classes. :lol:
     
  7. GaryPolter

    GaryPolter TPF Noob!

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    Say I have a lens set to a focal length of 110mm. Effectively, this means that the lens is 110mm away from the image sensor, right?

    Now say I have an object 300mm away that I want to focus on. Using the lens equation, I know that the distance from the lens to the object image plane is 173.68mm.

    How would I focus on this object? If I adjust the focal length, then the object image plane will change...

    My overall goal is to understand this diagram: http://http.developer.nvidia.com/GPUGems/elementLinks/fig23-02.jpg
     
  8. AdrianC

    AdrianC TPF Noob!

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    I don't know much about lenses, but I think you're confusing things.

    The focal length tells you how much the lens zooms, basically. Then you have the focus ring which allows you to focus on whatever you want. There are two rings.
     
  9. prodigy2k7

    prodigy2k7 No longer a newbie, moving up!

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    The beginning of that quote says "To focus a distant object" Its referring to the focusing, not the focal length. Focusing is just a small movement of lense(s) to ...focus

    Im no expert but that sounds right doesn't it? i hope so ha
     
  10. prodigy2k7

    prodigy2k7 No longer a newbie, moving up!

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    Ya thats what I thought, but how do you explain the EF 24-70 F/2.8L USM?

    "Of note is that the 24-70mm L reverse extends - it becomes longest at 24mm as shown above. Most lenses including the 24-105 L become fully extended at their longest focal length setting."

    Canon EF 24-70mm f/2.8 L USM Lens Review
     
  11. clanthar

    clanthar TPF Noob!

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    This is absolutely correct. I think what you're missing is this additional information:

    Lenses have a specific "focal length at infinity" where they are as close to the film/sensor as possible. A lens closer to the sensor than it's focal length at infinity isn't in focus on anything. As you focus a lens on subjects increasing closer to the sensor the focal length increases. At the point where the lens focal length is double it's infinity focal length it will be equal distant between the sensor and the subject and you'll be into the macro range of photography.

    Joe
     
  12. clanthar

    clanthar TPF Noob!

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    [/quote]
    Ya thats what I thought, but how do you explain the EF 24-70 F/2.8L USM?
    [/quote]

    Focal length as the distance from the center of the lens (nodal point) to the film is the basic principle for simple lens designs. That all changed when this happened: P. Angenieux 35mm f/2.5. Lens designers discovered how to move the nodal point behind and/or in front of the lens center.

    Joe
     

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