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  1. #1

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    What is Focal length?

    Sorry if this is in the wrong section.

    The title says it all. If someone could explain to me what focal length is, maybe with illustrations or something. You know, dumb it down a bit.

    Thank you very much.

  2. #2
    wiltw's Avatar
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    Have you ever taken a magnifying glass, hold it so the sun shines thru it onto a piece of paper? The bright spot is an image of the sun! And the distance from the center of the lens to the paper is the 'focal length'. Modern camera optics are much more complex that that magnifying glass, but the principle is identical!

  3. #3

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    Check out Jason's video on how to measure focal length www.jasonbrunner.com/videos.html

  4. #4
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    As wiltw said, it is a measure of the distance from the focal plane (the film, or digital sensor) to the optical center (where light rays come together to a point) of the lens.

    A lens of shorter focal length, like a magnifying glass or the capital letter X, accomplishes a wider field of view. A lens of longer focal length, like a telescope or ><, gives a narrower field of view that essentially brings a far subject closer. Because how much the lens has to pull the light rays together will differ for different film sizes (less strong for larger format, much stronger for small 35mm format), what we call "wide," "normal," or "telephoto" will differ accordingly.

    Does that help?
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  5. #5
    holmburgers's Avatar
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    The easiest way for me to understand focal length is with a pin-hole camera and forgetting about lenses for a moment. It is simply the distance from the hole to the back of the camera, where the film goes.

    Now imagine that you're standing inside this pinhole camera, or if you'd prefer, you're in a large dark room with a single window that looks onto a field. If you are standing on the opposite side of the room looking across to the window, you're going to see just a small bit of whatever's outside; maybe just the distant horizon. As you walk towards the window you start seeing more of the outside up until the point that your nose is pressing against the glass and you can see all around. Shorter focal length = wide angle, long = "zoomed in"
    If you are the big tree, we are the small axe

  6. #6
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    Gee, I always thought it was how many millimeters you could jam into a lens

  7. #7

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    OK, so is focal length is the point when the image is focused, or is st when the light beams converge?

  8. #8
    Ottrdaemmerung's Avatar
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    Quote Originally Posted by Jitterbug View Post
    OK, so is focal length is the point when the image is focused, or is st when the light beams converge?
    It is the distance from the film/sensor plane to where the light beams converge.
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    "Embrace the negative with absolution, your final positive reward." --IQ, "The Province," Frequency

  9. #9

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    hi jitterbug

    just to add to your confusion
    when a lens says "50mm" as a focal length
    ( or whatever number you want to say )
    when the lens is set to " infinity " (something 50 feet away ? )
    the lens will focus that thing off in the distance "50 mm"
    measured from where the fstop/diaphragm thingy is to
    wherever you are projecting the image.

    a 3 1/2 inch lens, will focus an "infinity" ( 50 feet away ) subject/image
    3 /2 inches away from a paper, ground glass, film plane &c.

    it is always the focus distance at infinity ... the closer the subject is, the further longer the distance
    to wherever it is you are focusing ... that is why macro photography on 35mm or mf camera requires "tubes"
    to because the lens won't focus at close distances ( unless it is a macro / micro lens ) exact size ( 1:1 reproduction )
    requires 2x the lens' focal length ( 100mm for a 50mm lens ) ...

    have fun !
    john

  10. #10

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    Quote Originally Posted by wiltw View Post
    Have you ever taken a magnifying glass, hold it so the sun shines thru it onto a piece of paper? The bright spot is an image of the sun! And the distance from the center of the lens to the paper is the 'focal length'. Modern camera optics are much more complex that that magnifying glass, but the principle is identical!
    Beautiful description. Thanks for that!

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