What is Focal length?

Discussion in 'Camera Building, Repairs & Modification' started by Jitterbug, Aug 26, 2011.

  1. Jitterbug

    Jitterbug Member

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    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. wiltw

    wiltw Subscriber

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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. bdial

    bdial Subscriber

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  4. Ottrdaemmerung

    Ottrdaemmerung Member

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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?
     
  5. holmburgers

    holmburgers Member

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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"
     
  6. flash26c

    flash26c Member

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

    Jitterbug Member

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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. Ottrdaemmerung

    Ottrdaemmerung Member

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    It is the distance from the film/sensor plane to where the light beams converge.
     
  9. jnanian

    jnanian Advertiser Advertiser

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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. Mike Crawford

    Mike Crawford Member

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    Beautiful description. Thanks for that!
     
  11. Jitterbug

    Jitterbug Member

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    Oh! I get it now, I wondered why my camera lens said 80mm. =)
     
  12. Leigh B

    Leigh B Member

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    Focal length is the distance along the lens axis from the rear node of the lens to the film when focused at infinity.

    The rear lens node is at the intersection of the lens axis and the second principle plane, which may be anywhere inside or outside of the lens. It's a design feature that's usually specified on the lens spec sheet.

    The rear lens node is the point within the optics from which all exiting paraxial rays appear to emanate.

    In contrast, the front lens node, at the intersection of the lens axis and the first principle plane, is the point on which all incoming paraxial rays appear to converge.

    The front and rear nodes are two distinctly different points. Both can be anywhere on the optical axis, and the rear node can be behind, on, or in front of the front node.

    In the Nikkor-T ED 270mm/6.3 for example, the first principle plane is 35.1mm in front of the front lens element, while the second principle plane is 9.6mm behind that, or 25.5mm in front of the front element.

    - Leigh
     
    Last edited by a moderator: Aug 26, 2011
  13. MattKing

    MattKing Subscriber

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    Practically speaking, the longer the focal length, the narrower the angle of view.

    So a lens with a short focal length will have a wider view of things than a lens with a longer focal length.

    The actual, practical angle of view also depends on how big the film or sensor is. That is why a 50mm lens designed for 35mm film is considered a "standard" lens whereas a 50mm lens for an APS-C digital sensor (which is smaller than 35mm film) is a moderate telephoto (i.e. has a narrower than standard angle of view).

    In each case though, when focusing at infinity, the nodal point of the lens will be 50mm from the film/sensor.

    And a big 8x10 or larger view camera will require a much longer focal length lens to achieve the same/similar results.

    Binoculars and telescopes also have focal lengths. So do prescription glasses.

    EDIT: as pointed out below, questions of angle of view will only be directly related to focal length if your camera doesn't offer movements (such as are available in many large format view cameras)
     
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  14. Leigh B

    Leigh B Member

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    I don't see where the OP asked about angle of view. :confused: :blink:

    Angle of view is not related to focal length except by general grouping. It's a function of lens design.

    - Leigh
     
  15. MattKing

    MattKing Subscriber

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    Leigh:

    It seemed to me to be one of those "illustrations" that the OP asked for, that might help the OP understand what effect focal length actually has.

    Thus the word "Practically".

    And for most people, who don't have the advantage of camera movements, format plus focal length determines the angle of view.

    I'll agree however, that I should have confirmed that I was referring to cameras without movements - and will edit my post accordingly.
     
  16. Leigh B

    Leigh B Member

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    OK. I'll go along with the 'illustrations' thing. :D

    The focal length and format define the angle of view, and thus are the specifications for the lens design.

    The comment to which I responded earlier did not mention format.

    As an example, in 150mm lenses you can find AoV from 8.2° (Olympus Zuiko 150/2) to 106° (Nikkor-SW 150/8).

    And yes, for any given format, a longer focal length will give a narrower AoV. That's simple geometry. That's why we have wide-angles and normals and telephotos.

    - Leigh
     
  17. John Koehrer

    John Koehrer Subscriber

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    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.

    That covered it! Don't know about how basic it was.
     
  18. moki

    moki Member

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    The explanation with a simple magnifying glass is probably the best to understand the principle. With modern camera lenses however, it can be more complicated than that, as Leigh B tried to explain.
    Look up "retrofocus" and "telephoto" at Wikipedia. These are the most useful (understandable for someone with little knowledge of optics) articles I could find on the subject. It's about how it's possible that you can have a f=15mm lens on your SLR which is a lot farther away than 15mm from the film (which would be logical if focal lenght equals lens-to-film distance). Or how it's possible to build a f=500mm lens that's actually shorter than it's focal length.

    I like playing around with various lenses and some ground glass and found out that building complex lenses (with more than 1 element) isn't that hard. I once built a 30mm/1,3 lens for 35mm film cameras with some old glass... getting it to be reasonably sharp and free of optical aberrations (look those up too, if you're interested in lens construction) is the hard part that makes development of new lenses so difficult.