calculation of focal length

Discussion in 'Digital Photography' started by TS, Aug 7, 2004.

  1. TS

    TS Guest

    I am trying to calculate the focal length of the lens in my digital camera.

    I understand that a smaller number such as 18mm is wide angle and a larger
    number such as 200mm is a tele-foto lens.

    If I calculate the angle of view of the camera, can I calculate the focal
    length?
    If so, how - and do I use the horizontal angle or the vertical plane
    (assuming the camera is used in landscape).

    Is this calculation an absolute or does it differ for 35mm cameras and
    digital cameras?

    Thanks
     
    TS, Aug 7, 2004
    #1
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  2. TS

    Matt Ion Guest

    18mm/200mm IS the focal length.

    --
    "Few people are capable of expressing with equanimity opinions which differ
    from the prejudices of their social environment. Most people are even
    incapable of forming such opinions."
    -- Albert Einstein


    "TS" <> wrote in message
    news:cf38p2$eic$...
    > I am trying to calculate the focal length of the lens in my digital

    camera.
    >
    > I understand that a smaller number such as 18mm is wide angle and a larger
    > number such as 200mm is a tele-foto lens.
    >
    > If I calculate the angle of view of the camera, can I calculate the focal
    > length?
    > If so, how - and do I use the horizontal angle or the vertical plane
    > (assuming the camera is used in landscape).
    >
    > Is this calculation an absolute or does it differ for 35mm cameras and
    > digital cameras?
    >
    > Thanks
    >
    >
     
    Matt Ion, Aug 7, 2004
    #2
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  3. TS

    TS Guest

    Also, If i add a 0.5x adaptor, does this half the focal length, or does it
    half the area (i.e. the square of the increase in focal length)?


    "TS" <> wrote in message
    news:cf38p2$eic$...
    > I am trying to calculate the focal length of the lens in my digital

    camera.
    >
    > I understand that a smaller number such as 18mm is wide angle and a larger
    > number such as 200mm is a tele-foto lens.
    >
    > If I calculate the angle of view of the camera, can I calculate the focal
    > length?
    > If so, how - and do I use the horizontal angle or the vertical plane
    > (assuming the camera is used in landscape).
    >
    > Is this calculation an absolute or does it differ for 35mm cameras and
    > digital cameras?
    >
    > Thanks
    >
    >
     
    TS, Aug 7, 2004
    #3
  4. Matt: The focal length is not a ratio but a dimension in mm.

    tspill:
    For conventional cameras:
    the distance to your object divided by the width of your object as you see
    it is equal to the distance to your film (sensor) inside the camera, which
    is the focal length, divided by the width of the film (which equals 36mm in
    35mm film)

    or
    Focal Length = 36mm x (Distance to Object/Width of Object)

    This should hold also for digital cameras if you think in terms of your
    focal length like that for a 35 mm camera. If you think in temrs of medium
    size cameras you use 60 mm instead of 36 mm and think of focal length in
    terms of the focal length for a medium size camera.

    "Matt Ion" <> wrote in message
    news:vJ9Rc.38137$gE.29243@pd7tw3no...
    > 18mm/200mm IS the focal length.
    >
    > --
    > "Few people are capable of expressing with equanimity opinions which

    differ
    > from the prejudices of their social environment. Most people are even
    > incapable of forming such opinions."
    > -- Albert Einstein
    >
    >
    > "TS" <> wrote in message
    > news:cf38p2$eic$...
    > > I am trying to calculate the focal length of the lens in my digital

    > camera.
    > >
    > > I understand that a smaller number such as 18mm is wide angle and a

    larger
    > > number such as 200mm is a tele-foto lens.
    > >
    > > If I calculate the angle of view of the camera, can I calculate the

    focal
    > > length?
    > > If so, how - and do I use the horizontal angle or the vertical plane
    > > (assuming the camera is used in landscape).
    > >
    > > Is this calculation an absolute or does it differ for 35mm cameras and
    > > digital cameras?
    > >
    > > Thanks
    > >
    > >

    >
    >
     
    Dankwart Koehler, Aug 7, 2004
    #4
  5. TS wrote:
    >
    > I am trying to calculate the focal length of the lens in my digital
    > camera.
    >
    > I understand that a smaller number such as 18mm is wide angle and a
    > larger number such as 200mm is a tele-foto lens.
    >
    > If I calculate the angle of view of the camera, can I calculate the
    > focal length?
    > If so, how - and do I use the horizontal angle or the vertical plane
    > (assuming the camera is used in landscape).


    I cannot determine from your post exactly what you want to do, but I
    am going to assume you want to determine what focal length you are
    using (or need to use) in a given situation.

    To do this, you will need three measurements, which are as follows:

    1. The horizontal (or vertical) size of an object you're going to take
    a picture of.
    2. The distance from that object to your camera when said object
    completely fills the image horizontally (or vertically if you used
    vertical in #1 above).
    3. The horizontal (or vertical, if you used vertical in #1 above)
    size of the sensor in your camera.

    Items #1 and #2 must be measured using the same units (i.e. both of
    them are measured in feet, or both of them are measured in meters,
    or furlongs, or whatever you want, as long as the units are the same).

    Item #3 should be measured in millimeters. Most manuals for cameras
    list the sensor dimension in the specifications section.

    Take the distance from the object to your camera, divide it by the
    object's size (#2 / #1). Multiply the result by the size of the
    sensor, and you have a pretty good approximation of your current
    focal length. For wide angle lenses, the calculation is less
    accurate, but it should be good enough for most purposes.

    For example, suppose I want to find the focal length that lets me
    just fit a one foot ruler in the frame while standing ten feet away.

    #1 = 1 foot
    #2 = 10 feet

    The camera I'm using is a Canon 300D. Its sensor measures about
    22.7 millimeters horizontally.

    #3 = 22.7

    #2 / #1 = 10 feet / 1 foot = 10
    That result multiplied by #3 gives us 10 * 22.7 = 227 millimeters.

    At ten feet, I would need a 227 millimeter focal length to fill the
    frame of my 300D with a ruler measuring one foot across. I would
    probably use a 200mm lens.

    > Is this calculation an absolute or does it differ for 35mm cameras
    > and digital cameras?


    As long as you completely fill the frame with the object you've
    measured, the calculation above should be independent of the type of
    film or size of sensor. Crop factor is irrelevant for the calculation
    above.

    BJJB
     
    BillyJoeJimBob, Aug 7, 2004
    #5
  6. TS wrote:
    >
    > Also, If i add a 0.5x adaptor, does this half the focal length, or
    > does it half the area (i.e. the square of the increase in focal
    > length)?


    As I understand it, a 0.5x adaptor (which is probably also known as
    a focal reducer) will cut your focal length in half, resulting in a
    linear field of view that is around twice as wide as before. If your
    field of view encompassed a region ten feet across when you are
    standing ten feet away, adding the 0.5x adaptor should give you a
    field of view measuring about twenty feet across when you are standing
    ten feet away.

    BJJB
     
    BillyJoeJimBob, Aug 7, 2004
    #6
  7. TS

    TS Guest

    I was giving these as examples. I need to calculate the focal length of
    another digital camera/lens to allow me to enter the lens details into a
    stiching/360 panorama application. And I asumed that you cna do this from
    the angle of the image from the camera.



    "Matt Ion" <> wrote in message
    news:vJ9Rc.38137$gE.29243@pd7tw3no...
    > 18mm/200mm IS the focal length.
    >
    > --
    > "Few people are capable of expressing with equanimity opinions which

    differ
    > from the prejudices of their social environment. Most people are even
    > incapable of forming such opinions."
    > -- Albert Einstein
    >
    >
    > "TS" <> wrote in message
    > news:cf38p2$eic$...
    > > I am trying to calculate the focal length of the lens in my digital

    > camera.
    > >
    > > I understand that a smaller number such as 18mm is wide angle and a

    larger
    > > number such as 200mm is a tele-foto lens.
    > >
    > > If I calculate the angle of view of the camera, can I calculate the

    focal
    > > length?
    > > If so, how - and do I use the horizontal angle or the vertical plane
    > > (assuming the camera is used in landscape).
    > >
    > > Is this calculation an absolute or does it differ for 35mm cameras and
    > > digital cameras?
    > >
    > > Thanks
    > >
    > >

    >
    >
     
    TS, Aug 7, 2004
    #7
  8. TS

    Paul W. Ross Guest

    You need to know the diagonal measurement of the sensor. The FL is
    then the distance a pinhole would be from this sensor to give the same
    ANGULAR view. TYpically, a "normal" lens is one whose FL is the same
    as the diagonal. WA is a larger angle, and TF is less. The specs for
    the sensor should be in the camera manual. Also, some digital cameras
    tell you the relationship between their lenses and a 35mm camera. Try
    that..
     
    Paul W. Ross, Aug 7, 2004
    #8
  9. TS

    Matt Ion Guest

    "Dankwart Koehler" <> wrote in message
    news:YAaRc.184314$...
    > Matt: The focal length is not a ratio but a dimension in mm.


    That's what I said. Okay, I typed it poorly: 18mm and 200mm (as specified
    in the original post) are focal lengths. They don't need to be calculated;
    they are specified by the lens or camera manufacturer.
     
    Matt Ion, Aug 8, 2004
    #9
  10. TS

    Matt Ion Guest

    You don't calculate the focal length; the camera or lens manufacturer tells
    you what the focal length is. 18mm and 200mm, in your example, are focal
    lengths. You can use the focal length to calculate the angle of view, not
    vice-versa.

    --
    "Few people are capable of expressing with equanimity opinions which differ
    from the prejudices of their social environment. Most people are even
    incapable of forming such opinions."
    -- Albert Einstein


    "TS" <> wrote in message
    news:cf3eok$p2g$...
    > I was giving these as examples. I need to calculate the focal length of
    > another digital camera/lens to allow me to enter the lens details into a
    > stiching/360 panorama application. And I asumed that you cna do this from
    > the angle of the image from the camera.
    >
    >
    >
    > "Matt Ion" <> wrote in message
    > news:vJ9Rc.38137$gE.29243@pd7tw3no...
    > > 18mm/200mm IS the focal length.
    > >
    > > --
    > > "Few people are capable of expressing with equanimity opinions which

    > differ
    > > from the prejudices of their social environment. Most people are even
    > > incapable of forming such opinions."
    > > -- Albert Einstein
    > >
    > >
    > > "TS" <> wrote in message
    > > news:cf38p2$eic$...
    > > > I am trying to calculate the focal length of the lens in my digital

    > > camera.
    > > >
    > > > I understand that a smaller number such as 18mm is wide angle and a

    > larger
    > > > number such as 200mm is a tele-foto lens.
    > > >
    > > > If I calculate the angle of view of the camera, can I calculate the

    > focal
    > > > length?
    > > > If so, how - and do I use the horizontal angle or the vertical plane
    > > > (assuming the camera is used in landscape).
    > > >
    > > > Is this calculation an absolute or does it differ for 35mm cameras and
    > > > digital cameras?
    > > >
    > > > Thanks
    > > >
    > > >

    > >
    > >

    >
    >
     
    Matt Ion, Aug 8, 2004
    #10
  11. TS

    Matt Ion Guest

    That would halve the focal length, which would give you a 4X increase in
    coverage.

    --
    "Few people are capable of expressing with equanimity opinions which differ
    from the prejudices of their social environment. Most people are even
    incapable of forming such opinions."
    -- Albert Einstein


    "TS" <> wrote in message
    news:cf3aoi$h3s$...
    > Also, If i add a 0.5x adaptor, does this half the focal length, or does

    it
    > half the area (i.e. the square of the increase in focal length)?
    >
    >
    > "TS" <> wrote in message
    > news:cf38p2$eic$...
    > > I am trying to calculate the focal length of the lens in my digital

    > camera.
    > >
    > > I understand that a smaller number such as 18mm is wide angle and a

    larger
    > > number such as 200mm is a tele-foto lens.
    > >
    > > If I calculate the angle of view of the camera, can I calculate the

    focal
    > > length?
    > > If so, how - and do I use the horizontal angle or the vertical plane
    > > (assuming the camera is used in landscape).
    > >
    > > Is this calculation an absolute or does it differ for 35mm cameras and
    > > digital cameras?
    > >
    > > Thanks
    > >
    > >

    >
    >
     
    Matt Ion, Aug 8, 2004
    #11
  12. TS

    Ken Burns Guest

    The focal length used for a particular shot is recorded in the EXIF data.
    Just use any software that can read this data and you will know exactly.

    KB




    "TS" <> wrote in message
    news:cf38p2$eic$...
    > I am trying to calculate the focal length of the lens in my digital

    camera.
    >
    > I understand that a smaller number such as 18mm is wide angle and a larger
    > number such as 200mm is a tele-foto lens.
    >
    > If I calculate the angle of view of the camera, can I calculate the focal
    > length?
    > If so, how - and do I use the horizontal angle or the vertical plane
    > (assuming the camera is used in landscape).
    >
    > Is this calculation an absolute or does it differ for 35mm cameras and
    > digital cameras?
    >
    > Thanks
    >
    >
     
    Ken Burns, Aug 8, 2004
    #12
  13. "Matt Ion" <> wrote in message
    news:GekRc.46314$M95.12442@pd7tw1no...
    > "Dankwart Koehler" <> wrote in message
    > news:YAaRc.184314$...
    > > Matt: The focal length is not a ratio but a dimension in mm.

    >
    > That's what I said. Okay, I typed it poorly: 18mm and 200mm (as

    specified
    > in the original post) are focal lengths. They don't need to be

    calculated;
    > they are specified by the lens or camera manufacturer.
    >
    >
    > For digicams, they are usually specified as the equivalent fl for a 35 mm

    camera.

    The formula for focal length has been discussed frequently on this and other
    NGs, it is surely avalable on the Web (sorry, I don't have time to search
    for it), and in most physics textbooks.
     
    Marvin Margoshes, Aug 8, 2004
    #13
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