Focal length manipulation

Discussion in 'Digital Photography' started by orelza@gmail.com, Apr 13, 2007.

  1. Guest

    Hello,

    Am working on my final year project and it involoves mosaicing some
    aerial photos taken in 1990 using an aerial photo camera with focal
    length 153. I scanned these at 800dpi, with dimensions 7242 x 7263
    pixels. The software am using requires a Camera calibration file and
    the focal length is expressed in pixels. Any body out there who can
    help me figure out this using the provided information? or if this
    post not clear or you may require some additional information, please
    let me know.


    Thank you.
    , Apr 13, 2007
    #1
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  2. cgiorgio Guest

    <> schrieb im Newsbeitrag
    news:...
    > Hello,
    >
    > Am working on my final year project and it involoves mosaicing some
    > aerial photos taken in 1990 using an aerial photo camera with focal
    > length 153. I scanned these at 800dpi, with dimensions 7242 x 7263
    > pixels. The software am using requires a Camera calibration file and
    > the focal length is expressed in pixels. Any body out there who can
    > help me figure out this using the provided information? or if this
    > post not clear or you may require some additional information, please
    > let me know.
    >
    >
    > Thank you.
    >

    Focal length in pixels (F =153 mm), 800 dpi

    153 / 25,4 * 800 = 4819 pixels
    cgiorgio, Apr 13, 2007
    #2
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  3. Paul Allen Guest

    On Fri, 13 Apr 2007 17:01:46 +0200
    "cgiorgio" <> wrote:

    >
    > <> schrieb im Newsbeitrag
    > news:...
    > > Hello,
    > >
    > > Am working on my final year project and it involoves mosaicing some
    > > aerial photos taken in 1990 using an aerial photo camera with focal
    > > length 153. I scanned these at 800dpi, with dimensions 7242 x 7263
    > > pixels. The software am using requires a Camera calibration file and
    > > the focal length is expressed in pixels. Any body out there who can
    > > help me figure out this using the provided information? or if this
    > > post not clear or you may require some additional information,
    > > please let me know.
    > >
    > >
    > > Thank you.
    > >

    > Focal length in pixels (F =153 mm), 800 dpi
    >
    > 153 / 25,4 * 800 = 4819 pixels


    This looks like an opportunity to learn something.

    How does it make sense to express focal length in pixels? Is this
    something peculiar to aerial photography? Or does the concept have
    wider applicability?

    Inquiring minds, and all that.

    Paul Allen
    Paul Allen, Apr 13, 2007
    #3
  4. Pat Guest

    On Apr 13, 11:29 am, Paul Allen <>
    wrote:
    > On Fri, 13 Apr 2007 17:01:46 +0200
    >
    >
    >
    > "cgiorgio" <> wrote:
    >
    > > <> schrieb im Newsbeitrag
    > >news:...
    > > > Hello,

    >
    > > > Am working on my final year project and it involoves mosaicing some
    > > > aerial photos taken in 1990 using an aerial photo camera with focal
    > > > length 153. I scanned these at 800dpi, with dimensions 7242 x 7263
    > > > pixels. The software am using requires a Camera calibration file and
    > > > the focal length is expressed in pixels. Any body out there who can
    > > > help me figure out this using the provided information? or if this
    > > > post not clear or you may require some additional information,
    > > > please let me know.

    >
    > > > Thank you.

    >
    > > Focal length in pixels (F =153 mm), 800 dpi

    >
    > > 153 / 25,4 * 800 = 4819 pixels

    >
    > This looks like an opportunity to learn something.
    >
    > How does it make sense to express focal length in pixels? Is this
    > something peculiar to aerial photography? Or does the concept have
    > wider applicability?
    >
    > Inquiring minds, and all that.
    >
    > Paul Allen


    I agree, interesting question. The OP is learning aerial photography
    but isn't learning Googling. How interesting is that? So here is
    what Google found:

    Field of View from EXIF Data

    In the images captured by some digital cameras, there is some data
    that is useful to determine the field of view for stitching panoramas.

    Ideally, this would be the focal length in pixels, but unfortunately
    the focal length is given in millimeters. In order to determine the
    field of view, it is necessary to know the density of pixels (in
    pixels per millimeter) on the virtual imaging sensor associated with
    the image. Again, this pixel density is usually given in pixels/inch,
    so conversion to pixels/mm is needed as well. The pixel density may be
    different for the horizontal and vertical directions.

    >From the focal length in pixels, and the number of pixels in each

    dimension of the image, it is possible to compute the field of view in
    each dimension.

    To convert from focal length in mm to focal length in pixels, where
    pixel density is given in pixels/inch, use:

    focalPixels = focalMM * pixelDensity / 25.4

    To compute the field of view from focal length in pixels, use:

    fov = 2 * atan( (H - 1) / (2 * focalPixels) )

    where H is either width or height, for the field of view in the
    respective direction. These equations are encapsulated into the
    calculator below.

    Field of View from EXIF Data Calculator

    E
    X
    I
    F

    d
    a
    t
    a FocalLength mm
    PixelXDimension pixels
    PixelYDimension pixels
    FocalPlaneXResolution pixels/unit
    FocalPlaneYResolution pixels/unit
    FocalPlaneResolutionUnit

    c
    o
    m
    p
    u
    t
    e
    d Pixel FocalLength in X pixels
    Pixel FocalLength in Y pixels
    Field of View in X degrees
    Field of View in Y degrees

    ----------------------
    So, to the OP, why are we doing your Googling for you. What are they
    teaching in college these days?
    Pat, Apr 13, 2007
    #4
  5. ASAAR Guest

    On Fri, 13 Apr 2007 08:29:16 -0700, Paul Allen wrote:

    >>> Am working on my final year project and it involoves mosaicing some
    >>> aerial photos taken in 1990 using an aerial photo camera with focal
    >>> length 153.

    . . .

    >> Focal length in pixels (F =153 mm), 800 dpi
    >>
    >> 153 / 25,4 * 800 = 4819 pixels

    >
    > This looks like an opportunity to learn something.
    >
    > How does it make sense to express focal length in pixels? Is this
    > something peculiar to aerial photography? Or does the concept have
    > wider applicability?
    >
    > Inquiring minds, and all that.


    It may make sense for certain types of aerial photography, such as
    when it's used for surveillance and resolving power is of primary
    importance. For instance, you may want to be able to read the
    numerals on a license plate, and one way to improve your chances
    would be to double the focal length of the lens. This would
    increase the image size by 2x, allowing 4x the number of pixels to
    be used for just the license plate. Or if the lens and the sensor
    size remains the same but the pixel width is halved (quadrupling the
    sensor's number of pixels), you'd also get 4x the number of pixels
    for the license plate. So a one-size-fits-all number that's used to
    characterize a lens/sensor system that's based on a function of
    focal length and pixel size is probably useful to get a rough
    estimate of the resolving power of such a system. It should use,
    however, the sensor's pixel size, and not a figure based on an
    arbitrary 800 dpi that was used to scan prints after the fact. The
    values used in the Camera Calibration File presumably are fixed, and
    wouldn't vary depending on how future prints might be scanned.
    ASAAR, Apr 13, 2007
    #5
  6. On Apr 13, 10:29 am, Paul Allen <>
    wrote:
    > On Fri, 13 Apr 2007 17:01:46 +0200
    >
    >
    >
    > "cgiorgio" <> wrote:
    >
    > > <> schrieb im Newsbeitrag
    > >news:...
    > > > Hello,

    >
    > > > Am working on my final year project and it involoves mosaicing some
    > > > aerial photos taken in 1990 using an aerial photo camera with focal
    > > > length 153. I scanned these at 800dpi, with dimensions 7242 x 7263
    > > > pixels. The software am using requires a Camera calibration file and
    > > > the focal length is expressed in pixels. Any body out there who can
    > > > help me figure out this using the provided information? or if this
    > > > post not clear or you may require some additional information,
    > > > please let me know.

    >
    > > > Thank you.

    >
    > > Focal length in pixels (F =153 mm), 800 dpi

    >
    > > 153 / 25,4 * 800 = 4819 pixels

    >
    > This looks like an opportunity to learn something.
    >
    > How does it make sense to express focal length in pixels? Is this
    > something peculiar to aerial photography? Or does the concept have
    > wider applicability?
    >
    > Inquiring minds, and all that.
    >
    > Paul Allen


    I admit I have not been involved in aerial photography for a long
    time, but this community, especially the military folks, were very
    conservative, and even specified focal length in INCHES, not even mm
    or cm. When I was involved, no one certainly worried about focal
    length in pixels.

    That being said, I certainly understand the utility of it. One of the
    features of an electronic image is that it has no inherent linear
    distance measurement. The true pixels per inch MUST be referenced to
    the image chip, not any file format or anything like that.
    Don Stauffer in Minnesota, Apr 14, 2007
    #6
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