How is the sensor size measured?

Discussion in 'Digital Photography' started by angus, Nov 30, 2003.

  1. angus

    angus Guest

    I've searched the archives and still can't figure out what an
    expression like 1/2.3" means when applied to sensor size.

    For example:
    A 1/2.3" sensor has actual dimensions of 4.54mm x 3.42mm according to
    one of the digital camera websites.

    1 divided by 2.3 equals 0.313", which I had originally presumed to be
    the diagonal dimension of the sensor.

    But 0.313" is equal to 7.94 mm while the true diagonal is only 5.68
    mm.

    So what does 1/2.3" actually refer to?
    Anybody have an answer?

    Thanks,
    Angus Cameron
     
    angus, Nov 30, 2003
    #1
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  2. The 1/2.3" is actually an old measurement used by the television industry to
    measure the outer glass diameter of vidicon tubes. The actual active
    surface is proportionally smaller.

    CCDs were first used to replace vidicon tubes in TV video cameras. Even
    though there's no glass surrounding a CCD, the measurement continued, partly
    to make sure video cameramen could continue to use the same calculation to
    determine perspective based on lens focal length.

    When CCDs began to be used for digital still cameras, the metric stuck.
    This, by the way, is also why most P&S digital cameras have an aspect ratio
    of 4:3 (TV aspect ratio) rather than 3:2, which is normal for 35mm.

    James
     
    James Akiyama, Nov 30, 2003
    #2
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  3. angus

    jam Guest

    A reasonably accurate rule of thumb that works with =most= popular CCD
    and CMOS sensors is to multiply the sensor size "fraction" by 16 mm to
    estimate the active diagonal. Thus 16 mm * 1/1.8 = 8.89 mm.
    Unfortunately, it's off by +22% with your CCD.

    The odd size designations have an arcane history going back to vidicon
    tube days. Someone else will have to fill in the details.
    --
    Jeremy McCreary
    Denver, CO
    www.cliffshade.com/dpfwiw/
    -------------------------------------------

    | I've searched the archives and still can't figure out what an
    | expression like 1/2.3" means when applied to sensor size.
    |
    | For example:
    | A 1/2.3" sensor has actual dimensions of 4.54mm x 3.42mm according
    to
    | one of the digital camera websites.
    |
    | 1 divided by 2.3 equals 0.313", which I had originally presumed to
    be
    | the diagonal dimension of the sensor.
    |
    | But 0.313" is equal to 7.94 mm while the true diagonal is only 5.68
    | mm.
    |
    | So what does 1/2.3" actually refer to?
    | Anybody have an answer?
    |
    | Thanks,
    | Angus Cameron
     
    jam, Nov 30, 2003
    #3
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