Pretty good article on medium format cameras

Discussion in 'Digital SLR' started by RichA, Aug 23, 2008.

  1. RichA

    RichA Guest

    RichA, Aug 23, 2008
    #1
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  2. RichA

    Guest Guest

    larger *sensor*.
     
    Guest, Aug 23, 2008
    #2
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  3. RichA

    Guest Guest

    there's no ambiguity about what 'sensor' means.
    where did you get the idea that 'sensor' means the front of the lens?
     
    Guest, Aug 24, 2008
    #3
  4. Not true.
    Observe:
    Take a large format lens and camera.
    Enter a completely constant lit room.
    Where the film should be, mount a 4x6mm 10 MPix sensor.
    Choose a exposure time, ISO setting and f-stop.
    Expose your sensor and read it out.

    Now replace the tiny 4x6mm sensor with a 60x90mm 10 MPix
    sensor, and expose it at the same exposure time, ISO setting
    and f-stop.

    Tell me, will the larger sensor have collected more photons?
    Will each pixel of the larger sensor have collected more photons?


    PS: Now remember that f-stops are independent of image circles.

    -Wolfgang
     
    Wolfgang Weisselberg, Aug 27, 2008
    #4
  5. RichA

    Me Guest

    At the appropriate f-stops required to produce an image at a standard
    size with the same DOF, or doesn't composition matter in theoretical
    photography?
     
    Me, Aug 28, 2008
    #5
  6. Oh, you are right, how stupid of me!

    I was just proving that "You will only get more photons if you
    use a physically larger lens" is wrong. Every time someone
    moves a full frame lens from a crop to a full frame body
    exactly that happens. (Yes, FF bodies are better at light
    gathering, all things being equal, see below.)

    But, as requested, let's do practical photography:

    a) Portrait, "normal" (50mm in 35mm equivalent) lens, 1 meter
    distance between camera and object, DOF ~10cm. Target is a
    20x30cm photo viewed at 41cm (the diagonal length)
    b) Landscape, "wide" (28mm in 35mm equivalent) lens, put everything
    from 2.5m onward in DOF. Target is a 2x1.5 meter print for
    the wall, viewed at ~1.56 meters distance (where the field
    of view is the same as the real landscape)

    Sensor used: 4x6mm 60x90mm
    Area: 24mm² 5400mm²
    a) Pixels needed * 2865x1910 i.e. 5.5 MPix *
    f-stop 1.0 16
    resulting DOF[1] 11.69cm 11.12cm

    b) Pixels needed * 4408x2938 i.e. 13MPix *
    f-stop 2.0 32
    resulting DOF[1] 2.17m 2.29m

    24mm² / 1.0 => 24
    5400mm² / 16 => 337.5

    24mm² / 2 => 12
    5400mm² / 32 => 168.75


    337.5 : 24 = 14:1
    168.75 : 12 = 14:1

    As you can see, the 60x90mm sensor enjoys a 14-fold light gathering
    advantage --- that's more than 4 stops --- over the 4x6mm sensor.
    Actually, 14:1 is the difference between 60 and 4mm, between 90
    and 6mm.

    Once again it's proven that quadrupling the sensor size needs
    doubling the f-stop (for the same DOF) and still doubles the
    light gathering capability.


    And practical photography also has a few problems with the
    necessary f/1.0 lenses for these tiny chips.

    -Wolfgang

    [1] Includes diffraction effects.
     
    Wolfgang Weisselberg, Aug 29, 2008
    #6
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