How to calculate the change of angle of view when to crop factor of the DSLR is known when using ful

Discussion in 'Digital Cameras' started by Mulperi, Sep 17, 2006.

  1. Mulperi

    Mulperi Guest

    For example the angle of view is 100 degrees in lens x with full frame
    camera and the crop factor is 1.5 in smaller frame camera . So what would be
    the final angle of view with that smaller frame camera with that lens x.

    --
    Juha Heinonen
    Seilimaki 17 B 8
    02180 ESPOO
    FINLAND
    E-mail:
    URL: http://personal.inet.fi/koti/juha.heinonen
     
    Mulperi, Sep 17, 2006
    #1
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  2. Mulperi

    Ben Brugman Guest

    One can calculate the angle, but it is far more
    easier to calculate the field of view. (And from there calculate the angle).

    All measurements are metric.

    Focal length = f (in mm).
    Distance = D (in meters)
    Sensor Width = w (in mm).

    Field of view = D*w/f

    Example
    D = 100 meters
    f = 45 mm
    w = 24 mm (Aps size sensor).

    Field of view (at 100) meters = 100 meters * 24 mm / 45 mm
    Field of view (at 100) meters = 100 * 24 / 45 * meters * mm / mm
    Field of view (at 100) meters = 2400 / 45 * meters
    Field of view (at 100) meters = 53.33 meters

    35 mm film is about 24mm * 36 mm and is diagonal about 43 mm.
    aps size sensor (1.5 factor) is about 16mm * 24mm and it's diagonal is about
    29 mm

    The angle is Atan(FieldOfView/(2*D))*2
    Or the angle is Atan(w/(2*f))*2

    You can use mm for the focal length and the sensor size and any
    other length for the Distance and Field of View. (For example yards or feet,
    as
    long as Distance and Field of View use the same).
    Field of view (at 100) feet = 100 feet * 24 mm / 45 mm

    The angle does not behave lineair, Field of view, distance, sensor size etc.
    do behave lineair.
    *) At very short distances be the field of view should not be used. (think
    macro).


    Good luck,
    Ben Brugman
     
    Ben Brugman, Sep 17, 2006
    #2
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  3. The tangent trig function converts angles into relative distances
    perpendicular to the optical axis (i.e. in either the subject or image
    plane). But you have to work with the angle from the axis to the edge,
    which is half the angle from edge to edge.

    For your specific example:

    1. the half-angle of 100 degrees is 50 degrees

    2. relative size on the full-frame sensor (really the ratio of half
    sensor width to lens focal length is tan(50 deg) = 1.192.

    3. portion of the image that falls on the smaller sensor:
    1.192 / 1.5 = 0.795

    4. half-angle at the edge of the smaller sensor: arctan(0.795) = 38.5 deg

    5. full angle: 2 * 38.5 = 77 degrees

    For the general case:

    small_sensor_FOV = 2 * arctan(tan(FF_FOV / 2) / crop_factor)

    Dave
     
    Dave Martindale, Sep 18, 2006
    #3
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