velocity of subject vs ISO speed

Discussion in 'Digital Photography' started by Desert Dweller, Jul 18, 2007.

  1. I've been reading and maybe thinking too much about this but have a
    technical question for the math geeks out there.

    Say your subject is traveling at a certain velocity through your field
    of view at a certain focal length and maximum aperture. And your camera
    is not panning. The subject is so fast you need an extremely high
    shutter speed. Even at this shutter speed you cannot get enough light
    for a proper exposure. And you cannot create artificial light on the
    subject. So you must increase the ISO. This means there is a
    relationship between the ISO film speed and the velocity of the subject.

    Given a certain ISO film speed, focal length, and amount of light, at
    what velocity can your subject travel through your field of view before
    you need to increase the film speed?

    Is there an equation that describes the relationship between velocity of
    your subject across your field of view and the ISO speed capable of
    exposing it quick enough?

    --
    DD
     
    Desert Dweller, Jul 18, 2007
    #1
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  2. Desert Dweller

    Alex Monro Guest

    Desert Dweller wrote:

    > I've been reading and maybe thinking too much about this but have a
    > technical question for the math geeks out there.
    >
    > Say your subject is traveling at a certain velocity through your field
    > of view at a certain focal length and maximum aperture. And your
    > camera is not panning. The subject is so fast you need an extremely
    > high shutter speed. Even at this shutter speed you cannot get enough
    > light for a proper exposure. And you cannot create artificial light on
    > the subject. So you must increase the ISO. This means there is a
    > relationship between the ISO film speed and the velocity of the
    > subject.
    >
    > Given a certain ISO film speed, focal length, and amount of light, at
    > what velocity can your subject travel through your field of view
    > before you need to increase the film speed?
    >
    > Is there an equation that describes the relationship between velocity
    > of your subject across your field of view and the ISO speed capable of
    > exposing it quick enough?
    >

    It's not really a direct relationship between the subject velocity and
    ISO speed, the lens aperture and light available comes into play too.

    I guess if you wanted to derive an equation for eliminating motion blur,
    you'd need to take in the pixel size, subject distance, and focal
    length, so you could calculate the shutter speed so that the subject
    would move less than 1 pixel in the image while the shutter was open.
     
    Alex Monro, Jul 18, 2007
    #2
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  3. Desert Dweller wrote:

    > I've been reading and maybe thinking too much about this but have a
    > technical question for the math geeks out there.
    >
    > Say your subject is traveling at a certain velocity through your field
    > of view at a certain focal length and maximum aperture. And your camera
    > is not panning. The subject is so fast you need an extremely high
    > shutter speed. Even at this shutter speed you cannot get enough light
    > for a proper exposure. And you cannot create artificial light on the
    > subject. So you must increase the ISO. This means there is a
    > relationship between the ISO film speed and the velocity of the subject.
    >
    > Given a certain ISO film speed, focal length, and amount of light, at
    > what velocity can your subject travel through your field of view before
    > you need to increase the film speed?
    >
    > Is there an equation that describes the relationship between velocity of
    > your subject across your field of view and the ISO speed capable of
    > exposing it quick enough?
    >
    > --
    > DD


    Its a simple trig problem. Find out the size of a pixel
    in your camera, and true focal length. Calculate the
    angular size of a pixel. For example, a 6 micron pixel
    and a 300 mm lens )6 microns = 0.006 mm):

    a = arctan (.006/300)

    Now for an object at some distance x traveling a velocity y,
    compute the angular rate: arctan(y/x) (keep units the same,
    e.g. at 20 meters traveling 1 meter/second gives the angular
    rate (e.g. degrees/second).

    For a sharp picture, the exposure time should be less than
    the angular size of a pixel. Example: if the subject angular
    rate were 1 degree/second and the size of a pixel with a given lens
    is 0.001 degree, then you want the exposure time less than
    1/1000 second (ideally about 4 times faster, so 1/3000 second).

    Roger
     
    Roger N. Clark (change username to rnclark), Jul 19, 2007
    #3
  4. Roger N. Clark (change username to rnclark) wrote:

    >> Is there an equation that describes the relationship between
    >> velocity of your subject across your field of view and the ISO
    >> speed capable of exposing it quick enough?


    > Its a simple trig problem.


    Seems to make sense. I guess I "over thought" that one and was trying to
    make it harder than it was.

    --
    DD
     
    Desert Dweller, Jul 21, 2007
    #4
  5. On Jul 18, 2:21 am, Desert Dweller <> wrote:
    > I've been reading and maybe thinking too much about this but have a
    > technical question for the math geeks out there.
    >
    > Say your subject is traveling at a certain velocity through your field
    > of view at a certain focal length and maximum aperture. And your camera
    > is not panning. The subject is so fast you need an extremely high
    > shutter speed. Even at this shutter speed you cannot get enough light
    > for a proper exposure. And you cannot create artificial light on the
    > subject. So you must increase the ISO. This means there is a
    > relationship between the ISO film speed and the velocity of the subject.
    >
    > Given a certain ISO film speed, focal length, and amount of light, at
    > what velocity can your subject travel through your field of view before
    > you need to increase the film speed?
    >
    > Is there an equation that describes the relationship between velocity of
    > your subject across your field of view and the ISO speed capable of
    > exposing it quick enough?
    >
    > --
    > DD


    People have actually determined the mft of subject movement. A Perkin
    Elmer engineer published an excellent paper back in the 70s showing
    the mtf curve vs angular velocity. You have to change linear movement
    velocity into angular velocity.
     
    Don Stauffer in Minnesota, Jul 21, 2007
    #5
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