iso numbers and image sharpness - theory

Discussion in 'Photography' started by Ruben, Dec 8, 2012.

  1. Ruben

    Ruben Guest

    Hello

    I'm very confused by the relationship to ISO numbers, sensitivity and
    sharpness of image.

    In film, high ISO film is more sensitive to light, but has few grains of
    light senstive salts, causing grain.

    I would think it would be the oposite. Film with a greater number of
    grains (and smaller) would be more senstive since there is more of them.

    If you have denser rod capacity in an eye, such as in an eagle, owl or
    cat, you have better sensivity to light and greater sharpness.

    Ruben
     
    Ruben, Dec 8, 2012
    #1
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  2. Maybe a fat grain will catch a photon of light? Smaller grains will miss
    available light so come out black in development.

    Eyes are a complicated subject (and some pre-processing is done in the eye
    and optic nerve before the signal even hits the brain). Resolution and
    sensitivity are really different things. The pattern of rods and cones in
    the human eye are just one thing that may be different in an animal plus the
    human eye has smaller pupils and the reflective layer that captures more
    light.

    Actually, I really hate explaining this stuff when there's a search engine
    out there and people have written basic introductions.

    http://www.inspirationline.com/Brainteaser/animaleyes.htm
     
    Charles E. Hardwidge, Dec 8, 2012
    #2
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  3. Ruben

    Alan Browne Guest

    In film, when ISO goes up, grain particle size goes up as well
    contributing to the 'grainy' look - and loss of sharpness. In film,
    there is both dimensional (x,y on the film/paper) grain and
    dynamic (z: colour and/or tone) noise as ISO goes up.

    In digital, the pixels of a given sensor are a fixed size so the
    dimensions of the grain can't go up with ISO so there is no dimensional
    loss of sharpness with increasing ISO. Of course the noise does cause
    colored speckling to increase (z), so wherever there is a contrasting
    edge it is broken up by that noise and there is overall loss of sharpness.
    I'd avoid such comparisons for various reasons.

    Density goes for sharpness but against sensitivity (rods: less acuity,
    more sensitivity; cones more acuity, less sensitivity).

    The size of these animals eyes, aperture/retina ratios, rod/cone
    ratios/distributions/arrangement, fixed sensitivity (different value
    than humans), accuity v. other "vision" aspects, make such comparisons
    interesting but not directly comparable.

    As an example, I've read that vultures can't 'see' carrion a few km
    away, but they do detect the flashing glints of flies buzzing around the
    carrion and that draws them towards the carrion. Animal vision is much
    more than the spec of the eye.
     
    Alan Browne, Dec 8, 2012
    #3
  4. Ruben

    Robert Coe Guest

    : Hello
    :
    : I'm very confused by the relationship to ISO numbers, sensitivity and
    : sharpness of image.
    :
    : In film, high ISO film is more sensitive to light, but has few grains of
    : light senstive salts, causing grain.
    :
    : I would think it would be the oposite. Film with a greater number of
    : grains (and smaller) would be more senstive since there is more of them.

    But because the grains are smaller, each sees less light in the time the
    shutter is open. Light is not an infinite quantity. A single photon, which is
    very small, reacts with a single grain of emulsion on the film, darkening it
    slightly more. A large grain catches more photons, and therefore darkens more
    quickly, but covers a larger area of the film; and this means less resolution.

    : If you have denser rod capacity in an eye, such as in an eagle, owl or
    : cat, you have better sensivity to light and greater sharpness.

    Denser rods mean greater sharpness, but not better sensitivity; there has to
    be another biological explanation for that. Humans, for example, have both
    rods and cones in their eyes. One, probably the rods (I'm too lazy to look it
    up), provide acuity for daytime vision, and the others provide sensitivity for
    night vision. A similar mechanism must be at work in the eyes of owls and
    cats.

    Bob
     
    Robert Coe, Dec 8, 2012
    #4
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