Why do pictures appear sharper than they should?

Discussion in 'Digital Photography' started by Robert D Feinman, Dec 22, 2003.

  1. There is a never ending discussion of resolution vs
    print size and capture media.

    The mathematics and empirical testing usually
    show that the usual expected resolution is in the range of
    40-60 lines per mm. A good print should have 6 to 8 lpm.
    So simple logic means that the maximum enlargement should
    be 5 to 8x. Thus the best size print one could expect from
    35mm would be on the order of 8x12 inches with correspondingly
    smaller sizes from digital cameras.
    In spite of this people still get prints that are "sharp" with
    much larger magnifications.
    Personally, I've been scanning 35mm color negative film with the
    new Minolta 5400 lately and can print out inkjets that look "sharp"
    all the way up to the 18x maximum scanning resolution.
    I'm not one of those who doesn't know what a "sharp" print looks like
    either, since I use formats all the way up to 4x5.

    So what's going on?
    My conjecture (a theory in progress):

    For people pictures shot at normal distances we are used to seeing
    detail only in limited areas of the face such as the eyes (lashes
    and reflections in the pupil) and perhaps loose strands of hair.
    For landscapes and the like, we can't see all that much detail in
    distant leaves and grass, but we do see bare branches, telephone wires
    and the like as sharp.
    For buildings and other man made structures the detail is seen in the
    building edges and things like window frames.

    In all cases the "sharp" things are not those with a lot of fine detail,
    but rather those with good edge contrast. In other words acutance.
    Most digital processing involves a certain degree of sharpening. This
    doesn't do much for real detail, but does increase acutance. This makes
    those features that we search for in real life appear "sharper" so we
    read the image as being sharp. We don't expect to see much fine detail
    so we are not surprised when it is lacking as long as those sharpness
    indicators have good edge definition.
    There are categories of images where the detail is important such
    as scanning electron microscope images and we always comment on
    how much detail we see in them when viewed. This shows that we don't
    normally expect to see the fine structures in an image.
    So I'm guessing that since the images conform to our expectations from
    viewing such scenes in real life we accept them as sharp even though
    the resolution figures would indicate that they are not really that
    detailed.

    As I said, a theory in progress, comments welcome..


    --
    Robert D Feinman
    Landscapes, Cityscapes and Panoramic Photographs
    http://robertdfeinman.com
    mail:
     
    Robert D Feinman, Dec 22, 2003
    #1
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  2. > In other words acutance.

    >
    > Most digital processing involves a certain degree of sharpening. This
    > doesn't do much for real detail, but does increase


    > acutance.


    ....any idea what OP means by this word? Acuteness, maybe?
     
    Robert A. Barr, Dec 22, 2003
    #2
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  3. Robert D Feinman

    Joe Guest

    In article <>, Not.for.@harvest
    says...
    > > In other words acutance.

    >
    > >
    > > Most digital processing involves a certain degree of sharpening. This
    > > doesn't do much for real detail, but does increase

    >
    > > acutance.

    >
    > ...any idea what OP means by this word? Acuteness, maybe?
    >
    >

    Nope.

    Acutance = In photography, the density gradient across an edge
    separating light from darkness, a physically measurable quantity that
    correlates well with subjectively observed sharpness of definition. By
    extension, in machine vision, a measure of the sharpness of edges in an
    image, as the average squared rate of change of the density across the
    edge divided by the total density difference from one side of the edge
    to the other.


    Now you have a computer, buy a good dictionary like OED or learn to
    Google :) I know what I would have bought first.
     
    Joe, Dec 22, 2003
    #3
  4. Robert D Feinman

    bmoag Guest

    Digital photography thrives for the same reason that MP3 compression
    survives in audio: neither is as good as the real thing but convenience and
    illusion make up for the actual technical limitations of the media.
     
    bmoag, Dec 23, 2003
    #4
  5. Robert D Feinman

    gsum Guest

    I'm mystified by this. I don't find that a scanned
    35mm negative can be printed to a greater size than
    a wet printed print as grain becomes dominant.
    The limit for ISO 100 film is about 8x10 inches.
    Obviously, there is much greater information in a
    6mp digital image than a 35mm scan (unless very
    slow film is used) and the limit is about 12x18 inches
    for 'photo' quality.

    Graham

    "Robert D Feinman" <> wrote in message
    news:...
    > There is a never ending discussion of resolution vs
    > print size and capture media.
    >
    > The mathematics and empirical testing usually
    > show that the usual expected resolution is in the range of
    > 40-60 lines per mm. A good print should have 6 to 8 lpm.
    > So simple logic means that the maximum enlargement should
    > be 5 to 8x. Thus the best size print one could expect from
    > 35mm would be on the order of 8x12 inches with correspondingly
    > smaller sizes from digital cameras.
    > In spite of this people still get prints that are "sharp" with
    > much larger magnifications.
    > Personally, I've been scanning 35mm color negative film with the
    > new Minolta 5400 lately and can print out inkjets that look "sharp"
    > all the way up to the 18x maximum scanning resolution.
    > I'm not one of those who doesn't know what a "sharp" print looks like
    > either, since I use formats all the way up to 4x5.
    >
    > So what's going on?
    > My conjecture (a theory in progress):
    >
    > For people pictures shot at normal distances we are used to seeing
    > detail only in limited areas of the face such as the eyes (lashes
    > and reflections in the pupil) and perhaps loose strands of hair.
    > For landscapes and the like, we can't see all that much detail in
    > distant leaves and grass, but we do see bare branches, telephone wires
    > and the like as sharp.
    > For buildings and other man made structures the detail is seen in the
    > building edges and things like window frames.
    >
    > In all cases the "sharp" things are not those with a lot of fine detail,
    > but rather those with good edge contrast. In other words acutance.
    > Most digital processing involves a certain degree of sharpening. This
    > doesn't do much for real detail, but does increase acutance. This makes
    > those features that we search for in real life appear "sharper" so we
    > read the image as being sharp. We don't expect to see much fine detail
    > so we are not surprised when it is lacking as long as those sharpness
    > indicators have good edge definition.
    > There are categories of images where the detail is important such
    > as scanning electron microscope images and we always comment on
    > how much detail we see in them when viewed. This shows that we don't
    > normally expect to see the fine structures in an image.
    > So I'm guessing that since the images conform to our expectations from
    > viewing such scenes in real life we accept them as sharp even though
    > the resolution figures would indicate that they are not really that
    > detailed.
    >
    > As I said, a theory in progress, comments welcome..
    >
    >
    > --
    > Robert D Feinman
    > Landscapes, Cityscapes and Panoramic Photographs
    > http://robertdfeinman.com
    > mail:
     
    gsum, Dec 23, 2003
    #5
  6. Robert D Feinman

    Jeff Guest

    "Robert A. Barr" <Not.for.@harvest> wrote in news:3FE7598C.856BF5A6
    @worldnet.att.net:

    >> In other words acutance.

    >
    >>
    >> Most digital processing involves a certain degree of sharpening. This
    >> doesn't do much for real detail, but does increase

    >
    >> acutance.

    >
    > ...any idea what OP means by this word? Acuteness, maybe?
    >


    Acutance
    Definition:
    In photography, the density gradient across an edge separating light
    from darkness, a physically measurable quantity that correlates well
    with subjectively observed sharpness of definition. By extension, in
    machine vision, a measure of the sharpness of edges in an image, as the
    average squared rate of change of the density across the edge divided by
    the total density difference from one side of the edge to the other.

    more detailed explanation at:
    http://www.pvinc.com/tutorial/tutorial-quality-acutance.htm
     
    Jeff, Dec 23, 2003
    #6
  7. Robert D Feinman

    Skee Guest

    On Mon, 22 Dec 2003 15:43:38 -0500, Robert D Feinman
    <> wrote:

    >There is a never ending discussion of resolution vs
    >print size and capture media.
    >
    >The mathematics and empirical testing usually
    >show that the usual expected resolution is in the range of
    >40-60 lines per mm. A good print should have 6 to 8 lpm.
    >So simple logic means that the maximum enlargement should
    >be 5 to 8x. Thus the best size print one could expect from
    >35mm would be on the order of 8x12 inches with correspondingly
    >smaller sizes from digital cameras.
    >In spite of this people still get prints that are "sharp" with
    >much larger magnifications.
    >Personally, I've been scanning 35mm color negative film with the
    >new Minolta 5400 lately and can print out inkjets that look "sharp"
    >all the way up to the 18x maximum scanning resolution.
    >I'm not one of those who doesn't know what a "sharp" print looks like
    >either, since I use formats all the way up to 4x5.
    >
    >So what's going on?
    >My conjecture (a theory in progress):
    >

    For many examples of pictures that appear "sharper than they should,"
    you need only look at some of the photos appearing in Newsweek and
    Time these days--this oversharpening fad seems to have become suddenly
    de rigeur. This too shall pass...hopefully.
     
    Skee, Dec 23, 2003
    #7
  8. Robert D Feinman

    Wdflannery Guest

    I think you are correct ... I noticed something similar when I had taken some
    pics of a redhead ... red skin, pale eyes, ...etc. ... that looked blurrier
    than I had anticipated ....... sharpening had the effect of increasing contrast
    in the face and all of a sudden the pic seemed acceptably sharp.
     
    Wdflannery, Dec 23, 2003
    #8
  9. Joe wrote:

    > In article <>, Not.for.@harvest
    > says...
    > > > In other words acutance.

    > >
    > > >
    > > > Most digital processing involves a certain degree of sharpening. This
    > > > doesn't do much for real detail, but does increase

    > >
    > > > acutance.

    > >
    > > ...any idea what OP means by this word? Acuteness, maybe?
    > >
    > >

    > Nope.
    >
    > Acutance = In photography, the density gradient across an edge
    > separating light from darkness, a physically measurable quantity that
    > correlates well with subjectively observed sharpness of definition. By
    > extension, in machine vision, a measure of the sharpness of edges in an
    > image, as the average squared rate of change of the density across the
    > edge divided by the total density difference from one side of the edge
    > to the other.
    >
    > Now you have a computer, buy a good dictionary like OED or learn to
    > Google :) I know what I would have bought first.
    >


    I tried a few resources -- not Google, though -- with no luck. I wasn't
    being a smartass, just curious. I even fed it to Microsoft's Bookshelf 2000,
    which is pretty thorough. Not perfect, but thorough.
     
    Robert A. Barr, Dec 23, 2003
    #9
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