Why do images appear sharper than they should?

Discussion in '35mm Cameras' started by Robert 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 Feinman, Dec 22, 2003
    #1
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  2. In general I agree with your analysis, but allow me to add a few
    observations (in line comments below).

    Your SE5400 is capable of extracting somewhat finer detail out of a low
    speed film. The combination of a good lens on a tripod mounted (or steady
    handheld) camera with "slow" film, allows, combined with the scanners
    optics and lightsource, to extract something like 70-80 lp/mm. And we're
    talking about measurable/quantifyable resolution (in addition to sharp
    grain).
    The human visual system responds primarily to edge contrast, especially
    horizontal and vertical edges (due to genetics or training combined with
    gravity and horizontal movement).
    The Unsharp Masking mainly affects high frequency detail, unless we
    deliberately choose to enhance lower frequencies in a separate pass.
    Shape recognition by the human visual system is a very fast but crude
    process. However, as you know from large format images, there's a quality
    to a good/life-like image that can be describesd as surface texture. This
    allows us to discriminate between, say, a piece of leather and a piece of
    cardbord. If the color and specular/diffuse reflection characteristics are
    similar, it takes surface structure to discriminate. For this, the human
    eye needs to resolve fine structures like fibers or dimples.

    Bart
     
    Bart van der Wolf, Dec 22, 2003
    #2
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  3. "Sharpness" varies with the viewer, of course, and the viewing
    distance (permitting lower unit resolution in larger prints to look
    OK). As you note, supplying good detail in limited important areas
    can suffice for some people (I'm always amazed by how bad
    image edge/corner resolution can be, and some people call the
    image sharp ;-). In digital, one can selectively sharpen (and
    selectively soften) areas to increase the impression of good image
    resolution and smoothness, if done carefully. For me, distant
    subject info MUST be sharp in a "landscape" or an "architectural"
    image - and the "almost-sharp" of stretched-DOF-covered images
    looks bad. Standards of sharpness can vary considerably by
    use, with direct-viewed images requiring greater resolution (I look
    at these close-up, regardless of size...;-), and with reproduced
    or screen images requiring less resolution to look good (I think
    the sharp screen introduced by the dot pattern in reproductions
    and pixels in monitors contributes to the look of sharpness that
    isn't real [as can noticeable sharp-edged film grain] and/or the
    expectations are lower...;-).
    Interesting area, image resolution perception...
     
    David Ruether, Dec 22, 2003
    #3
  4. Robert Feinman

    Jeremy Guest

    --
    x-no-archive: yes
    I remember seeing Carl Zeiss chart that showed the effect of diffraction at
    various f-stops. Once you go beyond f/8, as I recall, the number of lpm
    drops off significantly. So, stopping down for maximum DOF has a price, in
    terms of reduced sharpness. Unfortunately, this trade-off is not apparent
    just by viewing the scene in depth-of-field-preview.

    On the other side, opening the lens up to max or near maximum aperture often
    results in softening, especially at the edges. So there is a relatively
    narrow range of f-stops that will yield an image that is both sharp and does
    not sacrifice lpm resolution on the film. Maybe 2 stops, if that????

    Decisions, decisions . . .
     
    Jeremy, Dec 22, 2003
    #4
  5. I suspect you are at least mostly correct. Sharpness can't really be
    measured as it is perception. LLP can measure resolution but not sharpness.
    A quick read of your message indicates you have a good grasp of this.

    Please note that this author is not the same Joseph Meehan who is a
    professional author of Photograph materials.
     
    Joseph Meehan, Dec 22, 2003
    #5
  6. Robert Feinman

    Jerry Guest

    Robert,

    I think you're onto something there mate ... I agree with your theory.
    People's perception is not always reality - a very good point.
     
    Jerry, Dec 23, 2003
    #6
  7. Yes, though limiting apertures to f8 minimum is more limiting than necessary,
    since diffraction effects are not very bad (or observable in prints) for 35mm
    until somewhat past f11. While with a good lens of 50-90mm or so, the
    resolution in the center may peak around f4-5.6 (and at the edges maybe a
    stop smaller), the losses due to diffraction do not generally soften the image
    appreciably even at f16 (though I avoid f22-32, except for high-magnification
    macro work). With landscape work, it is necessary for the fine distant detail
    to be sharply-rendered (this is more important than having the larger-scaled
    nearer material technically equally sharp in setting up the focus distance for
    best DOF) - nothing looks worse to me than soft landscape photos (unless
    intended) resulting from use of too small a stop, or from poor choice of
    focus distance (using DOF scales and tables results in this, unless the guides
    for focus distance and aperyture are modified...).
    Top-class non-zoom lenses in the 50-90mm range for 35mm are sharp
    from wide-open to about f16, with the peak near the middle; lesser lenses,
    ones significantly different in FL (especially super-wides), or zooms may
    have much more restricted ranges of stops for good performance.
    Good super-wides may perform really well only at f11-16; many good
    zooms that include the WA range may perform well only at f8-16;
    lesser-quality lenses away from the 35-135mm range may have no
    really good stops at all. While it is fun to use the best lenses at only their
    best stops, often in reasonable-sized prints the visual differences in the
    prints between this stop and one with 1/4 or so less resolution may be
    minimal (and would preclude using the lenses wide-open, which can
    result in wonderful images...). Given the above, though, it is still amazing
    how much better the image will look if shot with a significantly larger
    format camera - the advantages in tonality can show easily, even if the
    unit resolution may not be much different...
     
    David Ruether, Dec 23, 2003
    #7
  8. Robert Feinman

    jjs Guest

    And so what? Of course edge contrast enhances the perceived sharpness.
    That's been known for hundreds of years, and in fact the effect was
    intentionally used and intentionally _avoided_ for specific esthetic
    reasons before we got techno-esthetic and obsessed by things that should
    only concern a recon maven. I hope you aren't suggesting we impose some
    kind of new metric on a photograph to discover whether, regardless of how
    appealing it might be to the eye, it must stand against some rez chart.
     
    jjs, Dec 23, 2003
    #8
  9. But what hasn't been mentioned is how easily some of these advantages get lost
    in publication - even in publications devoted to photography.
    For some time, the British weekly "Amateur Photographer" ran a series of
    landscape shots taken by one of the current masters of the art. For me, most of
    the offerings were simply a waste of space having blocked shadows etc. and being
    little better than my own attempts (35mm & 6MP DSLR) but also a lot more
    contrived, and after a short time I began to be annoyed by their inclusion.
    However, when I saw the originals at an exhibition I was completely bowled over
    by just how good the 30" x 40" and larger prints were, and realised then that
    the phrase "a good big un is better than a good little un" definitely applied to
    them. I think they were mostly shot on 5x4 or may be larger.
     
    Malcolm Stewart, Dec 23, 2003
    #9
  10. Uh, I believe I touched on this in my first post in this thread with,
    "Standards of sharpness can vary considerably by use, with
    direct-viewed images requiring greater resolution (I look at these
    close-up, regardless of size...;-), and with reproduced
    or screen images requiring less resolution to look good (I think
    the sharp screen introduced by the dot pattern in reproductions
    and pixels in monitors contributes to the look of sharpness that
    isn't real [as can noticeable sharp-edged film grain] and/or the
    expectations are lower...;-).
    --
    David Ruether

    http://www.ferrario.com/ruether

     
    David Ruether, Dec 23, 2003
    #10
  11. Um, but a person's perception (at the time) *is* their reality...;-)
     
    David Ruether, Dec 23, 2003
    #11
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