Schematic of Film Curling
in a) Scanner Holders
b) After market holders with
depressing bars
c) ScanScience wet mount
a


b


c)
FILM:  WHY IT CURLS
Film:
Naturally Unflat
    Left to its own devices film is seldom flat.  Even when properly stored in glassine
    envelops , -and between flat objects like books, it is not flat. It assumes a curvature due
    to its inherently unbalanced construction.
Plywood  
Construction
    Plywood consists for the most part of several layers, optimally an uneven number of
    layers. In consequence, any tendency of one layer to curl in a given way is balanced by
    another's tendency to do likewise but in opposite direction. Plywood never consists of
    only two layers for that reason. It has a tendency to warp because there are not enough
    layers to balance the forces that cause to curl.
Film Components
  • The emulsion which contains the light sensitive substances in a gelatin base.
  • The backing which is now polyester, similar to Mylar.
Emulsion Gelatin
Base
    The gelatin in the emulsion is water sensitive, it traps or releases water according to the
    surrounding environment. If it is humid, it traps water and if it is dry, it gives up water.  
    According to the environment it swells if it is humid and shrinks if it is dry.
Film Backing
    The backing is NOT wanter sensitive nor does it change in response to changes in
    humidity
Tug of War
    Evidently, the backing & the emulsion, do not have identical physical properties and do
    not respond identically to humidity.  
  • The backing is immune to changes in humidity but the emulsion readily absorbs it.
  • Since the swollen emulsion is bonded to the unyielding backing, a tug of war
    ensues:
  • In high humidity, the emulsion has the tendency to swell or grow in size but it
    cannot stretch the backing so it takes a detour: It adopts a position as the outer
    layer of a curve, in which the backing is the larger of the two. Now the backing is
    consigned to being the inner element in the curl.
  • In low humidity, prevalent during the heating season,  the reverse occurs: The
    emulsion shrinks but the backing does not. Now the emulsion becomes the inner
    element in the curve due to its smaller size.
Aged Film
    With time the gelatin hardens, -oxidizes and will not respond as readily to changing
    environmental conditions.  Straightening old film can then be more difficult and the
    surface tension of the scanning fluid may not be strong enough to keep the film flat. In
    such cases it may be necessary to fluid mount in between two glass plates tightly taped
    at the edges. In an imperfect world, the additional refraction from  the extra glass may be
    less objectionable that an out of focus scan or Newton Rings.
    Scanner optics are close-focusing systems. Those of you experienced in Macro Photography know that the plane
    of optimum focus is extremely shallow, and becomes shallower at higher magnifications. For this reason, a deviation
    in film flatness of a fraction of a mm can have result in un-sharp scans. Film un-flatness is the most common
    problem encountered in dry scanning with film and flatbed scanners.

  • If the film is flat, the scanner has a uniform target to focus on. In Fixed focus scanners the focus of a scanner is
    preset. If the film curls, portions of the film will be higher or lower than the scanner's plane of optimum focus and
    only those portions that are at the right elevation will be sharp and in focus.
  • With autofocus scanners, like the Nikon 9000/8000, film curl will result on the scanner optimizing focus at some
    point of the film frame, with other areas being out of focus.
  • A number of not- so -successful techniques have evolved in dealing with this problem, such as glass  holders, and
    after market holders that come with spacers that keep the film down at intervals. The problems with the latter are
    illustrated in the image to the right.
  • Any one that has studied physics knows that the lateral tension required to make a string suspended between two
    points adopt a perfectly straight shape, is infinite and likewise, film flatness can not be totally attained by tensioning
    the film.  
  • Overcoming the film's natural tendency to curl requires applying a perpendicular force to the film through the
    whole area, not just at the margins and lateral edges. Lateral tensioning devices or the interval spacers provided
    with after market holders can't not apply downward pressure over the center of the film. The surface tension of the
    fluid, however provides that perpendicular force uniformly.
  • Film flatness is not a problem at all with drum scanners, which all use fluid scanning where film flatness is a given.
  • The ScanScience Manual supplied with the kits provides additional information and techniques for dealing with film
    curling but the following gives a basic view of the problem. Film curling can lead to Newton Rings when the film
    comes near the glass. For more on Newton rings view the page in this section that deals with this problem.
FLATNESS IN DRY AND FLUID SCANNING
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