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One of the main physical measure of
sharpness is acutance.
It is however not the same as resolution
as many would have assume.
Acutance is actually
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The measure of the ability of film to
delineate critically between adjacent tones contained on a negative or
transparency.
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The clearer, finer, and more precise this
delineation is, the higher the acutance of the film.
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The actual process of measuring a film's
acutance level involves bringing an opaque knife edge in direct contact
with the film surface being tested and then exposing the film to a
well-collimated light source. The resulting image on film should be an
easily observable edge between dark and clear portions of the film.
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Comparing edge widths of different films
of the same general type produced under identical conditions gives us
some idea of different films' relative ability to distinguish edge
boundaries sharply.
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High acutance does not always go along
with high resolution. In the case of some films, there actually seems
to be an inverse relationship. Some high-resolution films appear to
exhibit substantially less acutance than similar film of lower
resolution.
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New thin-emulsion films (films that are
thinner from front to back) are able to generate higher levels of
acutance than their conventional counterparts.
Logically, one would assume that always
choosing a high-acutance film that also displays high resolution is the
thing to do. This isn't always possible. Film design involves a series
of compromises, and creating a film that is fast but still has all the
desired sharpness qualities may not be within the film-maker's ability.
It is a tribute to the state of the technology that we even come close
to the ideal combination of sharpness, acutance, and speed.
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