[pure-silver] Re: what is acutance?
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Mon, 18 May 2009 16:29:15 -0700
----- Original Message -----
From: "Shannon Stoney" <shannonstoney@xxxxxxxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Monday, May 18, 2009 9:05 AM
Subject: [pure-silver] Re: what is acutance?
Acutance is a term invented by Kodak for an effect
that enhances apparent sharpness. The effect is what
happens when the contrast at the demarcation of a bright
and dark area is increased by any of several effects
during development. Acutance is related to what are
called adjacency effects. This happens when reaction
products of development either restrain or accelerate
development in a small area adjancent to the development
because they diffuse sideways in the emulsion faster than
they can be removed by fresh developer diffusing from the
surface. In effect the effect draws lines around objects
in the image. Since the eye tends to interpret high
contrast edges as sharpness the effect will increase the
apparent sharpness of an image even though the resolution
may be fairly low. In fact, adjacency effect tend to
lower resolution somewhat but usually this is not
apparent except by measurement.
I am visualizing this by thinking about the photoshop
unsharp mask filter. It "draws lines" around objects too.
Is this the right way to visualize what accutance is?
--shannon
Its not quite the same as the unsharp mask, perhaps
closer to edge sharpening in photoshop although the visual
effects are similar.
You are probably aware of the use of masks to control
contrast. Essentially, a weak positive image is created
which is printed exactly in register with the negative. If
the positive is linear it simply reduces overall contrast
but the positive can be made so that its not linear, for
instance so that the highlights are much stronger than the
rest of the image. This will have the effect of burning in
the highlights. Masking is also used for color correction
but that is a bit to far removed for this discussion.
Now, if we make a contrast mask such either the mask
itself is a bit blurred or if a sharp mask is set up
slightly out of focus, for instance by spacing it away from
the negative with a sheet of clear material, the effect will
be to reduce the contrast of the mass light and dark areas
but will not affect fine detail, which will remain at nearly
its original contrast. Hense the unsharp mask reduces
overall contrast but increases the effective conrast of fine
details. Thus it enhanses the visiblility of textures, etc.
Acutance does not exactly do this because it does not
affect overall contrast. It increase the contrast of edges
but where there is relatively low contrast fine detail it
either has little effect or can acctually reduce the
sharpness slightly.
When a the density through a sharply defined light to
dark line is plotted it will have little "ears" on it at
both light and dark ends (sometimes only one depending on
the developer). The effect is to increase increase the rate
of slope. The little ears are what appear to be lines when
the effect is carried out to a high degree.
Again, there is no increase in actual detail but rather
the creation of an optical illusion.
--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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