Unsharp Masking
What is unsharp masking?
Unsharp Masking (USM) is the process whereby the edge contrast
is increased in a digital image. The name comes from a technique
used in printing and photographic processes where a soft or
out of focus "unsharp" negative copy of an image is
sandwiched with the original positive image with the effect
of increasing edge contrast.
Why use unsharp masking?
Every digital file, whether created via a scanner or a digital
camera, is produced in three primary colours: Red, Green and
Blue (RGB). In commercial printing these three colours are converted
into Cyan, Magenta, Yellow and Black (CMYK) colour channels.
Due to various factors one or more of those colour channels
will be less sharp than the others or the entire scan may be
less than optimal due to mechanical and optical factors during
the creation of the digital file. Every single digital image
ever produced will always benefit from the application of usharp
masking.
How does unsharp masking work?
Unsharp masking increases the edge contrast by finding the
areas in the file where changes in colour and density occur
and then creating an increase in contrast in the transition
areas. This has the effect of appearing to increase the sharpness
of the image.
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| Simulated raw scan,
no sharpening |
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Unsharp masking applied |
In the past why did I not have to apply unsharp masking in
traditional print jobs?
In the past all print jobs were sent to a printer operator
to produce scans from transparencies provided by the client
to suit their layout. When the printer operator produced the
scans they were made to fit the exact layout specifications
provided by the client. No resizing was necessary so unsharp
masking was applied during the creation of the digital file
or scan. In fact at the same time the file was also converted
from RGB to CMYK, the colour channels then relating to the
four printing inks used by commercial printing presses.
So the scanner produced a digital file "on the fly"
which was converted to CMYK with unsharp masking applied.
Why do the Marine Themes scans need unsharp masking applied?
As noted above, when a scan is produced to the exact dimensions
of your layout we can apply unsharp masking for you. However
in almost every case, scans are supplied larger than your
requirements to give you maximum flexibility as to how the
image is used in your project. In fact in most cases it is
only after the finished scan is received that accurate layout
and final design is achieved.
When using scans supplied by Marine Themes you can crop,
downsize, add text, add other images, invert, distort, transform,
make colour and density adjustments and many other changes
without any technical restrictions. If the scan has unsharp
masking applied before you make those changes you will have
to re apply unsharp masking as the adjustments reduce the
unsharp masking effect. However when unsharp masking is re
applied to a pre-sharpened scan you lose detail, increase
grain or "noise" and end up with a digital file
of lesser quality.
Another factor to consider is that any file with unsharp
masking already applied which is then saved in jpg format
for web delivery will to some extent have the sharpening effect
altered with the same degradation occurring when unsharp masking
is re-applied.
The simple solution is to avoid all sharpening until the
digital file is ready to print and then apply unsharp masking
as final step. Either you or, if you have no experience with
sharpening, your printer operator can perform that step.
With digital cameras rapidly overtaking film based cameras
we will soon be in the situation where no image will be scanned
as it is already in digital form. Conversion to CMYK and the
application of unsharp masking will then take place using
Photoshop or similar program on a computer and not within
a scanners software.
How do I apply unsharp masking?
In Photoshop the unsharp masking filter is found under the
Filters menu at the top of your Photoshop screen. With an
image open, select the unsharp masking filter and apply the
filter effect to your image.
How much unsharp masking should I apply?
There is no absolute answer to this question because the
amount applied will depend on how the scan is to be used (press,
internet, display print, etc) and the file size. Small low
resolution scans, such as those used in a web site, need less
sharpening than a large file used on a billboard or mural.
If you need help applying unsharp masking you can either instruct
your printer operator do it for you or we can advise you as
to which settings to use based on your finished layout.
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