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 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 |
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.