2.3.10 Sharpening Using Unsharp Masking (USM)

This last step will allow the masonry of the bridge in particular to make a sharper impression. The right tool for this job is USM (Unsharp Masking). The slightly confusing name of this tool originates from analog photo processing technology. There, a negative was masked with an unsharp positive to increase the contrast on the edges in the picture. The result was an increased brightness difference on edges, which increased the impression of sharpness.

The USM tool in SilverFast works in a similar way. Here, too, the brightness difference is amplified along edges. A good approach is first to pull the slider for Power excessively far to the right. That gives you a good overview of where and to what extent the unsharp masking will show effect in the image. Next in line is the slider for the radius. This is used to adjust how far the brightening or darkening of automatically detected edges will be carried out. If you set too high a value here, bright or dark "seams" will be visible on the edges. Once a good value has been found for the radius, the Power can be reduced to the point that an attractive result is obtained. If the sharpening is supposed to be a little subtler, the slider for the threshold value can be used. For example, if this is set to a value of 30, the unsharp masking will only be used on edges whose brightness has a difference of at least 30%. This allows you to ensure that the sharpening will work on masonry or other objects that are well delineated, but that parts of the motif with less pronounced brightness differences will be ignored.

If an image is being prepared for offset printing, more sharpening is necessary than if the planned output device is to be a monitor or projector. The type of color application in the offset process makes sharp edges a little softer again. So on the monitor, a picture to be printed can easily seem a little excessively sharpened, but the printed result should end up being exactly right.

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