Understanding USM (unsharp mask)

There are many factors that determine the quality of the final print, it's where the rubber hits the road and what can seem like a great image on a monitor can soon appear disappointing in print form. For many photographers the print is the last step, the grand reveal. Of those major criteria for a great print is the appearance of sharpness and a major consideration in the making of a digital photographic print is the sharpening stage or moreover the process by which to achieve it. Unsharp Mask (USM) filter in Adobe's Photoshop is an edge contrast  tool used to create the impression of sharpness in the image. The filter compares tonal changes in the image, then exaggerates the differences between them. This can lead to the appearance of a sharper, or in some cases where necessary a softer image.

Understanding the USM features

I'm often asked to explain the features of the USM tool and how each relates to the sharpening process,so here's an attempt to explain simply what they are and what they do.

Amount:how much of an increase in edge contrast should be applied.

Radius: how far in pixels on either side of a contrast change the sharpness will be applied.

Threshold: how much of a contrast change must be encountered before the filter is applied. 

An often overlooked aspect is that you do not have to apply USM to all three channels, many digital cameras suffer from noise in the blue channel of the color file, which can be exaggerated in sharpening. In Photoshop you can select one channel and sharpen it as required. You can choose to  sharpen selected areas of an image too.

There are no magic numbers for an “ideal” sharpening workflow. Factors such as the sharpness of the capture whether its via camera or a scanner, the resolution of the file, film grain, or digital noise; the paper you print on,i.e., matte fiber or glossy and the overall look you desire. Generally I start with a pixel radius of 0.3 pixels, then increase the amount until it looks correct to me.

When working on files from my medium format digital back which are very high resolution, I set the amount to 300, threshold to zero and radius to .3, If I were to do this with a Nikon or Canon DSLR file it would likely introduce really terrible halos. When scanning film try going up to 4 or 5 on the Threshold to avoid sharpening the film grain. There is no set rule; you need to experiment and discover the results that please you. Remember, over-sharpening  creates ugly halos around hard tonal edges.

When viewing images on screen for sharpening I set the sharpening preview window to 200% and then I view the final image on screen at 50% to make a final assessment before committing ink to paper. Again, these are personal preferences but try them and see if they work for you. Here's a little tip, after you sharpen your image use the Fade filter (under the Edit menu) through the Luminosity mode - Edit>Fade USM>Luminosity and back the slider off to about 70% to reduce possibility of emphasizing  color artifacts caused by the Bayer-pattern sensors.

Whether you use Photoshop or some other sharpening software, most sharpeners are essentially creating light and dark outlines around contrast changes, the trick is to create those outlines without being visible. If a dark or light outline becomes visible then the logical fix is to back off on the amount or radius.

An alternative sharpening technique within Photoshop:

Another variation on edge detection is to use the High Pass Overlay technique.

1. duplicate the image

2. Set Blending Mode to Overlay

3. run Filters/Other/High Pass on the just copied Layer. Start with a value of 2

4.vary the opacity of the Layer for effect you desire

5.erase parts of the new Layer where the effect is over done such as along high contrast edges, i.e., where sky meets mountains or perhaps erase the sharpening effect on the sky completely.

A final comment: focus your camera carefully to begin with, use a fast enough shutter speed to prevent camera shake. Digital"sharpening" is about creating an optical illusion, not about sharply focused photographs. The "Perfect Print" begins in the camera.