When images are captured either via scanning or digital photography, a certain amount of softness is introduced into the digital image. To achieve the most accurate reproduction, the images must be sharpened–usually through the use of one of the many sharpening filters in Photoshop.
The softness within images occurs at two points. First, when continuous tone information (either a real-life scene with a digital camera or a photo original with a scanner) is converted into pixel information, it is done so via sampling data on a regular grid pattern–this is essentially the resolution of the device. Any data that is finer than this grid pattern gets averaged, and it is these averaged pixels that creates softness within the image. Second, image softness is introduced when the image data is converted into halftone dots. Here again, when the pixel information is transformed into a screened halftone for press, data is averaged and even more image softness is introduced.
Unsharp Mask in Photoshop
There are lots of image sharpening tools in Photoshop, and one that I find that offers great control is Unsharp Mask. Unsharp mask introduces sharpness by evaluating and adjusting the contrast difference between pixels along an edge. These pixels along this edge are made lighter (and/or darker) giving the illusion of sharpness. Unsharp Mask offers three controls:
- Radius. The Radius setting affects the size of the edge to be enhanced and determines how far out from an edge contrast in increased. This is typically the most critical setting as high amounts can cause unwanted halos, especially in fleshtones.
- Amount. Amount determines how much darker and how much lighter the edge borders become where the light and dark pixels meet.
- Threshold. Threshold is the difference in tonal values necessary for pixels to be affected by the sharp ening. Low values sharpen more because fewer areas are excluded. Higher threshold values exclude areas of lower contrast.
An image without unsharp mask (top) and with unsharp mask (bottom).
So what’s the best unsharp mask setting to use? While there is no “magic bullet” for all images, the settings in the figure on page 33 are a good starting point with most images. This setting applies a very light application of unsharp mask, and can be reapplied if the image content allows. For example, images that contain fleshtones would benefit from one application of unsharp mask using these values, while images that contain many edges (a city scene, for example) would benefit from two or thee applications of unsharp mask. Some other points to consider when applying unsharp mask:
- Always view images for sharpness in Photoshop at 100%. Viewing images at any other zoom rate will make the image appear sharper that it actually is.
- When applying unsharp mask for print production, the image should appear to be slightly sharper when viewed on screen.
- Always apply unsharp mask after you have resized the image. Downsampling the image may discard the sharpened pixels, and you have to reapply the filter again.