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Perfect Sharpness Every Time - 2/9/09

Subtle sharpening via Photoshop’s LAB lightness channel

This Article Features Photo Zoom

Issues of sharpness—from too much to not enough—are always on photographers’ minds. While there are a million different methods for sharpening, and a million and one reasons to do it, there’s one rule that always holds true: understanding sharpening is important.

The issue of sharpening is nuanced enough to fill its own magazine, but it’s commonly agreed that sharpening should be the last step of the workflow, carried out by a photographer in preparation for final output. The amount of sharpening is often dictated by how an image will be used: an online usage requires a different type of sharpness than a magazine-printing, as does a gallery-displayed print. The point is, when you’ve finished all your other edits, don’t forget to sharpen.

There may be a million methods for sharpening, but there’s one simple approach that provides a powerful result—making it just right for inclusion as a Tip of the Week. It’s based on working in Photoshop’s LAB Mode color space to selectively sharpen an image subtly, but effectively.

LAB mode stands for Lightness, A and B, and it represents the three channels that make up an image in this color space. It was designed to approximate human vision, rather than to emulate mechanical output as with RGB and CMYK spaces. The how and why may not be easy to understand, but the effects are: working in LAB helps to effectively sharpen an image without amplifying the artifacts that can occur with normal sharpening. It essentially turns a normally image-degrading process into a lossless one.

To sharpen an image with the Lightness channel, first convert to LAB mode. Then deselect the A and B channels by clicking off the eyes next to them in the channels palette. Select and highlight the Lightness channel in the palette so your next modifications will occur only to the lightness channel. In the Filters menu, choose Sharpen>Unsharp Mask. When the dialog box opens, input an amount between 75% and 125%, usually, depending on the amount of sharpening you want. It helps if you’re looking at your image greatly enlarged—200% or more. This way you can see exactly how the subtle sharpness changes actually affect the pixels.

The default values for Radius and Threshold are 0 and 1, respectively. Leave these alone, as they’re probably perfect as-is. When you’re happy with the sharpness, click OK. Zoom out on the image and use the history palette to examine the before and after appearance of the photo. Assuming you’re happy, convert the mode back to RGB and your sharpened final image is ready to go.

By sharpening only the Lightness channel in LAB mode, you’ve avoided exaggerating the color fringing and artifacts that can (and do) occur when sharpening the whole image in RGB or CMYK. Isolating the Lightness channel avoids sharpening the colors of an image, meaning only the black and white brightness and shadow values are affected—eliminating the possibility of corrupting any of the color data, and avoiding the possibility of an unattractive, over-sharpened image.

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