In a previous column, I advanced the notion of using a two-pass approach to sharpening. Since then, I've taken a much longer, harder look at the whole issue of when and how to sharpen images. Disclaimer: Much of this article stems from research I undertook with the goal of producing a commercial product (Pixel Genius's PhotoKit Sharpener). But this piece isn't about that product. Rather, it's about a sharpening philosophy of which the product is but one implementation.

Sharpening is one of the most-overlooked yet most-essential components of the digital-image-reproduction workflow. I'm often asked when to sharpen, and what settings to use: The truth is that neither of these questions has a single, let alone a simple, answer.

Why Sharpen?
We need to sharpen for several reasons, each of which imposes its own demands. But more often than not those demands contradict each other.

Whenever we turn photons into pixels, we lose some sharpness, because no matter how high the resolution of our capture devices, they sample a fixed grid of pixels, turning the continuous gradations of tone and color that exist in the real world into discrete pixels.

Sharpening can be a creative tool. Sometimes we want to make the image sharper than it really was, to tell a story, make a point, or emphasize an area of interest.

When we turn pixels into halftone dots or stochastic dithers of inks, or even into continuous-tone dots on dye-sublimation printers or film recorders, we lose sharpness again. So our output devices also introduce some softness.

We need to apply sharpening to counteract the softness introduced by both capture and output, and to satisfy our creative goals.

The Sharpening Problem
The problem inherent in trying to achieve all sharpening goals in a single pass is that we need to sharpen images for at least three basic reasons, and they often present conflicting demands. Why is this?

The capture process, whether scanned film or direct digital capture, imposes its own noise "signature" on the image. We want to sharpen the detail without exaggerating the noise, so successful sharpening needs to take into account the relationship between image detail and the noise signature of the image source, whether film grain or a digital camera filter mosaic. Incorrect sharpening makes the grain or noise more rather than less obvious, and sometimes even obscures the detail instead of enhancing it.

Good sharpening is content-sensitive. Apparent sharpness depends on the contrast along what we see as edges. A close subject with soft detail such as a headshot has wider edges than a high-frequency image such as a forest full of trees. If we use the same sharpening for both, the results will be less than optimal on at least one of the images. Incorrect sharpening either obscures small details, or oversharpens textured areas such as skin tones.

The output process introduces softness too, and we need to sharpen to counteract it. But the sharpening required for a traditional ink-on-paper halftone may be fairly different from that required for an inkjet using error diffusion rather than a conventional halftone, and different again for a true continuous-tone printer such as a Durst Lambda or Fuji Pictrography. Incorrect sharpening either produces insufficiently sharp results, or makes obvious, objectionable sharpening haloes along high-contrast edges.

The Case for Three
I've never been happy with the results when I try to address all these problems in a single round of sharpening. Over the years, I've flirted with a two-pass approach to sharpening, performing a round of gentle sharpening close to capture time, and a second round tailored for the specific output process, but I was going as much by instinct as anything else.

Since we sharpen for three very different reasons, why not split the sharpening into three separate stages: one close to capture, one in the creative phase, and one round of sharpening tailored specifically for the final output? The obvious reason not to do so is that you'll end up with a hideously oversharpened mess. But based on extensive testing and research, I now believe that it's not only possible, but also optimal to break up sharpening into these three stages. Obviously, caution is required, but before we look at how to make such a sharpening workflow, let's look at the advantages.

First, applying a modest amount of sharpening close to capture time makes it much easier to make good judgments about fine-tuning contrast. Sharpening is essentially a localized contrast adjustment -- increasing the contrast along edges -- and global Unsharp Masking can often change the tonal balance of an image in unintended ways. Nudging the image towards reasonable sharpness early on helps the editing process, and gives you a solid floor to stand on when it's time to make creative sharpening decisions.

Second, uncoupling capture and creative sharpening from sharpening for output lets you keep a single master file to which capture and creative sharpening have already been applied, and from which you can create different iterations for different print processes. Once the particular version is at final output size, you can apply an output sharpen that's specifically tailored for that output process.

The Sharpening Workflow Solution
When I looked at the Big Three factors that sharpening has to address, I was struck by their resemblance to the color management problem. At the risk of drawing a parallel from one incomprehensible subject to another, we can make an analogy between the sharpening workflow and a color management workflow.

The conversion from a capture color space to a Photoshop working space for editing is analogous to applying a source-specific capture sharpen. The tone and color edits we make in the working space are analogous to applying creative sharpening. The color conversion we make to an output profile for output on a specific device is analogous to output sharpening. The master image contains "device-independent" sharpening that holds up well through resizing operations, and the final "device-specific" sharpen is applied to the output file at output resolution.

Translating this workflow concept to sharpening is actually rather simple. Instead of trying to handle all the different issues that affect sharpening in a single edit, the sharpening workflow splits sharpening into three stages:

1. Capture Sharpening is applied early in the image-editing process, and just aims to restore any sharpness that was lost in the capture process.
   
2. Creative Sharpening is usually applied locally to accentuate specific features in an image-for example, we often give eyes a little extra sharpness in head shots.
   
3. Output Sharpening is applied to files that have already had capture and creative sharpening applied, after they've been sized to final output resolution, and is tailored to a specific type of output process.

Dividing sharpening into these three phases lets us handle the competing needs separately. We compensate for the image source and image type using capture sharpening, without having to worry about output issues, We apply creative sharpening directly to the image at hand without having to worry about image sources (since they've already been addressed by the capture sharpener). So output sharpening can then concentrate solely on the needs of the output process since we've handled the other problems in the two previous phases.