Web DevCenter
oreilly.comSafari Books Online.Conferences.
MySQL Conference and Expo April 14-17, 2008, Santa Clara, CA

Sponsored Developer Resources

Web Columns
Adobe GoLive
Essential JavaScript
Megnut

Web Topics
All Articles
Browsers
ColdFusion
CSS
Database
Flash
Graphics
HTML/XHTML/DHTML
Scripting Languages
Tools
Weblogs

Atom 1.0 Feed RSS 1.0 Feed RSS 2.0 Feed

Learning Lab






The Ideal Digital Photographer's Workflow, Part 5

The Three Ideal Stages for Sharpening Your Images

by Ken Milburn, author of Digital Photography: Expert Techniques
07/20/2004

Would you believe that every image shot by a digital camera needs to be sharpened -- not once, but three times? Well, you shouldn't, because it's not always true--but almost always, it is.

The first time you sharpen, you do it because it is in the nature of most image sensors and of such things as dust-protecting and low-pass filters, which are built into most digital cameras (most especially those meant for pros and prosumers). The image sensors (except the Foveon sensor used in Sigma cameras) have to interpolate the colors in the image from something called a Bayer pattern. The Bayer pattern is the name for the way individual sensors are grouped and filtered for sensitivity to the three primary colors: red, green, and blue. Because the eye is most sensitive to subtleties in green, there are two green sensors in each RGB group. A computed algorithm has to be applied to this pattern so that the picture ends up being interpreted (somewhat) as though each sensor had three transparent layers (which is how color film works). In the process of sorting out all of this, the camera's computer actually makes the picture just a bit fuzzy.

Now, to be fair, most camera software gives you an option to sharpen the picture in the camera. If your objective is to end up with the highest quality image possible, don't take the camera up on its offer to do this work for you. It simply doesn't have the brain power that your hunky computer does. Nor does the camera have your own subjective judgement as to what you want to see. To make matters worse, the sharpening adjustment parameters that make one picture look superb may make another look just a bit (or very) coarse, or downright ugly. You need to use your own judgement to get exactly the look you want. It takes practice, technique, and perhaps some software tools you may not already own.

You'll understand all of this a lot better as we just jump into this article. I'll explain the three stages of sharpening that are almost always necessary. I call them rescue, effect, and output. Also, there are a few things you should do before you even start sharpening.

Before Sharpening

Start with as much image information as you can get for your picture. Set your camera for the lowest ISO setting that will give you the picture you want. The lower the ISO setting, the less noise you'll have in the picture. Noise is always the biggest contributor to loss of image quality in a digital image. Second, don't set your camera to use any color balance or shadow/highlight corrections (unless you have to shoot JPEG). If at all possible (and if your camera permits it), take the picture in RAW mode. In the camera's RAW interpreter, set the color balance, exposure, shadow, and midtone settings to the point that looks best to you. Do not use noise reduction or sharpness settings unless lack of time simply leaves you no other choice. Finally, save from the RAW interpreter at the original resolution (or twice that, if you use a Fuji Super CCD camera) and in 16-bit mode.

If you have to make any optical corrections, such as for barrel and perspective distortion, do so now. This is because these operations will contribute to softening the image and you want to keep the number of necessary sharpening stages to a minimum.

Rescue Sharpening

This is the stage that "corrects" the problems that result from the camera's behind-the- lens filters, Bayer interpolation, and (sometimes) even the fact that you can't afford the sharpest lens of the focal length you wanted to use to take the particular picture you want.

Note: There are two kinds of blurring that aren't likely to be helped by sharpening: out- of-focus blur and motion blur.

Theoretically, you should do rescue sharpening before you do anything else. In a sense, that's what you're going to do. However, I suggest a slight variation on this plan: first, use Adjustment layers to correct the image's brightness, contrast, and color balance. By using Adjustment layers, you'll still have the original intact as the Background layer, so you can always go back to your original. The big advantage you gain is that sharpening is always somewhat subjective. So you'll be able to judge your sharpening previews much more accurately if you have an image that's more representative of the end product you want than the original image is.

You should also do your "spot retouching" before you sharpen. That is, whatever techniques you use to eliminate skin defects, dirt on the sensor, or litter on the landscape. Do not, however, do major retouching that involves image compositing. That will come later.

Here's a good step-by-step routine for first-stage, or rescue, sharpening that you can paste on your wall:

  1. Do use Adjustment layers to do the various gamma adjustments (Auto, Levels, Curves).
  2. Do overall color balance adjustments (gray card, color balance).
  3. Do any color effects you're likely to want to use with a Hue Saturation Adjustment layer.
  4. Remove any image noise. I highly recommend using Nik Dfine, but if you can't afford it, you can download some effective noise removal Actions from Adobe Studio.
  5. Do basic retouching.
  6. Do basic sharpening.

At this stage, I don't recommend you use any of the more powerful sharpening tools suggested for use in the second (creative effect) stage of sharpening. Use Unsharp Mask (USM) or Simple Sharpener. The exact settings you want to use will vary depending on your camera, settings, and image size. However, here's a good opening suggestion for settings that typically work for a 6MP image with Photoshop's built-in Unsharp Mask filter. Be sure the preview window in your sharpening tool is zoomed in to 100 percent and that the Preview box (they all have one) is checked. Now, play with the adjustments until the image looks believably sharp, but not exaggeratedly so. If you start to see white or black "halos" around the edges, back off. Ditto if the edges start getting jagged. Figure 1 shows a 100 percent view of the same image before and after successful rescue sharpening.


Figure 1. Here you see the stages of initial sharpening using the Photoshop CS Unsharp Mask filter. The image on the left is the unsharpened interpretation of a high- resolution RAW file. At right is the USM initial sharpening result and the settings used.

  • Amount: 250 to 350
  • Radius: .2 to .5 pixels
  • Threshold: 0

Pages: 1, 2

Next Pagearrow