A monthly column by Mike Chaney
| December 2004 |
In a fog over sharpening?
What is sharpening? Simply put, sharpening is an image editing technique that allows us to make slightly blurry or out of focus images look in focus, clear, or "sharp". While sharpening cannot fix obvious focus problems where the subject in our photo is simply not focused properly, it can allow us to add that final "punch" to the photo to make the jump from just "in focus" to "tack sharp". Sharpening doesn't really add any real detail to images, however, it can accentuate details to make them more obvious on screen or in print. Our task with sharpening is to find that delicate balance that makes our photos look clear and sharp without making them look overdone.
Why do we need sharpening? The first step in understanding how to use sharpening is understanding why we need it in the first place. In a perfect world, our cameras would record every pixel in our images perfectly and those pixels would be rendered perfectly on screen and in print. Unfortunately, devices like cameras and printers have their limitations and one of these limitations involves sharpness. Due to the way cameras capture an image, for example, there is always some smoothing which "leaks" information from one pixel into surrounding pixels. Imagine a single point of light in an image such as a star in the distance in a perfectly focused frame. Even if that point falls on only a single pixel on the camera's sensor, the algorithms that put together the final image will spread that point of light into neighboring pixels making the point look less focused (blurry). Add to this the fact that most cameras have antialiasing filters that blur the image slightly before it even gets to the sensor and the fact that no lens is perfect, and we start to understand why there is a need for sharpening to undo the appearance of some of this blurring. Sharpening can take that point of light that was spread into neighboring pixels and bring some of that spread out information back closer to what should have been recorded (a single pixel of light). The mechanisms in the image capture process that cause blurring are not limited to point sources of course since edges/lines are also affected such as a sharp edge on a car door, power lines against sky, and other fine details. Sharpening can help reverse blurring in those areas as well.
Where to apply sharpening Sharpening should be viewed as a way to compensate for deficiencies in the way devices capture or render images. If we use sharpening to undo the inherent blurring effects of a device, we have done the best we can do because the result will be closer to reality with respect to sharpness. There is often a state of confusion with respect to where and when to apply sharpening so here are a few recommendations. Sharpening the original image:
Sharpening at print time:
We understand "when" but what about "How"? There are actually a lot of different techniques that allow sharpening of images. Sharpening can be as simple as clicking "Sharpen" in your photo editor, or as complex as converting images to Lab color space and sharpening only the luminance channel using unsharp mask. Let's try to keep it simple and just focus on the most common and one of the most flexible sharpening techniques: unsharp mask. Unsharp mask, unlike the name implies, is actually a method of sharpening. It is called "unsharp mask" because it uses a blurred copy (an unsharp copy) of the image to compare to the original in order to sharpen. Here are the parameters associated with unsharp mask:
Notes on sharpening:
There is a lot of information on the web regarding sharpening techniques, a lot of which can be confusing and even incorrect. Some of the best resources I have found are those at digitalsauces.com: If you would like more information on the Sharpening Equalizer in Qimage, see this article. In Summary: Sharpening is a technique that is so broad that you can make it as simple or as complex as you like. Unless you are sharpening for artistic expression, I recommend using sharpening functions to compensate for deficiencies that cause blurring in images. That is, we compensate for any blurriness or softness in the original by applying an unsharp mask to sharpen the image back to its original/intended clarity (or perception thereof). We also apply sharpening at print time to insure that our prints are as sharp as the image viewed on screen. Hopefully this article has given you a baseline understanding of when to use sharpening and has touched on a few of the methods of how to apply that sharpening.
Mike Chaney is president and chief software programmer for Digital Domain Inc and is the author of Qimage and Profile Prism software. He has a bachelor's degree in computer science from the University of Maryland, College Park. Mike worked for the federal government for 14 years as a senior software engineer, designing and deploying large scale workload tracking systems. He began developing digital imaging software in 1996 as a part time venture and due to the success of his software was able to resign from the government in 2001 to pursue his passion for digital photography and development of related software. Mike continues to develop and update his software and is also an active member of many online communities related to digital imaging. He enjoys helping others by discussing the latest topics, doing independent research, and sharing with others in order to take some of the "mystery" out of some difficult but key concepts in the field of digital photography.
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