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Sharpening

One of the determinants of final image quality is the how well an image is sharpened. In fact, as the print size is increased for any given image, the quality of the sharpening becomes ever more important. Yet, it is not uncommon for photographers, who spend thousands of dollars to get the best equipment available in hopes of getting very high quality images, to lose some of that quality at the end of the process by using less than ideal sharpening methods.

For many, the issue of sharpening starts and stops at Unsharp Mask (USM). Yet, USM is only the beginning. There are other sharpening tools. Perhaps more important, USM and the other tools can be used as the basis for far more sophisticated and powerful sharpening techniques.

It is important to set the purpose of this article up front. While this article will detail the proper use of USM and some of the other tools, its goal is not to simplify sharpening down to its easiest level. To do so would be to diminish the potential of the sharpening tools. In other words, the reader will not find advice to set the sharpening tools to such-and-such settings and click the OK button. While such advice may make life easier, it would not result in optimum quality images. Rather, optimum quality images are created when the photographer analyzes the sharpening options in relation to the content of the subject matter, the initial quality of the image, the method of image capture, and the output options (e.g., inkjet print, wet processing, or web display). Only such a careful analysis can determine the best approach to sharpening a particular image. Furthermore, what is optimum for one image is not likely optimum for another — different images require different tools, settings, and approaches. The purpose of this article is to delineate the tools and methods, their use, and their strengths and weaknesses in relation to the particulars of images. In short, this article is a thinking person’s guide to sharpening. To this end, this article will cover the following:

  • USM.
  • Smart sharpen tool.
  • High pass sharpening.
  • Lab based sharpening and the use of the lightness channel.
  • Use of the Fade tool.
  • Layer based sharpening and use of the luminosity blend mode.
  • Sharpening masks.
  • Edge masks.
  • Three pass sharpening.
  • Sharpening plug-in.

WHAT IS SHARPNESS

In practical terms, we understand the concept of sharpness. Even a person unschooled in photographic concepts can look at an image and tell you if it is sharp or not. However, the technical definition of sharpness is less well understood. Ask even an accomplished photographer to provide a precise, technical definition of sharpness, and you my get nothing more than a blank stare. Yet, understanding sharpness is integral to understanding sharpening tools and what they do. Further, understanding the sharpening tools is integral to creating the best possible photographic output.

Sharpness is actually determined by two factors: resolution and acutance. Resolution is more closely aligned with what most people think of as sharpness. Resolution is the ability to resolve fine detail. Resolution charts that have lines that get finer and finer are often used to test the resolution of lenses. Resolution is generally measured in line pairs per millimeter LP/mm). The more LP/mm that a lens can resolve, the greater the resolution of the lens. In practical terms, resolution is the ability to reveal detail. Photographic equipment of high resolution reveals fine detail while that of low resolution can not. Thus, resolution is largely determined by the camera and lens (actions taken on an image after capture can degrade resolution, but no actions can create resolution that was not previously there). Sharpening software has little to do with resolution.

Figure 1: Acutance

Acutance has to do with contrast. In particular, the contrast of adjacent, or near adjacent, pixels. The human eye and brain interpret light pixels lying next to dark pixels as an edge. The quicker the transition from light to dark (i.e., the greater the contrast), the sharper edges appear to be. This is demonstrated in Figure 1. In both Image 1 and Image 2, there is a light gray rectangle next to a dark gray rectangle. In Image 1, the transition between the light gray box and the dark gray box, as well as the transitions between both boxes and the green background, is quick. In this image, the contrast, and therefore the acutance, is high. As can be clearly seen, the edges appear sharp and well defined. Image 2 has the same configuration of boxes except that the transitions between the boxes, and the boxes and the background, have been made more gradual. The contrast along the edges has been lowered. Image 2 has a low acutance. This low acutance is interpreted by the eye as a lack of sharpness.

The important point here is that the appearance of sharpness in Image 1 and the lack thereof in Image 2 has nothing to do with resolution or detail. It has everything to do with contrast along edges.

We now come to one of the most important points about sharpening tools: sharpening tools increase the apparent sharpness of an image by increasing the acutance of the image. In other works, sharpening tools increase the contrast along edges. The corollary is that the sharpening tools do not in any way increase resolution or detail. They can only make detail that was already present in the image stand out by increasing the contrast along the edges of the detail.

This brings up an important aspect of sharpening. Sharpening can not bring an out of focus image into focus. An out of focus image has low resolution. Since sharpening can not increase resolution, it can not rescue the image from its focus problem. Thus, sharpening works best with images that have crisp detail due to the use of quality equipment and proper photographic technique.

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source: http://www.ronbigelow.com/articles/sharpen1/sharpen1.htm

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