Wednesday, March 5, 2008

Image Sharpening - A Reference and Tutorial

Image sharpening is one of the most important image enhancements in the digial world. This is especially true of scanned negatives or slides. However, the importance of sharpening is due to fuzzy images, but rather due to noise.

Noise Causes a Need for Sharpening

Noise is found in all digital images to some degree. Images shot in low light with high sensitivity set typically carry more noise than images shot in brighter light with lower sensitivity settings. Furthermore, all negative scans carry noise no matter what the speed of film and dots per inch scanned.

How does this noise cause sharpening problems? From both a digital and scanned negative perspective, the process of effective noise reduction causes the edges of an image to become softer. Removing noise in an image replaces pixels thought to be noise with a guess of the correct pixel color and luminance based on surrounding pixels. Thus, if an eyelash in a portrait has noise around it that is removed, some of the previously flesh toned pixels may actually now carry the eyelash color. I.e., the eyelash got a little wider than it was. In reality, the pixels probably carry a color in between the flesh and eyelash colors, causing a blurred efect.

Remove Noise, Add Sharpening - Why Do I Still Have Noise?

So we see that removing noise in an image causes the image to become softer. The next question is how we can add sharpening back, without causing the noise to reappear? If a sharpening algorithm looks at high contrast areas within an image and causes the pixels at either side to blend closer in color to the edges, doesn't that add some noise back? The answer is Yes.

This round robin of removing noise which blurs the image and then sharpening the image causing some more noise is what makes the image work flow so difficult. It also makes a successful noise reduction and sharpening workflow add significant value to your images.

An image that has noise removed and is extremely sharp is the type of image you see in advertising from printer manufacturers, film manufacturers, camera and software manufacturers. They all show this crisp, sharp and smooth toned print saying this is what you can do. They are correct, they just don't tell you how.

The Challenge Revealed

Now that we know what needs done, all that remains is to learn some techniques for accomplishing the workflow. I will leave you with several references here that I have found extremely in-deapth and useful. Following will be a list of topics we will explore over time here as it relates to sharpening and noise reduction. The good news is that once the technique is learned, you can easily replicate it with your images. Note that in order to achieve the best image, you will have to tweak the process for each image. However, understanding what happens will make that tweaking process relatively simple.


  • Bruce Fraser's book on Photoshop Sharpening - this book is by far the most detailed book on sharpening. More importantly, Bruce helps you design several sharpening routines that you can use in Photoshop over and over. I have created both Black & White as well as RGB sharpening routines based on Bruce's theories and practices - a great resource, thanks Bruce!

  • Neat Image Sharpening plug-in and stand-alone software - This software comes first for me because it has two great features - first, it works on black and white (greyscale) images as well as color and black and white is the majority of my work. Second, this program is both a Photoshop plug-in and a stand-alone software program. Thus, I use it directly in Photogshop and as an External Program in Lightroom. It also has a built in image that you can use to create noise removal profiles for your digiatl camera or scanner. I have had great success with the profiles for my Leica D-Lux3.

  • PhotoKit Sharpener by Pixel Genius - This program is produced in part by Bruce Fraser (see the book above) so it contains his workflow techniques. This is one reason I like this software so much. However, the drawback for me was that you are limited to a plug-in in photoshop and a 16-bit RGB image. No black and white (greyscale) images can be used. It is unfortunate, because this software actually leads you through a three step sharpening process - the source, image and printing. Kudos to Bruce again, and understanding his concept, I know it will be difficult to replicate for black and white, but I am sure the folks at Pixel Genius will find a way.

Sharpening Workflows

Why isn't Lightroom Listed?

Lightroom isn't listed here for the same reason Photoshop isn't. These programs have sharpening tools built in and the tools above can supplement them. However, I will review both my workflow and sharpening techniques for both Photoshop and Lightroom here. Keep looking for links that will take you to more practical information using the reference material above.

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