Sharpening Digital Images
Intro
All digital images need sharpening! This is because your camera has a anti-aliasing filter (AA filter) sitting in front of the cameras sensor that effectively blur's the image, removing certain unwanted artifacts like moire patterns and aliasing caused by repeated patterns in the subject.
Image sharpening can do a lot to improve an image, but it can't fix out-of-focus pictures! Unfortunately there isn't much you can do with a blurry image apart from binning it. This article is more concerned with images that are "in-focus" and need a certain amount of sharpening applied for enhancement.
Sharpening isn't an exact science, and varies from image to image. Experience helps greatly when it comes to how much sharpening you should apply to you images, but with a little knowledge of the best ways to approach sharpening, you will be well on the way to mastering the technique.
What is Digital Sharpening
In simplified terms, digital sharpening works by re-sampling pixels so the contrast between edges increases. Dark pixels darken, while brighter pixels brighten. This increase in contrast creates the illusion of "sharpening".
In Camera Sharpening
All cameras allow you to shoot JPEG's, while the more sophisticated cameras allow both JPEG and RAW and sometimes TIFF's. If you are shooting JPEG's, sharpening applied in camera is embedded in the image and cannot be reversed once the picture is taken. Over sharpening in camera can create ugly artifacts like halo's (edge effect where glowing separation of pixels appear between two contrasting tones) and other unwanted artifacts.
RAW capture is a different matter. RAW is unprocessed data, captured by the camera's sensor and needs to be converted to a file format in a bitmap graphic editor like Photoshop. Whatever sharpening applied on your camera won't be attached permanently to the images. All the adjustments made on camera are stored in the EXIF data attached to the RAW file, not on the image data. So RAW capture allows you to start with a blank canvas if you like. Sharpening and other adjustments can be added to you hearts delight in your editing software later, but you will still have the virgin RAW file to revert in the future.
As I said at the start, all files need some sharpening, even JPEG's that have had some light sharpening applied in camera. The best approach to sharpening images is to do it in a non-destructive fashion. What I mean by 'non-destructive' is that all sharpening adjustments applied can be reversed later.
Non-Destructive Sharpening Techniques in Photoshop
Sharpening an image should always be done in a non-destructive fashion. By this I mean that all sharpening adjustments applied to the image can be altered in your RAW convertor of your bitmap image editor after the master file has been saved. The beauty of this approach is that it gives you great flexibility when processing images for the web or print. Adjustments can be made and saved with the knowledge that they are not permanent, saying you time and reducing your work flow.
The two non-destructive sharpening methods discussed below are what I use most frequently in my digital workflow. These two methods offer me the flexibility I require to make easy sharpening adjustments in lightroom (or whatever RAW convertor you use) and Photoshop after I have saved the master file.
Non-Destructive Sharpening Techniques in RAW Convertor
Sharpening in Lightroom
Adobe Lightroom is a RAW convertor that allows you to edit your photos in a non-destructive fashion. All other RAW convertors are the same in that they allow you to make adjustments to the image without permanent effect. All adjustments made are stored in a separate file and updated as changes are made.
Located in the Develop module in Lightroom 3, the Detail panel contains sliders for adjusting sharpening and noise reduction. For this lesson I am only going to talk about Lightrooms sharpening adjustment sliders.
Lightroom Detail Panel
Amount -This is self explanatory. The higher the number the greater the amount of sharpening. I usually set the Amount to between 40 and 60. Again this really depends on the image I am working with.
Radius - How much sharpening around an edge. The default is 1.0, meaning Lightroom will only apply sharpening to one pixel around the edge. If the radius is set to the maximum value of 3.1, sharpening will spread over three pixels, resulting in wider, "contrasty" edges. I usually leave this slider at the default setting of 1.0.
Detail - As the name suggests this slider controls the amount of sharpening in the detail of the image. A small value only sharpens large edges, while a larger value would sharpen the smallest edges. Again, I usually leave this setting at the default, 25.
Masking- This slider masks out areas of the image that have little detail. It works on isolated subjects from the background and large areas of little detail like sky's and calm water. When moving the slider hold down Option/Alt key to see how much of the image will be masked.
In my normal workflow, I start my post-processing in Lightroom and then export my image into Photoshop before finishing off the image in Lightroom. Normally I leave sharpening in Lightroom until the second time round. In doing this I give myself the most flexibility in making changes in the future. If I were to export the image from Lightroom (or which ever RAW convertor you use) into Photoshop with sharpening embedded, I would have no way to remove the sharpening in the file once it has been converted to PSD file. This technique is all about flexibility, remember all changes you make in a RAW convertor are reversible.
Non-Destructive Sharpening Techniques in Photoshop
High Pass Sharpening
Your software package already comes with many sharpening filters attached, such as "Unsharpen Mask", "Smart Sharpen", etc., but these methods don't give you the flexibility of having your sharpening done on a separate layer than the background layer. The benefit of this method is that you can adjust this layer later if necessary by turning it on or off, or adjusting the opacity of the layer and consequently it's strength.
How to create High Pass Sharpening
1. Open your image in Photoshop and duplicate the background layer. Name the duplicated layer "High Pass".
2. With the "High Pass" layer selected, select Filter>Other>High Pass. Leave the Radius at the default setting, 10.0.
3. Set the Layer Blending Mode to Overlay. This blending mode exaggerates tone. A good analogy for opacity blending mode would be to imagine you are placing one slide over another. Since the overlayed layer is identical to the layer below, shadows will darken, while highlights will brighten.
4. Adjust the opacity of the layer to between 20% and 70% for best results. Normally I set opacity to around 20% to 30%. This is very much a visual decision that can easily be compared to your non-sharpened background layer by turning on and off the layer.
You can also avoid areas you don't want sharpening by applying a mask. Masks offer you great control over areas you don't want sharpened. The strength of the mask can also be adjusted using the opacity slider, offering you even greater control.
In conclusion
I advise you to play with both methods of sharpening discussed here. With time you will master the correct amount of sharpening required for your intended purpose, whether that image is viewed through a screen or in print form. I personally combine both of these sharpening techniques in Lightroom and Photoshop for my prints. I normally apply a High Pass in Photoshop first, and later apply a degree of sharpening in Lightroom. As I have repeatedly stated in this article, the beauty of this method is that all sharpening applied to the image can be altered later to your desired effect. I have found, with time, my attitude has changed to how I post-process my images. Like any discipline, knowledge and attitude changes, so having the flexibility to make those changes is a godsend. Happy Sharpening!