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Sharpening your images is a bit of a science and there are quite a few aspects that need to be considered. You can sharpen for printing, for web maybe for display on an iPad. On top of that, different resolution images will need different settings. Combine this with the fact that current incarnations of Photoshop have 5 different sharpen filters, you begin to see its not as simple as pressing sharpen.
Before You Get Started
In this brief tutorial we will take a look at some of the Photoshop built in filters and also a couple of more advanced sharpening processes. Before we start, it is important to know that sharpening should be the very last thing you do to the image, make your color/exposure corrections first, then any cropping and resizing then when you are happy that you have the final image, apply the sharpening. Also you should sharpen your images at 100% view on screen to keep an eye out for problems such as artifacting.
Using Native Photoshop Filters
Lets start with Photoshop’s own filters and say the the one not to use is the generic sharpen filter. This applies a sharpening algorithm that you have no control over and so will not necessarily do what you require it to do. The same applies to the Sharpen Edges and Sharpen More filters.
The most commonly used Photoshop filter is the Unsharp Mask. When you open this filter, you will see it has three settings, Amount, Radius and Threshold. Amount signifies how aggressively sharpening is applied to the image and should be determined visually, it is also related to the image size and resolution. The Radius control determines by how much around each pixel the filter is applied. The general consensus is that you should look for look for a value of between 0.3 to 0.5. Lastly the Threshold should be set to between 0 and 1. The best policy is to visually inspect the image at 100% as you adjust each of the sliders.