Multi-Level Sharpening an Image
- By: Luis Argerich
I can’t even start to count the number of books, articles and tips that are written about sharpening. There are so many recipes and so many methods that it gets confusing. In this small article I will describe a simple technique to control the amount of sharpening in an image. It’s a technique commonly used in astrophotography where the finest level of sharpening is needed and it can be applied to daytime images without any difference.
Using the HighPass Filter for sharpening
The tool we’ll use for sharpening is the high pass filter. The method is simple, duplicate the layer, apply the highpass filter to the new layer and set its layer mode to overlay. You will see how the new layer is a sharpened version of the previous one. To control the level of sharpening you change the radius of the highpass filter. A very small radius is useful to enhance textures without modifying the rest of the image. A larger radius can increase contrast in the edges. A very large radius will increase local contrast a lot but will probably create some artifacts.
The filter in action
To show you the effect of the filter I will show some images with different levels of sharpening applied.
You can see how increasing the radius changes the amount of sharpening applied to the image.
If you want to enhance local contrast in your photo textures you should apply the filter with a radius of less than 1 pixel. 0.3, 0.5 and 0.7 are good values to try. This values will enhance micro-contrast in your textured areas without impact in the rest of the photo.
This is a example:
If you look carefully you will see how the texture is enhanced.
Different Levels of Sharpening
As we have seen we need different levels of sharpning for different things. The best approach is to create several copies of your original photo and apply the highpass filter to each layer with a different radius. Then you can use masks and opacity to decide the amount of sharpening to use from each layer and where to apply the changes. I use a very low radius to highlight textures, a 2 or 3 pixel radius for general areas of the image, a 5 pixel value for edges and a 10 or 20 pixel value for things such as grass or clouds where a higher level of sharpening can be needed.
This is how the layers look while editing:
Once we have all the different levels of sharpening masks and opacities are used to decide how the final image will look.
Final Sharpening Tips
Sharpening should be applied only to the final image once resized. Save your master photos in full size without applying sharpening at all. Then when you want to print it or put it on the web resize the image, apply sharpening a save a jpg to be used for printing or publishing. Different sizes need different levels of sharpening.