a13 May 2012

Tutorial: Sharpening - Part 2: Unsharp Mask Sharpening

Photo of kids playing with giant bubble in the centre of Warsaw city.
Recently I wrote about sharpening using High-Pass filter. Today I will describe another very popular method of sharpening - using Unsharp Mask filter. Although it's as popular as the method described previously (or even more) it's a bit more difficult to use at the beginning as you will see shortly.

First of all, I know it might look weird to Unsharp an image (isn't it more about blurring it?) but the fact is that this name exists only for historical reason and it really is a sharpening tool. Another note is that this is destructive operation so many photographers suggest to do this as the last adjustment, just before saving your final image.

Unsharp Mask (often referred to as USM) can be found in a number of programs:
Here are the steps to apply it:
  1. Start by opening an image in Photoshop.
  2. Duplicate your background layer by pressing CTRL + J (I always duplicate base layer so I always have untouched version available).
  3. Make sure you view your image at 25%, 50% or 100% zoom level. It is because between these values the image seems to be interpolated in a quite ugly and blurry way.
  4. Navigate to Filter -> Sharpen -> Unsharp Mask menu item.
  5. Following dialog box will appear:
As you can see compared to the High-Pass filter there are not 1 but 3 settings affecting sharpness (what makes it a bit less intuitive and more difficult to use initially):
  • Amount - it is the strength of the sharpening effect. Think of it of as of how much contrast is added to the edges.
  • Radius - tells how wide the edges are. Smaller values enhance small-scale detail while large values enhance larger-scale detail. Very high values for Radius might produce ugly halo artifacts around the edges so it is generally not recommended to use them. To see what I mean change radius to eg. 70 - the effect looks more like a Grunge HDR effect than sharpened image.
  • Threshold - tells what brightness difference must exist between the pixels to enhance them. Using this value is important to prevent sharpening uniform areas like eg. sky (which have pixels of similar brightness). Also it is important to use it in case of noisy images to prevent unsharp mask from sharpening noise (this would have effect of making noise more visible).
As you move the sliders, the preview will update (I mean both the small image box in the Unsharp Mask dialog and your original image), provided you have "Preview" checkbox checked. I sometimes uncheck it for a short moment to see a change between sharpened and unsharpened image.

When you finish sharpening just click OK button.
Also note that Amount and Radius are linked together. Using lower value for one of them allows to increase the second one.

Now what settings should be used for images? The problem is - it depends on the image and here are some suggestions:
  • For close-up portraits I use values of Amount: 75% - 100%, Radius: 2.0, Threshold: 3+. These settings prevent sharpening of smooth areas like skin but at the same time they will sharpen eyes, eye lashes, hair, face edges, etc.
  • For full-body portraits and animals I use values of Amount: 100 - 125%, Radius: 1.0 - 1.5, Threshold: 3+. There isn't so many smooth areas like in case of close-up portraits.
  • For landscape I use values of Amount: 100% - 150%, Radius: 1.0 - 1.7, Threshold: 3+. Landscapes often (but not always) contain a lot of small details like sand, leaves, beautiful textures in the rocks etc. Using smaller radius allows to enhance them. However, when I have landscape with large detail and uniform textures (like silhouettes of mountains) I use higher values for radius.
  • For long exposure landscape shots I use values of Amount: 100% - 150%, Radius: 1.0 - 1.7, Threshold: 6+ (often 9 or 12). The reason for high threshold values is that in night and long-exposure shots there are more uniform areas (a lot of black areas) and also quite a bit of noise caused by the high-ISO and/or long-exposure.
  • If I have blurry or a little out-of-focus photo I might use high values for Amount and lower for Radius (eg. 200% and 1.0 respectively) or other way round (Amount - 50% - 75%, Radius: 3.0 or more). But I do this rarely.
But what if none of the settings work well (i.e. some parts are not sharp enough or some are oversharpened)? Well then the solution is to sharpen locally but I will describe this in the next part of this tutorial.

Also note that in some cases you will have to use completely different values, it really depends on the image, lens with which the photo was taken, etc. For instance in case of this photo I applied only minimum sharpening as the photo was very very sharp even without Photoshop work.

Finally here is before and after comparison. The left side of the image was sharpened using described method but right side wasn't.
Before/after comparison. Left side of the image was sharpened using Unsharp Mask filter. The right side was left untouched. Sharpening on the left side brings sharpness to grass and trees in the back.

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