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Posted: Tue Apr 7, 2015, 18:28 (1534 days ago)
Modified by Admin: Tue Apr 7, 2015, 23:14 (1534 days ago)
USM – A Basic Primer.
This is a basic guide to using USM (Unsharp Mask). The subject can be studied extensively on the web. I'm using Photoshop Elements; other programs may use different terms for the controls but the effects should be identical. This is one of those guides where you should read the whole thing before you try and experiment with the process. The key to using USM comes at the end.
I prefer USM to other sharpening tools because I feel it gives me more control over the result. I'm sure other methods of sharpening are as good or better in the right hands. Some say the disadvantage of USM is that the effect cannot be reversed. This is not true 1). I never work on an original file, I always make a copy to work on and 2). As you work you can always hit CTL Z (undo) to go back one or many steps.
The first thing to understand is that despite it's name, USM is a sharpening tool. The name is derived from a sharpening technique used to print from negatives. We used to make a 'soft' copy sightly out of focus and then sandwich it with the original negative. The out of focus negative would lend more contrast to some of the edges and make them more defined. This was a lot of work and was only used for very important prints and for customers who were willing to pay for the process.
Today the software does more or less the same thing but with controls available so that you can adjust the degree (the strength ) of the effect. It provides a second layer (blended with the original) that lends more contrast at any edges in the photo. Someone once said that it's like taking a dark pencil and drawing over all the lines in the photo. That's a good description of what USM does.
There are three controls available:
1). Amount: This is the strength of the effect. As you get to a higher number the effect becomes more obvious.
2). Sharpening Radius: This sets how large the effect will be around any edge. The bigger the number, the larger the area affected.
3). Threshold: This sets where the sharpening will be applied. It compares the tone difference between two adjacent pixels (an edge). In most areas of sky, there is little difference between adjacent pixels. At the edge of a table or other hard object there is a high difference. If a low number (zero) is chosen the USM will be applied to almost all areas (it will probably make the sky appear grainy). If a higher number, 6 for example is chosen then USM will only be applied to areas which have a tone difference of 6 or more. This is an excellent control to prevent USM from sharpening an area of sky or similar area. It also helps prevent USM from sharpening existing noise.
Be aware that these controls are interdependent. If you change one it will affect the other two.
Here is a sample image that you can download and work on. It has a large area of sky, some trees, some solid shapes as well some good textures in the coal and elsewhere. The settings that I am going to suggest apply to this image. After you see what they can do you will be able to apply them to your own images. This is the link where you can download the photo.
Here is the same photo for reference:
It's important to set the view on your monitor to 100% (actual pixels) before you use USM. This is because with ANY other view your monitor will combine (change) multiple pixels to fit the image on the screen. If you use 100% you will see the actual effect of the settings you apply with USM very easily.
I like to apply USM in four steps. 1. Make initial settings 2. Radius 3. Threshold. 4.Amount
Before you start, use the sliders to select an area of the photo that contains some sky, some trees or buildings. After you change any setting you can always use the sliders to see how it affects other areas of the image. In Photoshop when you have the controls on screen you also have a small preview box. If you click your cursor on different parts of the image the preview box will show that area.
Step 1. Use an initial setting of Amount = 300%. Radius = 3 Threshold = 0 The high Amount (strength) setting will help make any other changes obvious. Don't forget to set the view on your monitor to 100%
Step 2. Set the Radius: This is the most important setting. Use an abnormally high setting say 30 to see what this control does. A wide bright halo should surround all the buildings and trees etc. Sometimes you will also see a reverse halo where a bright line appears on a solid object. Use the preview box to switch the view on and off. The idea is to get the Radius to where you see a slight halo around most edges and then back it off a touch so the image is not degraded.
If you are using the sample I supplied, set the Radius down to about 3.5 – you should see narrow halos at this setting but the effect on the image should be obvious because of the high Amount (strength) we chose.
Step 3. Set the Threshold: First click the preview on and off so you know what the effect is with Threshold at zero. Now to see what it does, bring it up to 30 and the USM effect will practically disappear. Set it back to 2 and see how you like the overall effect.
Finally, bring the amount down to 105%. Now the USM should be evident but not overdone.
These settings should give a slightly sharper image. If you want it even sharper try increasing the Amount then increasing the Radius slightly and dropping the Threshold slightly as well.
This is where your own preference comes in. Choose your own settings.
I can only encourage you to use it sparingly so that the final image is not obviously over sharpened. Remember to set the Amount abnormally high only to help choose the radius and Threshold.
My usual settings have the Amount between 80 – 120%, Radius from 0.5 to 4 and Threshold from 0 to 5. You will find your settings will vary a lot depending on the subject. You also might want to choose specific areas to be sharpened. In portraits for example I usually only apply USM to the area of the eyes. Once again – never work on the original and use a separate layer for your sharpening work.
Sharpening should always be the last step in post processing since it can add noise. After you have finished make sure that you have not increased the noise in large smooth areas like the sky. Note if you use sharpening on a RAW or other very high resolution file it will not have as much effect.
Let me know if this was helpful. More importantly please tell me if there's something that you need clarified so that I can improve the guide for others. If you are having problems with a particular image I'd be glad to help. All questions/comments are appreciated.
Click HERE to send me an email message.
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Posted: Wed Apr 8, 2015, 08:39 (1534 days ago)
Mepo,What an excellent guide to USM, with easy to follow, step by step instructions. It seem your skills know no bounds................It was fascinating to see that you own, and drive your own train. :-)