USM is fast and easy and familiar to most photo editors, although its not the only option. It is the one we will explore first. Rightly or wrongly we live in a pixel peeping world. The Major Stock Photography site rules say no sharpening (that's a job for the people who end up printing it), but the stock sites are the exception to the rule. Everybody else seems to want it so sharp it can cut. This aims to show you how to do sharpening in the GIMP, use it wisely. If you want to know my position on pixel peeping and an example of how well GIMP works for stock, read the aside after the example image. But first lets tackle USM
1) First Lets zoom to 100% using the drop down list at the bottom right of the image and find something in focus. You can place the mouse over the image and hold down the space bar to move around quickly. Alternatively use the navigation bars, or alternatively hold the left mouse button on the 4 arrow icon at the bottom right of the image and move around. Why something "in focus" USM in GIMP (and all other software) can make sharp things sharper, but it can't make out of focus items in focus. So if you missed focus by a lot this won't fix it.
2) Click Filters >> Enhance >> Unsharp Mask, a small preview window will appear with the default settings of Radius 5.0, Amount 0.50, Threshold 0. You can make this windows as big as you like, the preview is always at 100%. Navigate again to the area that is in focus. The larger you make the window the slower it will run, but even on a modest machine it should be fine. Try turn the preview and and off to see what affect it will have when you press Ok. Most of the time these settings work well.
3) Radius, USM works by detecting edges and enhancing the contrast of those edges. The Radius Setting dictates how many pixels around the edge are affected. Too little and the sharpening is weaker, to much and you can end up with a halo around the edges and your pc will have to work harder to show you the results. 2nd image is with Radius 120, Amount 0.50, Threshold 0. A side effect of high Radius is that more of the image gets a contrast boost, so if you had a low contrast image it very well could make things look better, but this is not what you want to use USM for. Fix contrast with the contrast control under the Color Menu, enhance sharpness with USM. In this image it enhanced contrast, gave slight halos and was pushed so far it induced a little colour distortion, but surpisingly wasn't to bad. Each image is different so play around with yours.
4) Amount, 0.00 to 5.00 controls "how much" to much and you get artifacts (stray pixels and jagged edges), to little and your not sharpening. 3rd image is Radius 5, Amount 3.00, Threshold 3.00, you can see the smooth edges of the drops starting to turn into jagged edges. Noise has also come out very harsh.
5) Threshold, This slider allows you to set the minimum difference in pixels that indicates an edge to be sharpnened. You can protect areas of smooth tonal transition from sharpening, and avoid creation of artifacts in smooth color transistions, such as the sky. For a lot of people this stays at zero, for my image I put it at 10 (out of 255) after finding the limits with the radius and amount, the impact was very subtle, too much threshold and you get a sharp edge and nothing else, it looks very abstract!
6) I settled for Radius 7.0, Amount 0.60, Threshold 10 (4th image). This was my personal preference, a lot of other people would push the image much harder. It close to the radius 120 shot without having the contrast changed. Now you know what the sliders do, its up to you how you use them.
The Example 100% crops: Ignore the first box on top right with 5, 0.5, 0 its actually not sharpened at all, all the other values are correct.
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About the Image
The image I'm using is an unspectacular shot of a leaf with some rain drops on it. I picked it as it was shot at ISO400, and has a beautifully blurred background with the potential for noise to become an issue if over-sharpened as stated you can't make something "in focus" the issue with blurry background is that the noise pixels are usually the only sharp thing on the background, so they get enhanced the background does not, end result more noise.
Sharpness in general, Megapixels and GIMP with Stock:
The original image was taken with a Canon 40d at 10.1MP (3888x2592) and Sigma 70-300 APO wide open at f/5.6 - 1/80 - ISO400 through a glass window, with rain falling between lens and subject, tripod mounted. It was cropped in DPP to 4.3MP (1861x2326). In its un-sharpened state it was up-scaled to 200% or double size to 17.1MP (3721X4652) using the Sinc "(Lanczos3)" resize option. It has been uploaded and passed Alamy QC and is available for sale as of yesterday, so its "sharp enough" without anything applied.
Let me summarize, dirt cheap lens, less than favorable shooting conditions, and lots of up scaling with GIMP, Raw was converted with the free DPP. As it was intended for stock from day one, no sharpening has been done to it as per the rules. It was checked by the stock site and it passed, they consider it saleable. Anyone who says you can't make a saleable image without a full frame with a huge megapixel count, an L lens, the most expensive Raw converter and version of the latest Photoshop CS is likely misinformed or being a little dishonest.
Thats not to tell you not to get these things, just remember if your doing this for business and the competition is using those things, you will both end up with more or less the same results, however you will have lower business costs and more money in your pocket. For this example GIMP was only used for upscaling for stock and it did an amazing job.