Unsharp Mask

The truth is. We can't really sharpen an image any more than it already is. If it wasn't sharp when captured, there's nowhere the information needed can come from later on. so, capturing a sharp picture is very important. What we can do is create the illusion of sharpness by exaggerating contrast along edges in the image. This added contrast makes the edges stand out more, making them appear sharper.

Amount
controls how weak or strong the mask image that is blended in will be. Thus, it controls how much edge contrast will be added and how much apparent sharpness we will get. If set too low, we won't see any change at all, but if set too high, the inverted halos from the mask will be visible in the final image and it will look artificial. Most Photoshop users are familiar with the appearance of an over-sharpened image, having probably produced a few themselves while trying to figure out what to do with Unsharp Mask. You can't make an omelet without breaking a few eggs though.

Radius
controls the degree of unsharpness the mask image will have. Thus, it determines the width of the halos the mask will produce. The higher the radius the more evident the sharpening effect will be. Set too low, the effect will be invisible or too subtle to be effective. The appearance of the halo will also vary with the content of the image since the amount of contrast already present will play a role.

Threshold
selects how much adjacent pixels need to be to be considered an edge. Thus, it prevents sharpening of unimportant or incidental detail while reserving the effect for detail that should stand out. It separates signal from noise, if you will. Set too low, sharpening will be applied to every edge or change of tone throughout the image including such things as film grain. If set too high, no transitions will be viewed as being edges and no sharpening will result.

sharpening is best done at final output resolution so do all your editing including resizing for print or web before sharpening. If you are creating a web graphic or other image destined for screen viewing, sharpen until the image looks right at full size. If you are sharpening in preparation for printing an image, you may find that slightly over sharpening . sharpening is required before printing. Sharpening is also dependent on image content. An image less inherent detail will generally tolerate more severe sharpening, while many with a great deal of detail will only require minimal sharpening since they will naturally look sharper to begin with.

Even though Amount is at the top of the set of controls in the Unsharp Mask dialog, you should generally start with the Radius control. An image with fine detail will need a lower Radius setting while one with less inherent detail can utilize a larger Radius. Start with a Radius between about 1.0 and 1.5 and go from there, lowering the setting perhaps to 0.5 or even lower for images that start with fine details, or raising it to perhaps 2.0 to 4.0 for images with lower levels of inherent detail.

If you have selected a small Radius setting, the halos will be narrow, so you will need a high Amount setting for them to have much effect. Conversely, if you are using a high Radius value, the halos will be wide and you will need a low Amount setting to avoid them being obvious. Once you understand what the controls actually do, you should be able to tweak things visually using the 100% view. Play with a few of your own images until you get the hang of things.

After settling on ranges for Radius and Amount, decrease the Threshold value until unwanted noise or other artifacts start appearing where you don't think edges should be, then raise it slightly. The exact setting you end up with will be highly dependent on the content and source of the image you are sharpening.