There are three Sharpen effects that heighten the contrast in an image:

• Sharpen
• Sharpen More
• Unsharp Mask

The Sharpen effects are filters that produce the opposite effect of the Blur commands by increasing the contrast between adjacent pixels where there are significant colour contrasts, usually at the edges of objects. They lighten the light pixels and darken the dark pixels. Sharpen More applies the Sharpen effect with more intensity. 
You can use these effects to fix photographs that are slightly out of focus. You may get better results if you use the Unsharp Mask dialog box rather than the preset Sharpen and Sharpen More effects. With it, you can vary the thickness of the edges and the intensity of the contrast.

Notes: You can apply effects to greyscale and 24-bit colour images. If necessary, increase the colour depth of your image to make the commands available.To apply an effect to a selection, create the selection before choosing the effect.

To apply a Sharpen effect choose 

Effects > Sharpen > the effect