APPLY UNSHARP MASK TO CMYK IMAGES

If you've already converted your image to CMYK mode and you want to quickly sharpen your image without introducing color shifts or halos, go to the Channels palette (under the Window menu), click on the Black channel, and apply your Unsharp Mask there. Applying the filter just to the Black channel will enable you to apply a higher level of sharpening without damaging the image.

©ISTOCKPHOTO/ALAN GOULE