The Sharpness slider adjusts the image sharpness to provide the edge definition you want. The Sharpness adjustment is a variation of the Photoshop Unsharp Mask filter, which locates pixels that differ from surrounding pixels based on the threshold you specify and increases the pixels’ contrast by the amount you specify. When opening a camera raw file, the Camera Raw plug‑in calculates the threshold to use based on the camera model, ISO, and exposure compensation. You can choose whether sharpening is applied to all images or just to previews.