Unsharp masking makes the lines look sharper in an image by flipping light and dark pixels.

1. Choose Filter > Sharpen > Unsharp Mask.

2. For Amount, enter a value between 150% and 200% to determine how much to increase the contrast of pixels.

3. For Radius, enter a value between 1 and 2 to determine the number of pixels surrounding the edge pixels that affects the sharpening

4. For Threshold, enter a value between 2 and 20 to determine how different the sharpened pixels must be from the surrounding area before they are sharpened by the filter.

5. Click OK.

Here's an example of an image before (left) and after (right) sharpening:

Photo by Kelly Danforth