Halftoning using a Layer Style
This tutorial requires Photoshop CS, CS2 or CS3 and the Classical Halftone Patterns offered at this website.
Step One: Download three free sample patterns to follow along this tutorial.
Step Two: Start with an image in either RGB or Grayscale mode. This method only works in these two modes! If your image is in RGB mode use a Gradient Map to convert it to grayscale. After this press Ctrl/Cmd-E to flatten the layers.
Step Three: To be able to use Layer Styles hold the alt key down and double click on the Background layer. Click on the Add a Layer Style icon at the bottom of the Layers panel and choose Pattern Overlay...
Click on the arrow next to the pattern icon, then on the small arrow in the next window and choose Load Patterns. Navigate to the Halftone pattern file you've downloaded, select a pattern and click OK.Finally set the Blend Mode to Hard Mix and click OK. Don't scale the pattern.
Step Four: Usually you would now apply an Unsharp Mask filter to bring out the detail but if you want a softer look goto Filter > Blur > Gaussian Blur to blur the image to your liking.
Step Five: To colorize the image click on the Add a Layer Style icon again and choose Color Overlay. In the dialog set the Blend Mode to Screen then choose a color. Hit OK.