Creative Nerds shares a great tutorial for this simple technique.
Tagged: halftone
Free halftone Photoshop brushes
I just came across a really nice brush set over at BrushKing this morning. Halftone3 is a pack of 11 Photoshop brushes by DeviantArt member, env1ro.
Create a comic book halftone-dot effect for your Photoshop images
If you’ve got a particularly “uninteresting” image you want to use in your next design project and you’re looking for a way to spice it up a bit, consider adding a comic book style halftone-dot effect to your image. It’s quick and easy, uses only Photoshop’s built-in filters, and produces excellent results.
1. Choose your image.
Images with a decent amount of contrast and midtones work best, but virtually any image will do the trick. I chose this handsome little fella: 
2. Duplicate the image.
Select the background layer from the layers panel and hit Command + J (or drag the layer icon to the new layer icon at the bottom of the layers panel) to duplicate the image layer. Label the new layer “Comic effect” or something meaningful.
3. Prepare for the filter
Make sure the newly created Comic effect layer is on top of the original layer (if you left the original layer set as “background” you should be good to go). Now hit the “D” key to reset your colors to solid black & solid white. Then hit the “X” key to invert them so white is the foreground color and black is the background color.
4. Apply filter
With the Comic effect layer selected, go to your menubar and select Filter>Sketch>Halftone Pattern. For now, just leave the settings at their default, which is usually: Halftone Pattern Size: 1 Contrast: 5 Pattern Type: Dot Hit OK. Your image should now look something like this: 
5. Invert the image
With the Comic effect layer still selected, hit Command + i to invert the image. It should now look something like this: 
If we wanted to have a black & white image, we could probably stop here, but I want color, so let’s move on.
6. Adjust the blending mode
Go to your layers panel blending mode drop-down menu and select Linear Light. You can try any of the other blending methods, but I’ve found that Linear Light works with the most predictable results and requires less tweaking. Your image should now look something like this: 
The image is too “muddy” like it is, so we have one last adjustment to make.
7. Set the opacity
With the Comic effect layer still selected, adjust the opacity to your liking. For this image, I chose a setting of about 60%, but it really depends on the image.
Final Image:
I added a thick black comic-style border, and a thought balloon with a comic-style font to complete the theme. There are several variations of this technique, but I’ve found that just playing around with the filter settings and layer blending modes can produce a wide-variety of effects by themselves. For instance, in the Halftone Filter settings from step 3, you could choose Line instead of Dot. Play around with it and see what you can come up with.
Create a halftone-dot effect in Adobe Illustrator
A while back I posted a link that would show you how to create an interesting halftone-dot effect in Photoshop. Recently, a reader left a comment on the post asking if there was a way to achieve the same effect in Illustrator. I’m happy to report that there is, and I’ve put together a tutorial to show you how. 
Step 1: Create a shape in Illustrator that you wish to convert into the halftone-dot effect. I chose a star. To achieve the mixed-size dots, you’ll want to use a gradient, and to make it easy later, you should probably start with a simple black color. Step 2: Select the object and select Effect>Pixelate>Color Halftone… from the menu. The color halftone dialog box will open where you can enter some numbers to adjust the appearance of the effect. 
Set the Max. Radius to 10-15 pixels. This will determine the size of the largest dot in the effect. Go ahead and leave the screen angles at the default setting. Hit OK. The star is now converted to a raster image. The darkest areas of the star have the largest dots, and the lightest areas have smaller dots. We are now half way there. We need to convert the raster image to vector. Step 3: With the star still selected, go to Object>Expand Appearance in the menu. Now select Object>Live Trace>Make and Expand from the menu. The star is now a full vector object. You can select each dot individually if you choose because they’re all vector shapes (see step 3 in the image above). But you probably want to add some color. 
Step 4: If the star is on a white background, everything will appear to be perfect. But draw a box, fill it with color and send it to the back by hitting Command-Shift-[. You’ll notice that the inside of the star remains white. If you want it to be transparent, you have just a bit more work to do. See the image at the right for an example of what I’m talking about. Use your Direct Select tool (the white arrow) to select any white are in the newly created halftone dot area. Go to Select>Same>Fill Color in the menu. Now that all the white areas are selected (including the white bounding box around the star, hit Delete. You should be left with nothing but the dots now. Now select the star again and add a color to it as you would any other object. Because the star is a compound path, all the dots will be colorized. That’s it. You have your halftone-dot effect, and it’s a fully-editable vector object in Illustrator. Why would you want to do it in Illustrator? Because many sign shops and screen printers require everything to be a vector shape depending on their production methods. Plus, it adds a little more flexibility for later editing. 
I chose to use a one color object for the sake of keeping this tutorial simple, but you can apply the same technique to a full color object for different results. See the image above with the same star filled with a color gradient.