Rendering group> Depth of Field

Trapcode Particular mimics the look of a real camera by adding a Depth of Field (DOF) simulation.

How does Depth of Field work? A real camera cannot maintain focus in all depths of an image. DOF mimics the way that a real camera cannot maintain its sharp focus at all depths. DOF is useful for increasing the realism of a scene. It is also useful for matching your Particular effect to footage shot with a real camera where Depth of Field is apparent.

By default, DOF is turned on with the Camera Settings option. Choose the Off option to turn off DOF. Let's look at how to use it!

Version 1.5 Note: DOF was previously turned off by default, and turned on with two Render Mode settings. New to version 2.0, the DOF render is now automatic. You will notice Depth of Field referred to in some Particle Type options with a 'DOF' indicator.

Tutorial: Simulating Depth of Field
Make a new comp (Ctrl/Cmd-N). Use the NTSC D1 Square pixel preset with frame rate at 30 fps and the duration 5 seconds long. Create a Solid (Ctrl/Cmd-Y), click the Make Comp Size button to make the Solid the same size as the comp, and click OK. Apply Effect > Trapcode > Particular to the Solid.
Make sure your Time Marker is at Time 0:00:00:00 (the first frame) of the comp. From the Particular preset popup, choose the preset 't2_Grid Across'. Now create a camera (Layer>New>Camera) and use the 28mm preset. Go to Time 04:00 (4 seconds into the comp) so you can see the particles.
Make the particles appear even closer to the camera. Choose the Particular layer and go to the Effects palette. Set the Emitter >Y Rotation to 44 degrees (it was 55 in the preset). You should see this:

Depth of Field rendering is turned on by default in Trapcode Particular. But the rendering will not happen unless is the Camera > Camera Options > Depth of Field switch is set to on.

There is a Depth of Field Type control below the Depth of Field options menu. When using a simple Particle Type such as Sphere as we are doing here, it does not matter if Square or Smooth DOF mode is used. That's why this control is grayed out for the Sphere particle. Note that the Glow and Star particle types will NOT render with depth of field.
Now we need to enable DOF on the AE camera. Select the Camera layer in the Timeline and twirl down its Camera Options.

Set Depth of Field switch to On. Set Focus Distance to 150 pixels. Set Aperture to 100 pixels. This setting is unrealistically high, but useful for quickly understanding how DOF works.

The Camera Options with the Depth of Field switched to On
Now you should see the partially blurred image below. You can change the Focus Distance slider values to see how the focus plane moves closer or farther from the camera.