In photography, depth of field refers to the subject matter appearing in focus while objects in front and behind it are blurry. This effect gives a sense of 3D space and realism when used in a 2D graphic. In Photoshop, you can blur the foreground (top layer) and background (bottom layer) to achieve this effect artificially. The main drawback is: if there are many layers, then you may have to adjust each layer according to its distance from the subject. The further the object from your subject matter, the blurrier it gets. This could really take a lot of time and hinder production.
If you are a member of Adobe Creative Cloud, you can download Adobe After Effects and its tool that should save you time in achieving the depth of field effect. Adobe After Effects is not just for animation, but a great companion tools to Adobe Photoshop for creating special effects in 2D graphic.
To create depth of field in Adobe After Effects, follow these simple directions:
1) Import your Photoshop PSD file into Adobe After Effects. All of the layers in Photoshop are maintained in After Effects.
2) Separate the layers into 3D spaces by moving the Z axis — making sure you turn on the 3D Box Symbol in each layer.
3) Add “Camera” into the layer with “Depth of Field” being checked.
4) Set the “Aperture” to 600 pixels
5) Move the “Focus Distance” to the object where you want to focus. The rest of the surrounding objects will be out of focus based on their distances. No need to manually adjust each layer.
6) Export the still frame back to Photoshop layers and you are all set (Composition > Save Frame As > Photoshop Layers…).