This is a quick technique for introducing depth of field in post.
The image on the right has had a slight amount of depth blur added in photoshop.
Compared to the direct color render below it has a subtle imperfection that makes it appear more real.
The blur is controlled by a greyscale depth image that was rendered along with the final color image.
Below I'll go through how to set this up in Photoshop.
Unprocessed color rendering.
Greyscale depth rendering (Zdepth).
Open your image in Photoshop and make the Channels Palette visible.
Open the depth image and drag it onto the color image holding shift.
This should snap the image to the frame.
With the new layer active control click the RGB channel to make a selection based on its greyscale value.
Hide the new layer by clicking the eye icon.
Select and doubleclick the background layer to unlock it.
Click the layer mask icon to make the active selection into a mask.
Right Click the new layer mask and choose "Disable Layer Mask".
Go to Filter > Blur > Lens Blur.
Under Depth Mask choose "Layer Mask".
Click in the image where you want the focus to be.
By adjusting the sliders you can add noise and burn the image to your own taste.
Keeping it subtle is the key to succes.
Click OK when you are happy with the result.
In this final image I have gone all-in and also added a "red shift" effect to make it "hyper real".