Fog and Light Correction
Depth is always rendered and can be found in Render > BPR render passes (Fig.27).
The top layer is a constant color (blending mode Normal) with the inverted depth as a mask. It is a very subtle effect with low opacity.
The lower layer is a constant color (blending mode Normal) where I painted a mask to darken the torso in order to bring out the head visually. The reason for not multiplying this layer is to not reach black in the image. I prefer to have my lowest values around 2-5 in a dark image (Fig.28).
The current pass can be seen in Fig.29 and the combined passed in Fig.30.
Time to add lens blur in Photoshop. In order to do this you need to bake/combine all layers into one. We want to keep the order of the old layers so we can go back and do changes.
Combine: Create a new layer on top of the entire stack and press Ctrl + Shift + Alt + E to merge all visible layers into the active layer.
Put depth pass in a new channel: Copy the layer with the rendered depth pass. Go to Window > Channels and create a new channel. Then paste the depth information into this channel and name it "Z" or "Depth" so you know which channel is which (Fig.31).
Apply lens blur: Select the combined layer and choose Filter > Blur > Lens Blur. When the new window opens you should choose the channel you created as a source. Photoshop will blur depending on the luminosity in that channel. A good way to choose blur focal distance is to click on the image itself. Radius sets the amount of blur. There are a lot of other variables that you can play with to get the look you are going for. Generally you should apply a bigger amount of blur if the camera is really close to your model (like a close-up on a flower or bug) (Fig.32).
Fig.33 shows the model before the lens blur and Fig.34 shows it after.