Shallow Depth Of Field : First we create a layers which are near to camera from front to back. Horse head is near to camera and then lady on the horse and back background. Now see how i will create depth map for it.
Step:1) Create Depth Map
Select quick mask [Q] Take hard brush [B].
Paint on subject near to camera.
Similarly now create second third nearest subject.
create black layer and drag down to below layers.
Now leave near to camera 1 layer and reduce the opacity to 50% for remaining layers.
Step:2) Add Depth Map layer to Channels
simply merge all layers and make it one layer.
Add some blur to layer to make edges smoothen.
now copy black and white depth map layer and paste it in channels.
Step:3) Apply lens Blur
Now apply lens blur and select depth map what we just created
if u want effect more smoother then add more guassian blur to depth map
Before & after results