To put into simple terms, its a copy & paste effect. In reality its far more complicated than that!
For example, when someone is standing on the street, what makes them appear "grounded" if you will to that spot?
Its all about lighting. When I create a composite or put someone into a background I need to make it look real, which means creating shadows and highlights to ground the subject into that setting. Not only do I need to create shadows in the background, but often times I need to create shadows on the subject. Here are some before & after examples to give you a better idea of how I create composites!
To see more examples of my composite work, visit my webpage
Family | Maternity | Newborn Photographer.