This applies to photography as well as to 3D graphics.
The above image was created with two separate Reality/LuxRender renders blended using an awesome program called Combine ZM.
You can download the program free here. (No, I do not work for them nor am an affiliate, I just love the program. )
I was unable to get the gun and her face in focus without it. That’s physics for you.
Check out a simple tutorial on how to do this.