With a simple change in focal length, we know that we can drastically change and control the depth of field. In this lesson from The Slanted Lens, Jay P Morgan shoots a fashion portrait at the Vasquez Rocks in California. Not only does he cover how to control the depth of field, but you'll also learn how he uses this tip to enhance his work.
From Jay P: You can see from this comparison that the wider the lens the more depth of field you will have. Wider lenses give us more depth of field. Longer lenses give you a shallower depth of field. To achieve a shallow depth of field you need a longer lens with a smaller aperture number which gives us a larger aperture opening and less area of acceptable focus. A 200mm lens at f2.8 is going to give you a very shallow depth of field where as a 50mm lens at f2.8 still has a lot of depth of field. On the 5D mark 2 a 50mm lens at f2.8 twenty feet from the subject will give you 8.57 feet of acceptable area of focus. A 200mm lens at f2.8 twenty feet from the subject will give you 0.5 feet of acceptable area of focus. Even if you back away from the subject with a 200mm lens to 40 feet, the area of acceptable focus is only 2.04 feet. That is why it's easier to throw the background out of focus with longer lenses. There is no right or wrong choice when it comes to depth of field. It's a matter of what and how you want to communicate. There are many resources to get accurate information on Depth of Field. I use an app for the iphone called Depth of field master. Its a great app that lets you choose the camera lens and distance to get the depth of field. Here are two web pages that I have used as well that will give you the same information as the app.http://www.dofmaster.com/dofjs.html