Depth of field technique

Three main factors that can be used to control depth-of-field1 The aperture. 
There's a simple, direct relationship between aperture and depth-of-field - the smaller the aperture, the more extensive the depth-of-field. 
If, however, you want to concentrate attention on just one part of the scene, and throw the rest out-of-focus, you should select a large aperture. Exactly how large this can be depends on the maximum aperture of the lens you're using

2 The focal length of the lens.Attach a wide-angle lens and you'll benefit from extensive depth-of-field, which makes it easy to keep everything in focus. The wider the angle-of-view, the greater the depth-of-field. The longer the focal length, the more restricted the zone of sharpness is.

3 The Camera-to-Subject Distance
For various technical reasons, the closer you get to the subject the more limited the depth becomes. In fact, when shooting close-up subjects it can extend to just a few millimeters in front of and behind the subject.