Depth of Field 

Controlling Foreground/Background focusing:  The camera lens 'aperture' controls whether all or just part of the image will be in focus. Low aperture numbers (2; 2.8; 4; 5.6) create limited focus or 'depth of field'. Large aperture numbers (8; 11; 16; 22) give a greater focusing range or 'extended depth of field'.

Click to see some examples