DEPTH OF FIELD

Shallow depth of field. : It means subject is in focus and background is out of focus. 

There are THREE things that control DEPTH OF FILED. 

1. Aperture 
2. Focal Length
3. Distance from Subject.

1. Aperture: To get a shallow depth of field, you need a wide (large) aperture opening (a small aperture value). The larger the opening, the shallower the depth of filed.  

2. Focal Length: Focal length also have a great impact on depth of field. When focal length increases,  the depth of field decreases. 

3. Distance from the subject:  The closer you are to your subject, the shallower your depth of field will be.