Aperture Blur Definition

Aperture Blur
This is the focus of the image. By using specific settings, you can adjust the framing's blur, foreground and background to create a depth of field. 
For a blurry background you want to create a shallow depth of field. Use the 50mm lens and in AperturePriority mode (AV on Canon, A in Nikon), set a large aperture, which is a low f-number, around f/1.8 or f/2. Put your subject as far away from the book case as possible, 10-15 feet if the room allows.

These images link to the subject matter of aperture as you can clearly see the two different apertures within these images because of the different depth of fields.






Main F/stops:
F/2 - F/2.8 - F/4 - F/5.6 - F/8 - F/11 - F/16 - F/22
The lower F/Stops would focus on one small part of the image making a small part focused and sharp and the other part of the image blurred. This is a shallow Depth of Field.
The higher F/Stops would allow the entire image to be in focus with hardly and of the image blurred. This is a wide Depth of Field.

Aperture is extremely important when shooting an image. It can allow for a specific subject to be sharp and for a background to be out of focus/blurred. This allows for the audience to be drawn into the subject only and to not be distracted by the background.