How to really BLUR the background using DSLR camera?

I have a Nikon D90 that comes with a 18-105mm lens. How do I really, really blur the background?

Increase the aperature.

Use your widest aperture and zoom to your longest focal length.

There are four variables:

a) Decrease the f number
b) Increase your focal distance (up to 105mm) if you can
c) Get as near to your subject as you can
d) Crop your image as much as you can

Blur = decrease depth of field
http://en.wikipedia.org/...h_of_field

Marie, you should begin by reading a basic photography manual (not the users manual) in order to understand the function of basic camera settings such as aperture and focal length. You can't learn photography by just asking questions here by itself. Once you read a basic photography manual, you'll learn a lot faster AND you'll get more facts and details than what people have given you here so far.

The blur you're talking about is called a shallow depth of field. You control depth of field with your aperture, length of your lens and distance from lens to subject.

To get maximum depth of field (everything in the picture in focus) you stop down to f22, use a wider focal length (closer to 18mm) and shoot your subject far away, such as at infinity.

To get the minimum depth of field (shallow, lots of blur and only a little bit in focus), you open your aperture to whatever the maximum is, closer to f2, use a longer focal length (closer to 105mm) and shoot your subject very close, such as three feet.

Furthermore, when you talk about lenses, it's of very little use to call them by their length alone. The proper way to refer to a lens is by it's length, maximum aperture, brand and model. An example would be a Nikkor 105mm 1.8 AIS.

The lens that comes with the camera is not very good at creating a very shallow depth of field because it does not have a very wide maximum aperture. A wide maximum aperture (referred to as a fast lens) is f2.8 - f0.95. Fast lenses produce more out of focus blur than slower lenses like yours. Zoom lenses are also not usually very fast unless you pay quite a bit of money for them. Prime lenses can be faster, cheaper and produce the very shallow depth of field you are looking for.

Still, you can get more shallow depth of field by following the instructions above. Of course, you will have to be in manual or aperture priority mode, not full auto or program.

Good luck! And go read that basic photography manual!