A blurry background, also known as shallow depth of field, is desirable in photography because it helps to separate the foreground from the background.
Creating a blurry background with you dSLR camera is so easy, and can be achieved through the following:
1. Wide Open Aperture (Small f/number)
The most important thing to remember about aperture is that a large aperture (represented by a small f/number) produces shallow depth of field, while a small aperture (represented by a big f/number) creates a large depth of field with much more of the image in sharp focus.
A large aperture (small f/number) allows more light to reach the sensor, allowing you to chose a faster shutter speed. Using a large aperture is especially useful when shooting in low light conditions, or when you want to create an artistic blurry background.
To create a blurry background, chose the smallest f/number possible.
Read this article for more about Aperture.
Vintage wine bottle, shot in low light with wide open aperture
2. Choice of Lens
It is important to remember that the lens you use, will impact the amount of blurriness / depth of field. A wide angle lens will produce far less blurriness than for example a macro lens.
Flowers in Jam Jar, shot with a macro lens
3. Distance to Subject & Distance from subject to Background
Try to position yourself as close to your subject as possible, and create as much distance between your subject and the background as possible. The further the background, the more blurriness you will achieve.
Viewing Station at Prague Castle
4. Zoom in
If you are using a zoom lens, zooming into your subject effectively compresses the elements of your image, making the background of you image far more dramatic.
Zooming in on Cinnamon Sticks
5. Sensor Size
The size of your sensor will also impact the final quality of your photograph. Using a phone camera (with a tiny sensor) will produce FAR less depth of field than a dSLR camera with a full frame sensor.
Once you start experimenting with shallow depth of field, you’ll notice another beautiful phenomenon even more gorgeous than a blurry background – bokeh. Bokeh (“blurred” in Japanese) are those beautiful out of focus circular discs that appear when your lens is set to a shallow depth of field.
Elize with Bokeh in the background