Thursday, March 31, 2011

How do I make my background blurry?

That's a question I get asked a lot. The photographic term refers to DEPTH OF FIELD (or DOF).
Here's an example:
Here's what Ansel Adams said about DOF in his book The Camera:
"We can achieve critical focus for only one plane in front of the camera, and all objects in this plane will be sharp. In addition, there will be an area just in front of and behind this plane that will appear reasonably sharp… This total region of adequate focus represents the depth of field."
 The APERTURE of your lens controls depth of field (DOF)...
  • Aperture is the hole in the lens that light travels through to reach the film or camera's digital sensor.  
  • Aperture is comparable to the iris of your eye which controls the size of the pupil
  • Aperture is measured in f-stops
  • Wide aperture lets in more light
  • Narrow aperture lets in less light
  • Wide aperture (smaller f/# such as f/2) = shallow DOF
  • Narrow aperture (larger f/# such as f/22) = deep DOF
To get a shallow DOF set your camera to Aperture Priority.  (This means YOU set the aperture and the camera will choose the shutter speed to obtain proper exposure.  For Canon cameras Aperture Priority is Av on your camera mode dial.) Set the aperture to the lowest possible f-stop # (such as f/3.5).

In order to obtain a very shallow DOF you must have a relatively "fast" lens.  Your kit lens (the lens that came with your camera) may not be fast enough - meaning, the aperture may not open up wide enough to give you a really shallow DOF.  In that case, you might want to consider a "Nifty 50" lens. (article coming soon... in the meantime, just google your camera manufacturer plus 'nifty 50', example: "canon nifty 50")

TIP: If you have trouble remembering that a wide aperture (such as f/2) is a small f-number and a narrow aperture (ex: f/16) is a large number try thinking of it as a fraction. Just as 1/2 is larger than 1/16th, f/2 is larger than f/16.  

There are two other things that effect DOF:
1) Distance of subject to the lens. Get closer to obtain a more shallow DOF.
2) Focal length of your lens. Use a longer focal length for shallow DOF. (For instance, you will get more shallow DOF with a 200mm focal length than with 50mm.)   

Now you know!