Have you ever noticed that portrait and food photographers often use defocused backgrounds? The determining factor is called Depth of Field (DoF), the range of distance in front of and behind your focal point. Understanding the relationship between background and foreground is key to composition in photography. Much like the Rule of Thirds, depth of field can be a useful tool when you know how to use it.
Similar to how our pupils adjust the focus in our eyes, aperture is the opening of the lens that controls the depth of field in your photo. A wider aperture (F4.0 and wider) allows more light into your lens and makes your DoF shallower, creating a defocused background. A narrower aperture (F8.0 and narrower) reduces incoming light. It makes your DoF deeper and keeps more of your scene in focus.
WHEN SHALLOW DOF IS BETTER
Food photography and portraits are two specific instances where you want to use a shallow DoF since the photographer’s goal is to highlight the subject. Most photographers like to use a wide aperture to separate their subject from the background. Lenses with wider apertures are able to create shallower DoF, providing more separation between your subject and the background.
But keep in mind that depth of field is relative. The closer you are to your subject, the shallower your depth of field is going to be. Shooting with a F4.0 aperture setting and standing 50 feet away is going to separate your subject less than if you were standing 5 feet away. On the other hand, you don’t want your depth of field to be so shallow that only part of your subject is in focus–unless it’s intentional.
THE BIGGER (DEEPER DOF) PICTURE
Your aperture settings tend to matter less when you’re shooting landscape photography or situations where you’re further away from your subject. In the photo above, notice the depth of field as shown by the edge of the fence. The leaves and barn in the background are still in focus because the photographer was far enough away that the separation isn’t as dramatic.