Page 1: Depth of Field - the science bit. Page 2: Depth of Field - Close-ups & Bokeh.

Depth of Field

What is it and what can you do with it?

Depth of field (DOF) is the amount of your picture that is acceptably sharp from foreground to background. Take a look at the two photos on the right, you can see that, in the top picture much more of the ruler is in focus than in the bottom picture. Although there is no exact point where the ruler moves from in focus to out of focus, there are areas in the photo, especially the bottom one, that we would consider sharp enough, and areas we would consider definitely out of focus.

The Science Bit

The amount of DOF that you get in a photo depends on several factors which we will discuss in this article, the main one being the aperture setting. The wider the aperture the less DOF you get. The top photo on the right was shot at f22, the smallest aperture available on the lens I used, and the lower picture was shot with the aperture wide open, at f2.8.

Aperture settings are rather strange things, the larger the aperture, the smaller the number that describes it. f2 is a larger aperture (hole) than f11. This is because the number is a fraction. Basically it describes the size of the hole as a fraction of the focal length of the lens. So for example, f11 could describe a 9mm aperture in a 100mm lens, or an 18mm aperture in a 200mm lens. The reason for describing things in this way is that, in both cases, the same amount of light reaches the sensor, this is because the larger hole is further from the sensor. To understand this better, have a look at my inverse square law tutorial where there is a diagram that shows how it all works. The principle works the same both sides of the lens (or, if you get confused, just take my word for it). See more on apertures.

Using Depth of Field Creatively

Changing the aperture setting will, of course, affect the exposure, so you have to adjust the shutter speed or ISO setting to compensate. The best camera mode to use when you want to experiment with different apertures is 'aperture priority' or Av. In this mode, you set the aperture manually and the camera sets an appropriate shutter speed to give the correct exposure.