(Tip: you may want to click through to the image above once you've finished reading the rest of this tutorial, then click the download button to get the full effect).
In a nutshell, there are a number of factors that determine how much of a given scene your camera will render in sharp focus. Three of those factors are the length of the lens, the aperture inside the lens, and the distance at which the lens is focused. Generally, the following rules may be observed:
- The wider the lens, the deeper the apparent focus. The longer the lens, the shallower the apparent focus. In the image above, I've shot three scenes with three of my lenses, a 45mm (moderate wide angle), an 80mm (normal field of view), and a 150mm (moderate telephoto).*
- The wider the aperture, the shallower the apparent focus. The narrower the aperture, the deeper the apparent focus. I've demonstrated this by photographing each scene with four aperture settings for each lens: f/2.8, f/5.6, f/11 and f/22.
- The closer the plane of focus is to the camera, the shallower the apparent focus. The farther away, the deeper the focus. Each scene above, top to bottom, represents a subject focused at 5 feet from the camera, 10 feet, and 100 feet.
*A quick note about those lens lengths: the camera I used to create these images is a medium format digital camera. The "normal" lens length of 80mm has an equivalent field of view to a 50mm lens on a 35mm system, or a 35mm lens on a camera with an APS-C sensor. For reference, my moderate wide angle lens would be equivalent to about 28mm on a 35mm camera, while my 150mm lens would be roughly 95mm. In fact, format (sensor/film size) is the fourth determinant of depth of field, but since I only have the one camera, I can't demonstrate the difference between the smaller formats, the medium formats and the large formats. You'll just have to take my word for it that f/2.8 on my 80mm lens is shallower than f/2.8 on a 50mm small format lens.