Composition: Part IV Isolating the Subject - Page 3
© Wendy Folse
The distance at which you focus on an object also controls the amount of depth of field. The farther away you focus, the larger the depth of field. The closer in you focus, the shallower the depth of field. There is a rule of thumb that says that from the focal point you have about one-third of the distance in front of the focal point and two-thirds of the distance behind the focal point. When shooting landscapes at small apertures this factor is not critical because everything will be reasonably sharp. However, when working with smaller objects at closer distances and using larger apertures, this does become very critical. It helps to remember that a photograph is a one-dimensional representation of a three-dimensional scene.
For example, say that you wish to take a close-up portrait of a person and you want to blur the background. After you work out the correct lighting, you decide you need a relatively larger aperture in order to blur the background but you don't want the face to be blurry either. What do you do and where do you place the focus point in order to assure that the face is in sharp focus? Think of the human head as being divided into three planes, from front to back. The nose is in the first plane, the eyes fall in the second plane and the ears and back of head are in the third plane. If you chose to place the focal point on the nose, one third of the distance in front of the nose would be in focus and two thirds of the distance behind the nose would also be in focus. That should mean that the entire head would be in focus. Maybe, maybe not depending on how large an aperture you are using.
By choosing the nose as the front plane you are accenting it and moving the other planes backwards. Most people would not find this pleasing. So where else could you place the focal point? The desired place to focus on a human face is the eyes. The reason is the same as mentioned above. The human head is divided into three planes with the eyes being located in the second third. This means that if you focus on the eyes, one third in front (the nose), and two thirds behind (the ears and then hair) will be in focus. The eyes are the critical points that must remain sharp.
