Production: Understanding Depth of Field

Properly understanding depth of field is part of what makes a great photographer.  This understanding comes from practice with cameras.

Space

"Depth of field" is a term that describes how much of the space in your image is in acceptable focus. For example, you are on a football field and there is a player at each 10 yard line. If you set your focus to the 50 yard line, then maybe you will also get the players at the 40 and 60 yard-lines acceptably sharp. However, the players at the 30 and 70 yard-lines are soft. This means that the space between the 40 and 60 yard-lines represents your depth of field.

Focal Length

There are three factors that affect the depth of field in your image. The first is your focal length. If you put on a wide lens, such as an 18mm, and point the camera at a book that is 3 feet away, your entire book will easily be in focus. However, put on a long lens such as a 100mm and leave the camera position the same. You will obviously have a much tighter image, but you may not be able to get the entire book in focus with a single setting. The greater your focal length, the less depth of field you get.

Distance

The next factor is distance from the camera to the subject. In the first example with the football field, you were able to get 3 players in focus when setting the focus to 50 yards. However, if you were to then set your focus to 20 yards, you may only be able to get one player in focus. This would also mean that with your focus set to 80 yards, you might get 4 or 5 players in focus. The farther the distance from the subject to the camera, the more depth of field you get.

Aperture and Aesthetic

The final factor is the size of your aperture. With your aperture closed down to a F22, you will have plenty of depth of field. However, filmmaking has developed an aesthetic of isolating the focus using narrow depth of field. So it is usually ideal to shoot at a wider aperture such as a F2.8. This way, you can keep only a portion of the image in focus, thus drawing the attention of the viewer to the subject. To control this in high light situations such as bright sunshine, neutral density filters are often used. These are shaded pieces of glass put in front of the lens in order to reduce the amount of light entering. This allows you to open up your aperture to acquire a narrower depth of field.

There are many ways to achieve varying degrees of depth of field. How much is an aesthetic choice. Use the known relationships to reach the desired effect.

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