Controlling Depth of Field

The majority of people who take photographs do not understand the importance of depth of field. By definition, depth of field is the focal area that is acceptably sharp in each photograph. This distance of what is sharp could range from a fraction of an inch to hundreds of feet. There are four important factors that control the area of sharpness. They are the focusing setting, aperture size, focal length, and distance of the subject to the camera. Many imagine that they would want everything in the picture to be equally sharp throughout the entire picture. While this is true for many landscape pictures, many other types of photography are actually more visually pleasing with a narrow area of focal sharpness. By making only a select portion of a picture sharp our eye naturally knows what is being emphasized.
Narrow Depth of Field
Wide Depth of Field
Focusing Setting

The optics involved with focusing is a fascinating study all by itself, but when dealing with depth of field, the setting of the lens’ focal point is critical. The focal point is the exact distance set on the lens. Whatever the focal point is set at will always be the sharpest portion in the scene. The average focusing distance usually ranges between several feet to infinity. Most SLR lenses show the distance in feet or meters on the side of the lens. From the initial focal point, one third of the range in front and two thirds of the range in the back of the scene will be the depth of field. For example, if we set our lens to focus at 10 feet the acceptably sharp range could be 5 feet to 20 feet. The total distance range of what is sharp is determined by several other factors that are listed below. 

Aperture Size 

The aperture works the same way as the human iris regulating the amount of light that reaches the image recording sensor or film. The term f-stop represents the size of the aperture opening. As the f-stop number becomes larger, the size of the aperture becomes smaller. As the aperture becomes smaller, the amount of depth of field increases in the photograph. For example, a photograph at f/4 will have a narrow depth of field (sharper focusing) than a photograph at f/16. 

Focal Length

The focal length is the physical length of the lens to the focal plane (sensor or film back of the camera). A larger focal length will make the subject larger in the frame. If you stand at the same location and zoom in (use a bigger millimeter number), the picture will have less depth of field than if you zoomed out (use a smaller millimeter number).

Distance of the Subject

If you use the same focal length and aperture size and only change the distance of the subject and focusing, the amount of depth of field will also be different. As the distance to the subject becomes closer, the depth of field becomes narrower. This factor becomes extremely critical when doing macro photography. Most close-up and macro photography can have less than an inch of focus range that is completely sharp. The reverse is also true. When photographing landscape photographs, the vast distance allows for much more depth of field.

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