Depth of Field in photography is the distances, from the nearest to the farthest, where the image is acceptably sharp. The focusing system of a camera will focus on a specific object. From this object, depth of field extend about one-third in front of that object and two third further away from it.
This object is on a plane of focus where all objects are sharp. The plane of focus is generally parallel to the sensor plane. On wide-angle lenses, because of curvature of some lens element, the plane of focus might look as a sphere.
Multiples variables are impacting depth of field.
- A shorter focal length as found in Wide-Angle Lens will lead to a greater depth of field.
- A greater subject distance will create a greater depth of field;
- A smaller sensor size is also responsible for greater depth of field;
As a result of these properties, a greater magnification, which is the result of increasing the focal length and reducing the subject distance, leads to very shallow depth of field.
Another resulting impact of these properties, very small sensor, like those found on cellular phone, will produce a very large depth of field. In most circumstances, all objects in the scene will be in focus. This is not always desirable.
Advances techniques in photography leads the photographer to decide what will be in the sharp zone and what will be out of the sharp zone. This will in turn, naturally lead the viewer, on the sharper areas of the picture first.
Depending on the blur properties of other objects in the pictures, they might be recognizable or not. The viewer is then drawn then to the recognizable objects. The other areas of the picture will have no interest and will push back the viewer to sharp and recognizable objects.
As you can appreciate, advance use of depth of field in photography can help the photographer create a picture that lead the viewer from one area to another. In order to do so, the photographer might rely on experience or use tools to calculate the exact depth of field based on the above parameters. The properties of the out of focus areas cannot be calculated though. Only experience will help the photographer predict this. When taking the picture, using live view on recent camera, might help appreciate the properties of out of focus areas. This will be an approximation only since the LCD at the back of the camera is too small to truly appreciate it.
You have multiples options if objects in your scene are not in the proper sharp zone:
- Changing the aperture to increase or decrease the depth of field;
- Moving objects around if possible;
- And moving the camera around the subject.
These three options create hundreds of composition possibilities. Taking the time to look at your subject and test the various combinations will lead you to great pictures.
Creating a picture is the art of putting together in the frame, the various objects that will lead the viewer from one part of the picture to others of that same picture. The resulting viewing experience creates an emotion.
A few years ago, there was a desirable look where the depth of field was very shallow. This created interesting pictures and high demand for full frame camera and large aperture lenses. These days, the look is moving to a sharp zone big enough to include all important element and pushing the rest of the picture in the blur zone. This involves trial and error, calculation and moving objects, the camera or changing the aperture to meet the desired effect.
The blur zone quality, also called bokeh and the impact of the aperture blades on the blurred zone when shooting at a smaller aperture will create small differences. These differences shall not impact the picture quality.
Only photographers care about the blurred zone quality. In some situation, for examples small lights in the background of the foreground, cheap lenses can produce distracting patterns in that zone.
About this image:
- Canon EOS 7D
- Canon 100mm f/2.8L Macro IS
Although chess pieces are usually very small, these pieces where three feet tall. In this case, the magnification was not the factor leading to the depth of field. This is why the sharp zone and recognizable zone are so big compared to the same picture, taken at a shorter distance with normal size chess pieces.