Depth of Field
One of the most important things to learn in photography is the concept of Depth of Field. Depth of Field refers to how much of the picture is in focus. When you focus on a particular subject, other objects in the photo are going to be out of focus if they are at a different distance from the focused subject. A shallow depth of field means that only the subject is in focus while everything else is out of focus. A deep depth of field means everything is in focus.
Controlling Depth of Field
The Aperture is the size of the opening in the lens through which light enters the camera and exposes the film. The aperture also controls how much of the photo is in focus. If the aperture is small, everything will be in focus, while a large aperture will make objects, even slightly far from the subject, blurred. The aperture is usually expressed as an F-number and is a ratio of the focal length to the diameter of the aperture. When the camera is set to F/2, it means that the diameter of the aperture is half the focal length. At F/20, the diameter is 1/20th the focal length. Hence F/20 (with a smaller aperture) will given a very deep focus, while F/2 (with a large aperture) will give a shallow depth of field.
With a Large Aperture, objects get out of focus quickly
With a small Aperture, all objects will be in focus
Here’s another example. The focus of the lens is on the letter ‘J’. In the shot with a large aperture (small F-number), the rest of the keys quickly fall out of focus and are blurred. In the shot with a small aperture (large F-number), almost all the keys are in focus –
Using Depth of Field
So, now we know how to control the depth of field, but the question is “why is it important to control depth of field?”. Why don’t we want all our pictures to have a very high depth of field and everything being in focus? Well, it turns out that often times, the background to a photo can be very distracting. To reduce the distraction in the photos, a photographer will make the depth of field shallow. This is often used in Portraits and Macro shots. Experiment with different depth of field settings to see how it affects your photos.
Advanced Topic – Depth of Field and Relative Distance
The rate at which the objects become blur depends on the relative distance from the camera and the object focused on and the other objects. If the subject which is in focus is very close to the camera, then the sharpness will fall very rapidly with distance. For example, if the focused subject is 1 foot away from the camera, a object 1 feet behind it will be much more blurred than the same object when it is one foot behind the subject at 10 feet distance. This is what makes Macro photography so challenging. The following diagrams will illustrate this concept –