Tutorial: What is Depth Of Field?
There is bound to be someone who is going to point out something that isn’t technically accurate with this post, be it with my definitions or diagrams. So, before we start, I would like to state that my sole intention is to simplify, as far as possible, some of the fundamentals of photography. My intention is not to swamp the reader with technicalities but rather to outline the basic principles.
What is Depth of Field (DOF)?
Let's start by talking a little about focus. Before you take a photograph, the first thing that you need to do is make sure that your subject is ‘in-focus’. To do this, you would usually turn the focus ring on your lens or half press the shutter button on your camera.
When you focus the lens, you are moving what’s known as the focal plane. This focal plane runs perpendicular to your camera's sensor or film. This focal plane carries with it an area which appears to be in-focus. The size of this area is known as the ‘Depth of field’.
The easiest to imagine how the depth of field works is to think of the depth of field as being a liquid and light as a container. If we pour this liquid into a glass, then stretch it, we alter the level of the liquid.
This analogy is true when looking at the relationship between light and depth of field with a camera. If we change the angle of the light reflected from our subject (incident light rays) the depth of field is affected in the same way that the liquid was changed above. To change this angle, we open or close the aperture.
Diagrams exaggerated for illustration purposes.
How can we control the Depth of field?
Three factors affect the depth of field:
1) The size of the aperture.
The simplest way to remember how aperture affects the depth of field is to say:
- The bigger the f-number (i.e. f/32), the smaller the aperture, and the more will be in focus.
- The smaller the f-number (i.e. f/1.4), the larger the aperture and the less will be in focus.
Many 'diehards' will snivel at the simplicity of the description above, but this is what’s happening:
2) The Focal Length of the lens.
The ‘Focal Length’ of a lens is what determines whether it is a 16mm, 24mm, 28mm, 35mm, 40mm, 50mm, 85mm, 100mm, 120mm, etc.
These numbers represent the distance between the cameras sensor and the beginning of the lens. Moving the lens closer to the subject changes the angle at which incident light enters the camera, thus altering the depth of field.
With the camera in the same position in both examples and with the same aperture, we can see how the DOF is affected by the change in lens size.
3) The distance between you and your subject.
The effect here works the same way as the example above; only the camera is the one being physically moved. Moving the lens closer to the subject changes the angle at which incident light enters the camera, thus altering the depth of field.
With the camera in the same position in both examples and with the same aperture, we can see how the DOF is affected by moving the lens closer to the subject.