When we take a photograph the aperture (f/stops) controls how much light is allowed into the camera. It also controls the Depth of Field (DoF). The point at which the camera is able to get focused (in the foreground), through to the point where it loses its sharpness in the distance is called DoF. In other words the DoF is where the lens can focus.
Reviewing Depth of Field
A wider aperture lets in more light. However, the wide aperture creates a shallow DoF – the distance between points where the focus is sharp is close together.
When the aperture is small only a small amount of light is allowed in to the camera. In this case the depth of field is deep – the distance between points where the focus is sharp are far apart.
The terms Shallow/Deep can be changed for thick/thin or narrow/wide. The words are just describing the relative distance in which the focus is sharp.
The video below shows three ways you can affect the DoF. I have mentioned aperture above. The other two are revealed in the video. After the video are my tips for today.
As you can see from the video the three methods for controlling depth of field are…
- Changing the diameter of the aperture
- The distance you are from your subject
- The lens you are using
Tip no. 1
Changing your aperture size affects the near and far edges of focus differently. As the aperture is made smaller, the DoF extends faster into the distance than it does in the foreground. The DoF extends 2/3rds further away in the distance and only 1/3 toward the camera in the foreground. When placing your subject you should remember this. You will need to move the subject much further back to widen the DoF, than you would need to bring them forward to narrow the depth of field.
Tip no. 2
The further you are from an object the wider the DoF can be. With a 50mm lens at f/1.8 (Wide aperture) you may not get enough DoF to have a persons whole head sharp if they are near to you. The face may be in focus, but the hair and ears may well be out of focus. The DoF is only a few centimeters. If you keep the focal length the same, but move the person further away from you, the DoF increases. So, if you are having trouble getting enough depth of field when close, move back a bit. Simple!
Tip no. 3
The type of lens you are using does not actually affect the DoF. In fact it is the way most people use particular lenses that give the impression of a changed DoF. Using a wide angle lens people tend to view a wider perspective and so will tend to narrow the aperture to create sharpness in the distance. When using a telephoto lens they tend to use it to magnify. In doing so they magnify the out-of-focus area (bokeh) in the distance which makes it appear the DoF is closer. In other words, the degree of magnification causes an apparent (not actual), closer DoF.