Differential focus is a term used to describe how an image has been designed by the photographer. The photographer intentionally keeps specific aspects of the image in sharp focus whilst allowing other parts to fall out of focus.
This is usually done to give separation between the subject and the background, so the subject literally pop out of the image at the viewer. However on occasion you may wish to have the main subject somewhat out of focus and have a secondary subject sharp. A good example of this is in wedding photography when photographing the bride and groom with the wedding car. The front of the wedding car will be in sharp focus whilst the newly weds will be further back in soft focus. This creates atmosphere in the image and is much more artistic and powerful than simply having everything in sharp focus, which will create a flat appearance with litter impact.
Using differential focus is another tool in the photographer’s arsenal to create the illusion of a 3-D image in a 2-D print. Other tools include placing your subject in an environment with a foreground and a background. Using lines either real or implied also gives cues to the viewer about the 3-D environment.
Differential focus actually replicates the way we view the real world. You can easily test this by looking at an object that is very close to you and then become mentally aware of the surrounding areas, which will be out-of-focus.
You can also test how much depth-of-field you have
Hold both of your hands around 12 inches, 30cm, away from your face with your forefingers extended. Focus on your forefingers. Now slowly move one of your hands further away until you become aware that it has started becoming less sharp. The distance between your fingers is effectively your depth-of-field for viewing objects that are close to you.
How to we create differential focus?
Differential focus is created by controlling the depth-of-field, which is the area considered acceptably sharp. Depth of field is controlled by three components; lens aperture, lens focal length and distance between you and the subject.
See http://en.wikipedia.org/wiki/Depth_of_field for formulae for calculating precise depth-of-field in specific circumstances.
However from a practical point of view all you need to know is you can reduce the depth-of-field by:
- moving closer to your subject
- using a longer focal length lens
- using a wider aperture
Using a combination of these will significantly reduce the depth-of-field.
A great example of this is a bridal portrait taken with a 70-200 lens at f/2.8. If you’re around 2 metres from your subject with the lens at 200mm and set to f2.8, you’ll have a depth-of-field of approximately 2cm! This is when focus becomes critical. Focus must be on her eyes and the camera parallel with her face to render the eyes and lips in focus. This then creates a natural soft focus effect on her skin removing and softening any blemishes. The background will then be rendered completely out-of-focus – the quality of this is referred to as bokeh.
The effect of these depth-of-field controlling factors is the film size or size of digital sensor. The larger the size of the medium capturing the image the less depth-of-field is realized for any given combination of the variable factors. In other words all things being equal the larger the film/sensor the shallower the depth of field; and conversely the smaller the film/sensor the deeper the depth-of-field becomes.
Therefore it is difficult to use differential focus with cameras that have very small sensors such as point and shoot pocket cameras and mobile phone cameras. So it is ironic that their images often appear unsharp everywhere! Perhaps this is caused by craming so many pixels in a tiny area, which increases the digital noise to such a level that it is unable to render anything sharp.