Cinematography: Depth of Field

We joined the photography students again to learn about Depth of Field (DOF).  Essentially, Depth of Field is a visual phenomena which occurs in film or photographs.

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In photography the depth of field is mostly affected by aperture.  This is demonstrated in the exercise below.  Each of these photos were shot with a different aperture, which was balanced to the correct exposure by adjusting the shutter speed.  The change in aperture causes different areas of the scene to sharp, or in focus.  A low aperture causes a shallow depth of field, while a high aperture causes a long depth of field.

This can be seen most clearly when comparing particular photos in the sequence.

The other change that can be seen in these photos is how movement is recorded.  This is due to the changes in shutter speed, as movement will appear to blur more the longer the shutter is open for. 


Applying this to film:

Shutter Speed is less of a variable in film because usually film will be shot at 1/50 shutter speed or 180 degree shutter angle.  Referring back to the exposure triangle we can see that the main variables must then be aperture and ISO.  When manipulating DOF in film the ISO and the sensor size of the lens become more important (the smaller the sensor, the greater the DOF).

The other key variable in causing changes in the depth of field is the use of Neutral Density Filters.  ND filters reduce the amount of light entering the lens, but do not affect the colour temperature.  This allows for the lowest aperture to be used while maintaining correct exposure.  

Depth of Field is usually used in film for stylistic or narrative purposes.  It can make something look visually appealing but can also be used to manipulate the audiences gaze to where the director wishes them to look, or even to conceal important hints.