Moving Images, Understanding Media

128 Moving Images: Making Movies, Understanding Media

Figure 4-8 Actress Aunjanue

Ellis is in focus while the

bright background outside is

out of focus in this still from

Side Streets. (Photo by Seth

Rubin, Courtesy Market Road

Films)

Factor Two: Exposure

Exposure is the process of allowing light to make contact with a fi lm negative

or other light-sensitive recording surface for a specifi c amount of time. Two

primary features aff ect the amount and quality of light necessary to produce a

visible image. One feature—the aperture—is the adjustable opening that allows

the light to enter the lens. Th e other feature—shutter speed—is the amount of

time that the opening admits the light into the camera before closing.

Setting the Iris

Aperture is the opening in the lens regulated by the lens iris (or diaphragm

iris) to allow a specifi c amount of light to pass through the lens. To admit

more or less light by increasing or reducing the aperture of the lens, the iris

is adjusted in precise measurements known as f-stops.

Th e f-stop is a number that expresses the relationship between the focal

length and the iris of a lens (from the optical concept of f-number). Th ese

numbers make up a logarithmic scale because each single step made to open

the lens by one f-stop doubles the amount of light passing through the lens.

To understand this, think of a fl ashlight being used to light up a pitch-

black cellar. If you turn on a second fl ashlight, you double the amount of

light in the space. To increase the light by the same proportion as when

you lit the second fl ashlight, you will need to turn on two more fl ashlights,

or double the amount again. If you want to increase the light by the same

proportion again, now you will need to turn on four more fl ashlights. Many

of the principles of light and photography work on concepts of logarithms

such as this example.

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