Practical Artistry: Light & Exposure for Digital Photographers

20 Understanding Exposure

The Exposure Equation

The Exposure Equation

I shot the original photo used in this composite sandwich of endless doors in the officer’s

quarters in historic Fort Point in San Francisco.

I knew that for the photocomposite to work, I needed to preserve as much detail as possible,

as well as get as much of the image as possible in focus. The need for preserving detail meant

I needed to use the lowest available ISO (100). The need to keep as much of the photo in focus

(high depth of field) meant using the smallest available aperture (f/22). With two of the three

components of the exposure equation preselected, I chose the shutter speed (10 seconds) to

match the average light on the scene.

Photocomposite, 18–200 VR zoom lens at 95mm, 10 seconds at f/22 and ISO 100, tripod mounted.

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The exp

osure equation is another way of saying that

the light reflected by a scene towards a camera is cap-

t

ured at a given dynamic intensity by shutter speed

combined with aperture and sensitivity.

The phrase “at a given dynamic intensity” means the

average overall level of brightness of a scene, and im-

p

lies that this overall brightness is kept constant. If the

overall brightness changes, then of course so must the

related exposure settings.

As I’ve already mentioned, shutter speed is a unit of

time, often a fraction of a second, for example 1/125 of

a second, or 4 secon ...