To understand exposure, you need to understand the six aspects of light that combine
to produce an image. Start with a light source—the sun, an interior lamp, or the glow
from a campfire—and trace its path to your camera, through the lens, and finally to the
sensor that captures the illumination. Here’s a brief review of the things within our con-
trol that affect exposure, listed in “chronological” order (that is, as the light moves from
the subject to the sensor):
■Light at its source.Our eyes and our cameras—film or digital—are most sensitive
to that portion of the electromagnetic spectrum we call visible light.That light has
several important aspects that are relevant to photography, such as color, and harsh-
ness (which is determined primarily by the apparent size of the light source as it
illuminates a subject). But, in terms of exposure, the important attribute of a light
source is its intensity. We may have direct control over intensity, which might be the
case with an interior light that can be brightened or dimmed. Or, we might have
only indirect control over intensity, as with sunlight, which can be made to appear
dimmer by introducing translucent light-absorbing or reflective materials in its path.
■Light’s duration.We tend to think of most light sources as continuous. But, as
you’ll learn in Chapter 12, the duration of light can change quickly enough to mod-
ify the exposure, as when the main illumination in a photograph comes from an
intermittent source, such as an electronic flash.
■Light reflected, transmitted, or emitted. Once light is produced by its source,
either continuously or in a brief burst, we are able to see and photograph objects
by the light that is reflected from our subjects towards the camera lens; transmit-
ted (say, from translucent objects that are lit from behind); or emitted (by a candle
or television screen). When more or less light reaches the lens from the subject, we
need to adjust the exposure. This part of the equation is under our control to the
extent we can increase the amount of light falling on or passing through the sub-
ject (by adding extra light sources or using reflectors), or by pumping up the light
that’s emitted (by increasing the brightness of the glowing object).
■Light passed by the lens.Not all the illumination that reaches the front of the lens
makes it all the way through. Filters can remove some of the light before it enters
the lens. Inside the lens barrel is a variable-sized diaphragm that produces an open-
ing called an aperturethat dilates and contracts to control the amount of light that
enters the lens. You, or the D7000’s autoexposure system, can control exposure by
varying the size of the aperture. The relative size of the aperture is called the f/stop.
(See Figure 4.3.)
■Light passing through the shutter.Once light passes through the lens, the amount
of time the sensor receives it is determined by the D7000’s shutter, which can
remain open for as long as 30 seconds (or even longer if you use the Bulb setting)
or as briefly as 1/8,000th second.
David Busch’s Nikon D7000 Guide to Digital SLR Photography98