Nikon manual David Busch’s Nikon D7000 Guide to Digital SLR Photography

98 David Busch’s Nikon D7000 Guide to Digital SLR Photography

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 aperture that 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.