Exposure Amount of Light and Speed of Shutter

Exposure = Amount of Light and Speed of Shutter

It's important to realize that exposure determines far more than just the tonal range in the image, and that even the assessment of tonal range is subjective. In Figures and , Figure features an optimum exposure, with strong tonal detail across the entire tonal range. Figure emphasizes the shadows and is more expressive. Both are acceptable, depending on your intentions.

Bench I
Nikon D2H
1/30 sec, f/6.7
Focal length 24mm

Bench II
Nikon D2H
1/80 sec, f/11
Focal length 24mm

This chapter looks at the variables that determine how an image is captured by a digital camera and how to manipulate those variables to achieve a wide range of results. Along the way, we'll look at ways of measuring light, light meter techniques, and even apply the classic zone system to the digital process.

The concept of exposure can be simply expressed as light's ability to describe a scene as it falls on a piece of film or a digital chip. Variations in exposure are determined by the amount of light, how long the lens is open, and the sensitivity of the film or recording chip. The amount of light is determined by the aperture or lens opening, which is measured in f-stops. The duration for which the shutter is open is measured in fractions of seconds (or whole seconds for long exposures), and is referred to as the shutter speed. The third variable in the equation is the sensitivity of the film or chip, which is sometimes referred to as the speed at which it records light. This sensitivity is described in an ASA/ISO rating.

The combination of these three variables determines the tonal range and sharpness for the image. If the aperture is too small, not enough light passes through the lens to capture the scene, and the image is dark and underexposed. If the opening is too large, the film receives too much light and the image appears too bright, with washed-out highlights and no shadow detail. Shutter speed plays an equally important role. If the shutter is too slow (open too long), you can let in more light, but the scene is blurry and soft. Fast shutter speeds freeze the action in crisp focus, but require a lot more light in order to render an image that's tonally balanced. And of course, the ASA/ISO setting for the film or digital chip is the baseline that determines the overall speed of the exposure.

PHOTOtip: ASA/ISO, F-stops, and Shutter Speeds

The easiest way to understand the relationship between the three primary exposure components is to think in terms of halves or doubling. As settings increase in any area, the amount of exposed light doubles or halves. This approach forms the basis of gaining reliable control over exposure.

For example, ASA/ISO ratings refer to the sensitivity of film (or the recording chip on a digital camera) and set a baseline for how fast the light in the scene is recorded. The standard ASA/ISO settings are 25, 50, 100, 200, 400, 800, 1,600, and 3,200, although many digital models don't offer any settings lower than 200. Each setting is twice as fast as its predecessor, as is reflected by the numbers themselves. (Some cameras include intermediate numbers between these settings, which do not reflect the doubling/halved equation described here.) The good news is that higher settings let you shoot in lower light conditions. The bad news is that as the settings increase, the image sharpness declines (gets more coarse and grainy).

Shutter speed ratings also follow the doubling/halved equation as they move up and down the dial. The good news is that lower settings (longer exposure times) allow you to shoot in lower light. The bad news is that at lower speeds, moving objects blur and overall image sharpness is subject to camera shake if you're handholding the camera.

F-stops are the most cryptic settings of the group, and although the settings seem arbitrary, they still follow the doubling/halved equation. The bad news is that as shutter speeds drop lower, so does the depth of field, which defines the range of focus within the image.

Because all these options follow the doubling/halved equation, they are somewhat interchangeable in how they impact the amount of light in the exposure. Increase the shutter speed by one setting or increase the f-stop, and the amount of light in the exposure is the same. Of course, you have to manage the side effects of each setting, taking the grainy resolution, blurred movement, or short depth of field into account for how it impacts the final result.

Each of these images was shot at f/4, but the shutter speed was set at 2,500, 1,250, and 640 from left to right, creating a one-stop difference between each image.

Another characteristic controlled by exposure is depth of field. Depth of field refers to the areas that remain in focus in front of and behind the primary focus point. Depth of field is covered in greater detail in Chapter 3, "Focus and Depth of Field."

In summary, exposure controls three primary variables: sharpness, tonal range, and depth of field. The "perfect" exposure is the one that manipulates these variables to serve your intentions for a given shot. Blurred subjects are dynamic and convey motion, sharp subjects convey depth of detail and information. Controlling the background and overall focus point plays a key role in overall composition and expressive narrative in the image. The perfect exposure is determined by the scene in front of the lens in combination with your artistic intentions.