Seeing the Light

 John Muir, the eminent naturalist and wilderness explorer, recognized the important part that light played in creating a memorable scene. In his book, The Mountains of California, he wrote, “It seemed to me the Sierra should be called not the Nevada, or Snowy Range, but the Range of Light.” During his many years of travels in the Sierra Nevada, he had numerous opportunities to see how the high country light could transform a landscape. He recognized that, as powerful as the mountain vistas were, the light, ever changing, filtered by mist, clouds, rain, and snow, added mystery and majesty to the jagged peaks and sheer walls of granite. In photography, light is everything. It brings a scene to life. It establishes a mood and influences the emotional impact of an image. Even photos made in the dark of night, far from artificial light sources, are the result of moonlight or starlight building up over time to form an exposure. The character and quality of light can have a great influence on a photograph, and understanding how a camera “sees” light is fundamental to producing a good image. One of the most important skills a photographer needs is the ability to visualize the photograph—seeing the picture in your mind’s eye and understanding how the camera will interpret the scene. Beginners see what they know is there and are often disappointed that the picture doesn’t convey the mood or meaning that they saw when they took it; photographers learn to see what the camera sees and to work with the light and camera controls to craft the image they see in their mind’s eye.

  Digital cameras offer an impressive array of automatic features and can almost always be relied on to produce a decent picture. But making a good photograph requires an understanding of the principles of photography and knowing how a camera translates the light in the scene into the photo captured by the image sensor. The difference between an ordinary picture and a good photograph is the difference between just pointing and shooting, and consciously working with composition, light, and camera controls to create a memorable image.
 
  Measuring the Light
 The proper exposure for a photograph is achieved through a combination of aperture, shutter speed, and the ISO sensitivity of the image sensor. A light meter, either in the camera or an external, handheld model, is used to determine the optimal exposure settings by evaluating how much light is being reflected back from the scene being photographed. Most cameras have builtin light meters that do a good job of calculating an adequate exposure setting, and higher-end cameras offer more sophisticated meters that produce excellent results. For photographers, this feature is definitely a case of “better living through technology”; you can concentrate more on the composition of the image and changing dynamics within the scene while knowing that you can rely on the camera’s light meter to do a good job in most lighting situations. Although you may trust your camera’s light meter when it comes to exposure decisions, it’s still important to know how the light meter evaluates light, so you can better anticipate how the exposure settings it recommends will affect the image.

    How Light Meters See Light
 The light meters found in modern cameras are very good at analyzing the light in a scene and selecting an aperture and shutter speed combination that will yield an image that is neither drastically underexposed nor overexposed. In most cases, the exposure may actually be quite good, but it’s not necessarily the best exposure for a given scene. A light meter doesn’t know what you’re photographing, nor can it determine if you’re using it the right way or even pointing it at the right place. A light meter doesn’t give you the correct settings to use; it just gives you the settings to create a certain type of exposure based on its very narrow interpretation of the scene before your lens. This limitation of the device is due to the fact that all light meters can see only luminance (brightness) in the form of how the light is being reflected from a scene. They can’t see color, evaluate contrast, or even tell the difference between black and white, so the reflected light from every scene they analyze is averaged into a shade of medium gray. A light meter’s view of the world is so limited that the gray tone seen by light meters is not just any gray, but a very specific, 18 percent gray. This precise percentage comes from the fact that most scenes tend to reflect approximately 18 percent of the light that falls on them. When you point a light meter at a scene—no matter if it’s a snowy hillside, a dark cave, or a casual portrait—and you take a reading, the meter assumes that it’s pointed at something that is 18 percent gray; the meter is calibrated to calculate an aperture and shutter speed that will record the average reflected luminance of the scene as a middle gray.

   Types of light meters
There are two types of light meters used in photography: those that are built into the camera and external, handheld units. Handheld light meters usually can be set up to measure the light as either incident or reflective light. Incident refers to the light falling onto the subject, and reflected is light reflected off the subject. When used in reflective mode, handheld light meters are very useful for taking precise reflected light readings in landscape photography. Special spot meter attachments with viewfinders can be fitted onto some handheld meters to provide the capability to take measurements from very small areas in a scene. This allows a photographer to precisely calculate the contrast range in a scene, make an exposure that will contain the tonal information needed to make the best exposure, and create a file that is more easily processed and will also print well.

    Metering Modes
With the exception of entry-level compact models, most digital cameras offer a choice of metering modes. Metering modes tell the light meter to analyze the light in different ways. The three most common are Matrix, Center-Weighted, and Spot . Working with the appropriate metering mode allows you to get the best exposure in a variety of situations. You usually select metering modes either from within the camera’s menu system or from a control button or selector switch located on the camera body.

     Matrix
Depending on your camera, this mode may be called Multi-segment, Pattern,or Evaluative metering. We feel it’s the best mode to use for most situations, and it’s the one that we use most often. The Matrix metering pattern divides the image into sections, or zones (anywhere from 30 to well over 200, depending on the camera), and takes a separate reading for each zone. The camera then analyzes the different readings and compares them to information programmed into its memory to determine an optimal exposure setting. High-end digital SLRs that offer a range of different autofocus (AF) zones will also factor the active autofocus zone into the metering pattern, giving that area of the image more weight in the final exposure calculation. In most cases, Matrix metering works very well, and we use it for most images simply because it does such a great job of calculating an optimal exposure, letting us concentrate on image making. Matrix metering may not be the best choice in all situations, and recognizing when it’s not can help you decide whether it’s time to use another metering mode or adjust the exposure. Common situations where Matrix metering may be fooled include heavily backlit scenes, bright areas such as snow or the beach, and dark subjects that you want to record as dark. To get a sense of how your Matrix metering system responds to different lighting situations, you should take a series of pictures in as many different lighting conditions as possible—a sunny day, a cloudy day, dawn, heavy shade, open shade, high contrast, low contrast, indoors—and carefully evaluate the results. Pay particular attention to the shots you take in very bright daylight where there is an extreme contrast range between deep shadows and bright highlights. This is the type of lighting that’s most likely to cause problems for a Matrix metering system. To be fair, we should point out that this type of lighting could cause problems for any metering system, but since Matrix metering bases its exposure on many different areas of the scene, it may not be able to distinguish which area of a high-contrast scene is important to you (remember that a light meter has no idea what you’re photographing). Your goal with these exposure tests is to try to determine how the meter handles the bright highlights and deep shadows. Does it tend to preserve good shadow detail at the expense of blown-out highlights? Or does it do a good job of controlling bright highlights but fail when it comes to capturing detail in the deep shadows? Using the histogram feature on your camera (covered later in this chapter) can help you evaluate problems on the highlight end, and to some extent, you can also use it to evaluate the shadows. To properly assess the integrity of very subtle shadow detail, however, you should view the images in an image-editing program on a calibrated monitor.

   Center-Weighted
The Center-Weighted metering pattern has been used in cameras for years and was around long before Matrix metering came on the scene. The mode meters the entire frame, but as the name implies, it gives more emphasis to the center area of the frame. The ratios will vary among different cameras. But typically, a Center-Weighted meter will base 60 to 75 percent of its metering calculations on the center circle (this is usually shown in the viewfinder) and the remainder on what’s happening in the rest of the frame. Center-Weighted metering is common for use in portrait situations, since the reflectance values at the center of the frame have more influence in determining the exposure. Although this is the most common type of light meter in more consumeroriented digital cameras (if your camera doesn’t offer a choice of metering patterns, then it probably uses a Center-Weighted meter), the main drawback is that it makes the assumption that your subject is centered. While that may be the case for general snapshot photography, it certainly isn’t true for all images, especially if you’re a photographer who’s already familiar with the major tenets of photographic composition—one of which emphasizes not centering the subject for every shot. For this reason, Center-Weighted metering often produces results that are adequate but not the best for many situations .

    Spot (Partial)
Whereas Matrix metering looks at many different areas of the image to evaluate the lighting in a scene, a Spot meter is designed to measure only the light in a very small area. The exact size of the spot may vary from camera to camera, but it’s typically a 3-to-10-degree circle in the center of the frame (degree refers to the angle of view—3 degrees is about as large as a dime looks on the sidewalk between your feet). This can encompass anywhere between 2 to 10 percent of the entire scene. Some pro cameras also have Spot meters that link the metering spot to the active autofocus zone for more precise control where the metering will follow the subject that’s in focus. On cameras that feature user-selectable AF zones, this allows an incredible degree of control for metering and focusing using the same area of the viewfinder. Spot metering is appropriate when you want to measure a specific part of the scene, and you want the camera’s exposure to be based on the luminance (brightness) of that area. Since meters want to place everything into a zone of 18 percent reflectance, keep in mind that a Spot meter is no different in this regard; it just measures from a much smaller area. A classic situation where you might use Spot metering is for a scene where a relatively small foreground subject is juxtaposed against a very bright or dark background. Matrix or Center-Weighted metering would factor a bright background into its calculations, causing the foreground subject to be underexposed. By framing the image so that the spot area is on the subject, a correct meter reading could be obtained for that area of the image.