In the last column, I presented a number of definitions pertinent to determining proper exposure of an image.

Before discussing the mechanics of setting exposure, by selecting appropriate aperture and shutter combinations, it is important to understand the basic qualities of light as relates to photography. In this column, I limit the discussion of light to ambient, or natural, reflected light. Artificial light introduced by the photographer, flash for example, and direct light, the sun directed at the camera lens for example, are left for another column.

The properties of light of interest to photographers are intensity, quality, direction and contrast.

All objects reflect light. It is this reflected light that is captured on film to form an image. Those objects in a scene reflecting a high intensity light are brighter than those parts of a scene reflecting less intense light.

Intensity is, therefore, a measure of the brightness of available light.

As mentioned, all objects reflect light, but rarely does an object reflect 100% of the light that is striking it. If that were so, it would indeed be a very bright world to live in. Sort of like always having your pupils dilated at the eye doctors. Properties of objects determine the percent of light reflected yielding its color and brightness. A white shirt, for example, reflects more light than a black pair of pants.

It is lights intensity that is measured by a camera meter. The meter measures the total intensity of the reflected light entering through the lens and striking the film plane. The meter then calculates an appropriate combination of aperture and shutter to render a proper exposure of a scene.

Most tourists take a ride along the Kancamagus Highway in the White Mountains during foliage season in the middle of the day, thinking that the best light is when the sun is overhead. “Make sure the sun is over your shoulder,” we were told when taking pictures. Turns out, that is when the quality of light is at its worst. Commonly called hard light, this type of light creates a lot of contrast; some trees are in bright light while others are cast in shadow. Return to the same scene on an overcast day and notice that there are no hard shadows. Being able to recognize the quality of light and when it best matches your visualization of a scene is an important step on the way to taking good photographs.

Quality of light is impossible to control. When you arrive at a scene, what you see is what you get for light. I remember many a sunrise excursion where the light just wasn’t worth it. Some of the hardest work that goes into making a photograph is visualizing a composition and returning to the scene again and again until the light is right. You know you are on your way to becoming a photographer when you decide to leave without taking a picture because the light just isn’t right. Light at sunrise is different than light at sunset. Mid day sun is hardly ever worth it. Overcast days provide soft light with minimal shadows. You, the photographer, decide when the light is right.

The source of ambient light, usually the sun, is positioned either to the front, back, side or top of a scene. A photograph can be taken with the light source at any direction relative to a scene, but the look and feel of the image will be different. Keep in mind that light direction is relative to the subject, not the camera. For example, front light is light that hits the subject from the front. This is the kind of light we all think we need. Front light usually makes the subject squint and the subject, or scene, is blasted by the light, leaving no texture or dimension in the resulting image. Backlighting, coming from behind the subject, tends to render the subject in silhouette. Side light provides the most texture and the most dimension, adding depth and feel to the subject. The photographer is usually able to control the source of light by moving the cameras’ angle of view relative to the light source. Is your subject backlit? Simply move to the other side. Want your scene side lit? Move your angle of view until the light source is at a 90 degree angle to you and the subject.

All scenes contain objects that have different reflectance values. Contrast is a measure of the range of brightness reflected by the darkest object and brightest object in a scene. This is easy to see through your viewfinder. Point your camera at a day lit scene. With your camera set to automatic, look through the viewfinder as you move the camera around the scene. Notice how the suggested aperture and/or shutter values change. The change represents different reflectance values in a scene. If the reflectance values vary considerably then there is a great amount of contrast.

Contrast is an important property of light and, unfortunately, has different effects on pictures depending on the type of film, or digital sensor, you are using. Different films have different contrast properties as do digital sensors. An important property of film is its ability to show detail in a specific contrast range. That is, what are the brightest brights and the darkest darks in which some texture or detail is visible? At some point the bright parts go pure white with no detail and the darkest darks go black with no detail.

In the next column we learn how to measure contrast and intensity of reflected light and convert that measurement into exposure. Until then, keep shooting.