Photographic Exposure Explained

For amateur or hobbyist digital photographers, the concept of proper exposure, what affects it and how to achieve it is a rough one to understand. Thanks to today's digital camera technology, decent exposure occurs more by chance than by having a complete understanding of the variables affecting it. That is okay for "photo album" photography, but for images to consistently be remarkable, you should develop a better understanding of what affects and creates proper exposure.

Proper exposure is equivalent to having the optimal amount of light in your image. Some photos should be evenly lit, while others should have high levels of light contrast within the scene. Your camera has a built-in light sensor that it uses to assess the average level of light in the scene and then sets the exposure level accordingly. The camera’s goal is to have approximately the same amount of brightness and darkness in each image, or in other words, average light everywhere. It works fine some of the time, but wouldn’t work well in the case of an image where there are high levels of both brightness and darkness, such as a vividly lit sundown with a silhouette of a barn or tree in the foreground.

For cases like these, a handheld light meter would be a better solution. It allows you to set the exposure based upon the light level in a certain area rather than the average of the entire frame. The camera’s automatic exposure settings would result in the vivid sky being weakened, while the silhouette would be lightened so that there isn’t as much contrast between them, even though the contrast is what makes the image remarkable. A professional photographer would probably shoot this scene with a little underexposure so that the colors would be rich rather than washed out.

Shutter speed, aperture, and ISO settings are the three pillars that drive exposure levels by ultimately determining the amount of light coming through the lens opening and hitting the camera’s image sensor.

Most easily to understand for most is shutter speed. Shutter speed is the length of time that the light is allowed to reach the sensor. High light levels equate to very fast shutter speeds, often faster than the blink of an eye. Darker conditions, such as at nighttime, have much longer shutter speeds, often as long as 10 seconds or more. For those instances where longer shutter speeds are necessary, using a tripod can improve image quality by minimizing camera movement while the shutter is open.

Aperture, also known as f-stop, sounds ominous but really isn’t. Aperture settings determine how large the opening in the lens is, just like the pupil in your eye does. Large apertures such as f2.8 or f4.0 let a lot of light in (allowing for a faster shutter speed) but also create a small depth of field. The depth of field determines how much linear area within the scene is in focus. An aperture value of f2.8 might only have a few inches that is in perfect focus with the rest of the scene blurred, while a smaller aperture like f16 might have vast distances in focus. Nothing in photography is free, however, so the tradeoff with the increased range of focus is that much less light is allowed through the lens, requiring a much longer shutter speed and the need for a tripod.

The third exposure pillar is your camera’s ISO setting. ISO is nothing more than a setting that tells your camera how sensitive it will be to light. In today’s digital cameras, available settings range from 100 to over 25,000 in some cases. 100 is the least sensitive setting and is used for brightly lit shots, like those taken outside on a sunny day. A higher ISO setting would be necessary for lower light periods such as cloudy days, indoor settings or sunrise and sunset periods. Without changing the ISO to a more sensitive setting, the shutter speed would be so long that images would most likely be blurry at best. Making the sensor more sensitive to light reduces the shutter speed and allows for sharper images with less motion-induced blur, but it can introduce digital noise (graininess) into your images that will need to be dealt with later using photo-editing software.

The best way to learn how to best use these settings is in the field taking images in a variety of light conditions. You will quickly get a feel for what settings work best in certain situations, and when to avoid certain settings. Having a solid understanding of what exposure is and how to affect it will make digital photography more productive and enjoyable for you. Now all you have to do is grab the camera and start shooting…