Introduction

An understanding of exposure is without doubt the most critical part of the photographic process. Automatic exposure systems found in many sophisticated camera systems calculate and set the exposure for the photographer. This may lead some individuals to think there is only one correct exposure, when in reality there may be several. The exposure indicated by an automatic system, no matter how sophisticated, is an average. Creative photographers use indicated meter readings for guidance only. Other photographers may interpret the same reading in different ways to create different images. It is essential the photographer understands how the illuminated subject is translated by exposure into a photographic image.

Exposure

Exposure is the action of subjecting a light-sensitive medium to light. Cameras and lenses control the intensity of light (aperture) and the duration of light (time). The intensity of light is determined by the size of the aperture in the lens and the duration of light is determined by the shutter.

Exposure is controlled by aperture and time - intensity and duration.

Too much light will result in overexposure. Too little light will result in underexposure. It makes no difference whether there is a large or a small amount of light, the same amount of light for an appropriate exposure is still required.

A digital camera set to manual mode cannot automatically alter its sensitivity to light. Exposure must be adjusted manually. This is achieved by adjusting either the intensity (aperture) or duration of light (time) or a higher or lower ISO chosen. Increasing the size of the aperture gives more exposure, decreasing gives less. Decreasing the duration of the shutter speed reduces exposure, increasing gives more.

Overexposure

Correct exposure - Line Mollerhaug Underexposure

To calculate correct exposure the intensity or level of light has to be measured. Light is measured by a light meter. Understanding the metering techniques of both hand-held light meters and those found in most cameras using their own metering systems is an important skill. All light meters inform the photographer of the amount of light available to obtain an appropriate exposure. This information can be used to set aperture and shutter speed settings in a variety of combinations. Each combination will give different visual outcomes whilst retaining the same overall exposure. Aperture will determine depth of field, shutter speed will determine motion blur. Working in a creative medium such as photography ‘correct exposure’ can sometimes be a very subjective opinion. The photographer may want the image to appear dark or light to create a specific mood.

Exposing for highlights - Mark Galer

Appropriate exposure

A light meter reading should only be viewed as a guide to exposure. Digital image sensors are unable to record the broad range of tones visible to the human eye. If the camera frames a subject with highlights and shadows the light-sensitive medium may only be able to capture the highlight tones or the shadow tones, not both. An extremely bright tone and an extremely dark tone cannot both record with detail and texture in the same framed image. Underexposure and overexposure in one image is therefore not only possible but common. It is often necessary for the photographer to take more than one reading to decide on the most appropriate exposure. If a reading is taken of a highlight area the resulting exposure may underexpose the shadows. If a reading is taken of the shadows the resulting exposure may overexpose the highlights. The photographer must therefore decide whether highlight or shadow detail is the priority, change the lighting or reach a compromise. A clear understanding of exposure is essential if the photographer is to make an informed decision.