The Exposure Triangle

What is the exposure triangle?

The goal of exposure is to accurately render the tonal values of the scene you are photographing. In order to accomplish this goal the correct exposure must be attained. Exposure relies on a balance of three elements: ISO, Aperture and Shutter Speed. The relationship of these elements is often referred to as the Exposure Triangle.

The exposure triangle is a useful way of describing the relationship between the three aspects of exposure. Each corner of the triangle represents one of the three variables: aperture, shutter speed and ISO. Adjusting just one of these will make the photo darker or brighter and will change the appearance of the photo based on what you have changed. For example, using a longer shutter speed will introduce motion blur to your photo but also make the photo brighter (increased exposure) due to more light hitting the sensor. The easiest way to understand it is to see the picture.

The three parts of the Exposure Triangle:

1. ISO – the measure of a digital camera sensor’s sensitivity to light. In addition to sensitivity influences the quality of the image. Low ISO values produce images with better detail and color.  High ISO images tend to degrade the image quality.
2. Aperture – is an opening in the lens that controls the quantity of light hitting the sensor. Changing the aperture size will have an effect on the Depth of Field. Depth of field is the zone in front of and behind the subject that are in acceptably sharp focus.
3. Shutter Speed – duration or amount of time that the shutter is open. The shutter speed can control the effect movement has on the recorded image.  Fast shutter speeds will stop  or freeze movement.  A slow shutter speed may result in a blurred image intentionally or not.

In order to achieve a properly exposed photo, you need to make sure that all three elements are working in harmony.

When determining the proper exposure a photographer must take into consideration these three elements and decide which element is most important to the creation of the photograph. Adjusting the other two elements will then be determined on the photographer’s choice.

For example a sports photographer would more than likely choose a fast shutter speed to freeze the subject.  This may require compromising the quality and/or sacrificing depth of field when adjusting ISO and Aperture.

A landscape photographer on the other hand would opt for the best color and detail in an image. Selecting a low ISO and a smaller aperture for more depth of field to achieve this could result in a long exposure requiring the use of a tripod to support the camera.

Fortunately your camera has a built-in exposure meter to help you achieve the correct exposure, either automatically making adjustments or providing a scale to help the more advanced photographer make exposure decisions. Todays mirrorless digital cameras also provide another feature known as exposure simulation.  When you view the image on the LCD or through the eye level viewfinder it will be immediately apparent that the image is either too light or too dark. The Exposure Meter will be covered later in the Advanced Camera Settings Section of this course.