The Exposure Triangle

Bryan Peterson defines the relation between Shutter speed, aperture and ISO by using the word “The Exposure Triangle ”. It means that if we place shutter speed, aperture and ISO on three corners of a triangle the correct exposure lies at the centre of the triangle. It may sound confusing at first but it is easy to understand once the relation is understood.

Shutter speed – Time for which the light is allowed to enter the camera.

Aperture – Size of the opening through which light enters.

ISO – Sensitivity of sensor or film to light.

Now here’s a metaphor. Consider a water hose and a bucket. Objective is to fill the bucket completely without any water overflowing. The factors we will consider are

• The duration for which the water flows through the hose.
• The width of the hose
• Pressure with which water flows.

If the width of the hose is increased then we will need less time or less pressure to fill the same bucket. If the pressure is increased then the time water flows can be less or the width of the hose can be reduced.

Consider Duration of flow of water to be the shutter speed. Water to be light. Width of the hose to be aperture and pressure to be ISO. If any one thing is increased by one stop and one of the other can be reduced by one stop keeping the exposure same.

Consider another way. Once the exact exposure is calculated by the camera and photographer decides he needs the shutter to be 2 stops faster, then consider stopping down 2 stops on other two. Either stop down ISO or Aperture (one of these) by 2 stops, or one stop each.