What is Exposure Triangle in Photography?


Exposure Triangle
In photography, exposure is the amount of light captured by the camera sensor or the photographic film. When the sensor or the film is exposed to light, a photograph is made. Exposure triangle refers to the three parameters that determine the exposure of a photograph. These parameters are: Aperture, Shutter speed, and ISO speed.

Achieving correct exposure is very similar to collecting rain water in a bucket. There are three factors under our control that determine how the rain water collects in the bucket. They are: the width of the bucket, the time for which it is kept in the rain, and the quantity of rain water to be collected. Collecting too little (underexposed) and collecting too much (overexposed) are not desirable. There are many combinations of these three controllable factors that will achieve this but how to achieve it depends on the person. For example, a wider bucket fills faster than a narrow one for the same quantity of water. It would not be wrong to say that the rainfall is analogous to the light entering the camera. Heavy rainfall would connote more light (i.e. daytime) and light rainfall would represent less light (i.e. night time). Therefore, in a way, aperture, shutter speed and ISO speed are analogous to the width, time and quantity mentioned above.

Each setting has a different effect on exposure control. Therefore there are many combinations of the above three settings to get the same exposure. With each setting, there is a certain trade-off and certain advantages. For example, aperture has an effect on the depth of field i.e. the part of the image in focus, shutter speed affects motion blur, and ISO speed governs image noise.

The term aperture tells about the size of the aperture (or hole, in crude terms) sending light to the sensor or film. The actual size of the aperture depends on a control called a diaphragm- a circle of blades that open up to make a large hole (letting in lots of light) or overlap each other to reduce the aperture and restrict the amount of incoming light. Aperture is denoted as f/number or f/stop e.g. f/2, f/5.6, f/8, f/11 etc. The larger the number, the smaller the opening i.e. an aperture of f/8 allows less light to pass than an aperture setting of f/2. As mentioned earlier, aperture affects the depth of field in a photograph. Depth of field of a photograph is the range of distance over which the objects are in focus. Smaller aperture (i.e. higher f/number, say, f/11) translates to a wider depth of field in the photograph when compared to that of a larger aperture (say, f/2). Typical values of f/number are: f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32. With each higher f/number in this series, the amount of light passing through the aperture is being reduced by a factor of 2.

Shutter speed is the amount of time for which the sensor is exposed to light. It is measured in seconds e.g. 1/30 second, 1/500, 10, etc. A higher shutter speed lets the sensor capture more light i.e. a shutter speed of 1 second captures more light than a shutter speed of 1/60 second. Shutter speed affects the way motion is captured in the photograph. A slow shutter speed (say, 1 second) can capture trails of light in the photograph whereas a fast shutter speed (say, 1/500 second) can freeze the motion of a high speed formula one car. Shutter speeds can be used to creatively change the way a scene is photographed. If a shutter speed is doubled (i.e. changed from say, 1/100 to 1/50), the amount of light captured also gets doubled.

ISO speed refers to the sensitivity of the sensor to light. It is denoted by a number e.g. 100, 200, 400, etc. The higher the number, the higher the sensitivity of the sensor to light i.e. at an ISO speed of 400, the sensor can capture the same amount of light in less time when compared to a lower ISO speed setting of say, 100. A drawback of using high ISO speed is that it amplifies noise in the image when compared to that with a lower ISO speed. An obvious advantage of higher ISO speed is that it allows one to use a faster shutter speed at the same aperture setting, which makes shooting hand-held photographs in night possible. Typical values are: 100, 200, 400, 800, 1600, 3200, 6400. With each higher ISO speed, the amount of light that is captured gets doubled.

The table below will help better in understanding how these settings affect exposure in a photograph.

Aperture Shutter Speed ISO Speed Exposure
f/5.6 1/100 200 Same Exposure
f/8 1/50 200 Same Exposure
f/5.6 1/400 800 Same Exposure
f/11 1/50 400 Same Exposure

Author: Ritesh Saini, IIT Mumbai (India)

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Edited by: Rajesh Bihani ( Find me on Google+ )

Disclaimer: The suggestions in the article(wherever applicable) are for informational purposes only. They are not intended as medical or any other type of advice