THE EXPOSURE TRIANGLE

The exposure triangle is a concept that involves three of the major functions of any camera, ISO, Shutter speed/shutter angle and Aperture. Let’s breakdown these three components to help us understand exactly what they do and how the relate to each other.

ISO

ISO, or film speed, refers to how sensitive a sensor or film is to light. Higher ISO values, like 1600, will be more sensitive to light than lower ISO values like, 200. As such raising the ISO in camera can help brighten a dark scene. This isn’t without its drawbacks however as increasing the ISO will also increase the amount of noise within an image.

Shutter speed/shutter angle

A shutter, when talking about cameras, is traditionally a mechanical component (although many digital cameras now use digital shutters) that exposes the sensor to light.

The term shutter speed is most commonly associated with stills photography. The shutter speed is measured in fractions of a second, with this fraction denoting how long the sensor is exposed for. For example, a shutter speed of 1/100 means that the sensor was exposed for 1/100th of a second.

Shutter angle on the other hand is a way of describing shutter speed relative to the frame you are currently capturing in. The term shutter angle originates from legacy film cameras wherein a disk with an angled opening would spin in front of the sensor, allowing light through the opening one per revolution. As shutter angle increases, all the way up to 360°, the sensor is exposed for longer.

To convert from shutter angle to shutter speed, as the latter is more common nomenclature, you would multiply your frame rate by 360 and then divide by the a known shutter angle. For example, if we wanted to know what the shutter speed that corresponds to my 180° shutter angle when I am filming at 25 frames per second I would do:

(25x360)/180 = 50 or 1/50th of a second.

Although exposing the sensor for longer will capture a brighter image, any motion that occurs whilst the sensor is exposed will create motion blur. In some cases large amounts of motion blur may be desired in some scenes however when recording video we often want a sharp image.

Aperture

The aperture is the diameter of the opening of the iris within a lens. Closing or “stopping down” the iris results in a smaller aperture, meaning that less light can get through. Inversely Opening or widening the iris allows more light to pass. Stopping the lens down increases the depth of field meaning that more elements of the background will start to come into focus, which isn’t always desired.

*This is a very difficult topic to cover in text alone. RED do what they call “REDucation” and have very good resources on their website. Similar graphics can help better understand the conecpts.

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