The Basics of Exposure

What is exposure?

In photography, exposure refers to the amount of light that reaches, and is collected by, the sensor to create an image.  Like with a sunburn, if the sensor receives and collects too much light, overexposure occurs.  If not enough light reaches the camera sensor, the final image is too dark or underexposed.

The key to understanding exposure lies with understanding how light reaches the sensor in the first place and what the photographer can do to affect that process.  When a photographer aims the camera at an object, the light passes through the lens, into the camera body, and reaches the shutter in front of the camera sensor.

Amount of Light Getting into the Camera


Inside the lens is a structure  called the diaphragm which can create different size openings for light to pass through.  The size of the opening  through which the light can pass is called the aperture.  The aperture size can be set by the photographer, or by the camera, and controls the amount of light that gets through to the camera body.  Specific apeture opening sizes are called f-stops.

For now, the important point to understand is that the photographer can change the amount of light getting through the lens by changing the size of the opening in the lens.

How Long the Light Available is Allowed to Reach the Sensor


While the size of the aperture (size of the opening inside the lens that lets light pass through) controls the amount of light getting into the camera, the shutter controls how long that light is allowed to land on the sensor for collection.  Like shutters on a house that can be opened to let light in or closed to block light, the shutter on the camera provides the same functionality.  The photographer can choose to set the shutter speed used by the camera or allow the camera to set it by itself.  Most DSLR cameras can be set in increments ranging from 30 seconds to 1/4000th of a second.

Sensor Efficiency at Collecting Light

Sensor efficiency  simply means how quickly the camera sensor is able to collect the correct amount of light to create a properly exposed image.  One may ask, why not always use the best sensor setting?  As will be seen in lessons on ISO, the term for the sensor sensitivity setting, there is a tradeoff between how quickly the sensor can collect light  when using higher settings and the image quality.

Canon EOS Rebel T7 Back Display

Based on the camera settings shown on the left, we know the photographer has set a shutter speed of 1/125th of a second, an aperture setting of f/5.6, which will be learned more about later, and an ISO setting of 6400, which will also be learned about more in future lessons.

The exposure metering mode label in the bottom left reflects the type of process the camera uses to suggest what it believes to be the values necessary to produce a correct exposure.

Future Lessons

The video to the right provides a somewhat more detailed explantion of shutter speed, aperture, and ISO as the variables controlling exposure.  It also introduces a few new concepts related to each that will be covered later on.  View the video  now if you choose, but do not be concerned about topics that have yet to be discussed.

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