Basics of Exposure on the Canon Rebel T5/1200D - dummies

Basics of Exposure on the Canon Rebel T5/1200D

By Julie Adair King, Robert Correll

Photographs taken with the Canon Rebel T5/1200D, are created by focusing light through a lens onto a light-sensitive recording medium (exposure). In a digital camera, the image sensor is an array of light-responsive computer chips.

Between the lens and the sensor are two barriers, the aperture and shutter, which together control how much light makes its way to the sensor. The actual design and arrangement of the aperture, shutter, and sensor vary depending on the camera, but the figure below offers an illustration of the basic concept.


The aperture and shutter, along with a third feature, ISO, determine exposure — what most people would describe as picture brightness. This three-part exposure formula works as follows:

  • Aperture (controls amount of light): The aperture is an adjustable hole in a diaphragm set inside the lens. By changing the size of the aperture, you control the size of the light beam that can enter the camera.

  • Aperture settings are stated as f-stop numbers, or simply f-stops, and are expressed with the letter f followed by a number: f/2, f/5.6, f/16, and so on. The lower the f-stop number, the larger the aperture, as illustrated in the figure below. The range of available aperture settings varies from lens to lens.


  • Shutter speed (controls duration of light): Set behind the aperture, the shutter works something like, er, the shutters on a window. When you aren’t taking pictures, the camera’s shutter stays closed, preventing light from striking the image sensor. When you press the shutter button, the shutter opens briefly to allow light that passes through the aperture to hit the image sensor.

  • The exception to this scenario is when you compose in Live View mode — the shutter remains open so that your image can form on the sensor and be displayed on the camera’s LCD. In fact, when you press the shutter release in Live View mode, you hear several clicks as the shutter first closes and then reopens for the actual exposure.

  • The length of time that the shutter is open is the shutter speed and is measured in seconds: 1/60 second, 1/250 second, 2 seconds, and so on.

  • ISO : ISO, which is a digital function rather than a mechanical structure on the camera, enables you to adjust how responsive the image sensor is to light. The term ISO is a holdover from film days, when an international standards organization rated each film stock according to light sensitivity: ISO 100, ISO 200, ISO 400, ISO 800, and so on. A higher ISO rating means greater light sensitivity.

    On a digital camera, the sensor doesn’t actually get more or less sensitive when you change the ISO — rather, the light “signal” that hits the sensor is either amplified or dampened through electronics wizardry, sort of like how raising the volume on a radio boosts the audio signal.

Distilled to its essence, the image-exposure formula is this simple:

  • Aperture and shutter speed together determine the quantity of light that strikes the image sensor.

  • ISO determines how much the sensor reacts to that light.

The tricky part of the equation is that aperture, shutter speed, and ISO settings affect your pictures in ways that go beyond exposure:

  • Aperture affects depth of field, or the distance over which focus appears sharp.

  • Shutter speed determines whether moving objects appear blurry or sharply focused.

  • ISO affects the amount of image noise, which is a defect that looks like tiny specks of sand.