Aperture, Shutter Speed, and ISO Definitions – Photography Theory

Understanding aperture, shutter speed, and ISO is crucial for beginner photographers. They are the foundations of digital photography.

Aperture Definition: The adjustable opening in a camera lens that light passes through and reaches the camera’s shutter and image sensor.

Shutter Speed Definition: The amount of time the shutter (a flap covering the image sensor) is open to control the amount of light passing through the lens to the image sensor and creating a photo.

ISO Definition: The camera image sensor’s sensitivity to light. A lower ISO means your camera is less sensitive to light. ISO stands for International Standard Organization.

aperture definition

Aperture Size Example: An aperture of 16 (f/16) is smaller than 4 (f/4).

I’ll elaborate on the definitions without going into the science. Besides, I’m no optical physicist.

Understanding Aperture Theory

The size of the aperture or lens opening can change and is measured by “f-numbers.” Lenses have a particular set of f-number sizes called f-stops.

F-stops are inversed. A large f-stop leads to small aperture; a small f-stop leads to a large aperture.

The part of the lens that controls the aperture size is the diaphragm, similar to the pupils of your eyes.  It’s made up of small blades which rotate synchronously.  A large aperture admits more light and a small aperture admits less light to the shutter and image sensor.  Aperture is one component in controlling depth of field.

Understanding Shutter Speed Theory

The camera’s shutter remains closed until you press the shutter release button (to take picture) at which time the shutter opens and closes to allow the camera to image a photo.  The shutter can open and close in less than 1/3000 seconds but can also be programmed to remain open for 30 seconds or more.

Most images will be taken with a shutter speed less than a second. Specialized photography such astrophotography will require a shutter speed of more than a second.

Understanding ISO Theory

A lower ISO is used in daylight settings because the camera’s image sensor doesn’t need a lot of light to process an image.

A higher ISO is usually used in darker environments. A common issue is higher the ISO, the more noise (grain) your photo will have. Think of noise in this sense; imagine you took the best photo in the world but then someone sprinkled a little bit of salt and pepper on the photo. The salt and pepper is noise!

Ideally you want to have your ISO as low as possible but can be challenging at night. Typically ISOs ranges from 100 to 12800.