What is the relationship between aperture, shutter speed, and ISO?

Two controls affect the amount of light that comes into the camera and strikes the image sensor - aperture and shutter speed. The ISO affects how much light is needed to produce a correct exposure.

Aperture
The lens aperture is a diaphragm that is in the lens itself or immediately behind it. It affects the amount of light that passes through the lens and is usually made up of a set of blades that open or close depending on the aperture setting selected. The aperture settings are commonly referred to as f-stops and have a specific numerical sequence, such as F5.6, F7.1, F8, F11 and so forth. Lower f-stop settings (such as F5.6) have a larger diaphragm opening, allowing more light through the lens. On the other hand, Higher f-stop settings (such as F11) have a smaller diaphragm opening, allowing less light through the lens.

IMPORTANT: Changing the aperture also affects the depth of field .

Shutter speed
Shutter speed also can affect the amount of light that comes into the camera by controlling how long the camera shutter remains open. The longer the camera shutter is left open, the more light that is allowed to enter the camera; this is achieved by using slower shutter speeds (such as 1/60). If the camera shutter is left open for a shorter duration, less light is allowed inside the camera; this is achieved by using faster shutter speeds (such as 1/250 or even faster). Shutter speeds can vary from fractions of a second to several seconds in duration.

IMPORTANT: Changing the shutter speed also affects motion blur .

NOTE:  There is a reciprocal relationship between shutter speed and aperture. You can get the same amount of light if you change the shutter speed and aperture settings at equivalent amounts. For example, 1/30 at F5.6 is the same as 1/8 at F11. Essentially, using a fast shutter speed with a wide aperture can provide the same amount of light to the image sensor as when using a slow shutter speed with a narrow aperture.

ISO
ISO determines the sensitivity of the camera sensor to light. A higher ISO number increases the sensitivity; this means that less light is needed to produce a correct exposure. A lower ISO number decreases the sensitivity and requires more light to produce a correct exposure. Therefore, in well-lit environments, a lower ISO (200 or below) should be used. When shooting in low-light situations, a higher ISO (400 or above) is recommended. It may also be necessary to use a higher ISO setting when shooting with a narrow aperture or high shutter speed - since a narrow aperture and high shutter speed reduce the amount of light that strikes the image sensor.