What is a stop of light?

What is a stop of light? A stop is a measure of light. The term 'stop' is most closely associated with aperture settings but is also used to describe shutter settings and ISO settings.

How big is a stop of light? Each stop of light is a doubling or halving of light. In other words, to increase the amount of light by one stop is to double the light. Conversely, to decrease the amount of light by one stop is to half the light.

What do the stop settings look like? Here are some shortened stop lists for Shutter, Aperture and ISO.  By the way, the settings in these lists are considered 'whole' stops - most cameras can also set 'half' or 'third' stops.

Shutter:
30s
15s
8s
4s
2s
1s
1/2s
1/4s
1/8s
1/15s
1/30s
1/60s
1/125s
1/250s
1/500s
1/1000s
1/2000s

Aperture:
f/1
f/1.4
f/2
f/2.8
f/4
f/5.6
f/8
f/11
f/16
f/22

ISO:
3200
1600
800
400
200
100

Briefly, what are shutter, aperture and ISO?  Shutter is how long the camera's shutter (similar to a window blind) is open to take the picture.  Aperture is the opening inside the lens (similar to the iris of the eye).  ISO is the sensitivity setting of the camera's sensor (the sensor is what records the image).

Examples, by text:
A shutter of 1/2 second allows one stop more light to hit the sensor than a shutter of 1/4 second.
An aperture of f/2 allows one stop more light to hit the sensor than an aperture of f/2.8.
An ISO of 1600 is one stop more sensitive to light than an ISO of 800.

Examples, by picture:

In these examples only the shutter setting was changed - the aperture and ISO were constant.  From top to bottom, each image halves the light to the sensor from the image above it.  To reinforce that point,

• the first image is shot with a 1/2 second shutter - let's call this the base image
• the next image is 1/4 second shutter which is half the light of the base image
• the next image is 1/8 second shutter which is half the light of the image directly above it and 1/4 of the light of the base image
• the next image is 1/15 second shutter which is half the light of the image directly above it and 1/8 of the light of the base image
• the next image is 1/30 second shutter which is half the light of the image directly above it and 1/16 of the light of the base image

Real world application.  Stops of light apply equally to shutter, aperture and ISO.  In other words you can mix and match - if you take an image and discover it is underexposed by 2 stops, you can add a stop of shutter and a stop of aperture to get the 2 stops of light you need.  A stop of light is a stop of light no matter the source, shutter, aperture or ISO.

Intent:  The intent of this post is to give a brief introduction to some of the terminology of photography, particularly the use of the term 'stops.'
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