The exposure triangle

describes the relation between aperture, exposure time and ISO speed

Stops is a common term in photography. Each stop means halving or doubling the amount of light collected by the camera.

Typical stops used in photography

• Aperture stops (of f-stops): f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22
• ISO-stops (typical): 100, 200, 400, 800, 1600, 3200, 6400 
• Shutter speed: (in seconds): 1, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, 1/2000, ...

Each stop change is a factor of 2 in light, so you need to adjust the exposure time by a factor of 2 to compensate. 

Examples: 

• f/2 and 1/100 second corresponds with f/4 and 1/25 second (2 stops difference)
• f/11 and ISO400 correlates f/5.6 and ISO100 (also 2 stops). 
• Each of these settings on the left would achieve the same exposure results.

For more details see 

• Shutter-Speed-Chart-Dave-Morrow-Photography.pdf (davemorrowphotography.com)
• What is a "Stop" of Exposure in Photography? | Photography Mad

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