Stops, Values, and Sensitivity (Exposure Series Part 2 of 5)
Myfotoguy series on exposure part 2 of 5
What’s a stop? What does “stop down” mean? What do all these numbers mean? If you’re confused, no worries, you’re definitely not alone. In this post you’ll learn what it all means.
A quick review from Part 1
Aperture controls the size of the opening that light travels through.
Shutter speed controls how long the image sensor is exposed to the light.
ISO is how sensitive the image sensor is to light.
What’s a stop?
The difference in value between one setting to the next (aperture, shutter speed, or ISO) is known as a 'stop'. Changing a setting by one full stop allows you to increase or decrease exposure by one stop.
Aperture and f-numbers
The size of the opening light travels through. A larger aperture means a larger opening; a small aperture means a small opening. The aperture is set using and f-number, sometimes called an “f-stop”. The numbering is counter intuitive because the larger the number, the smaller the opening. For example, f/22 has a much smaller opening than f/8.
The range of f-numbers follows a standard sequence with each number being half as bright (allowing half as much light to pass) as the previous one. A typical aperture range:
The amount of time the shutter is open controlling how long the image sensor is exposed to light. The shutter prevents light from reaching the film until the moment of exposure, when it opens
Shutter speeds follow a standard sequence with each number being half that of the next, allowing half as much light to pass through. A typical shutter speed range:
1sec; 1/2sec; 1/4sec; 1/8th; 1/15th; 1/30th; 1/60th; 1/125th; 1/250th; 1/500th; 1/1000th; 1/2000th
Shutter speeds are expressed in seconds or fractions of a second. If you change your shutter speed from 1/125 to 1/60th you will gain one stop more exposure.
The higher the number the more sensitive the image sensor is and the less light it needs to create an image. The lower the number the less sensitive it is and more light will be required. ISO setting follow a standard sequence, a typical range:
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