2. ISO

ISO is a measure of how sensitive the sensor of your camera is to light.  The term originated in film photography, where film of different sensitivities could be used depending on the shooting conditions, and it is no different in digital photography. The ISO sensitivity is represented numerically from ISO 100 (low sensitivity) up to ISO 6400 (high sensitivity) and beyond, and controls the amount of light required by the sensor to achieve a given exposure

At ‘low’ sensitivities, more light is required to achieve a given exposure compared to high sensitivities where less light is required to achieve the same exposure.  To understand this, let’s look at two different situations:

Low ISO numbers
If shooting outside, on a bright sunny day there is a lot of available light that will hit the sensor during an exposure, meaning that the sensor does not need to be very sensitive in order to achieve a correct exposure.  Therefore, you could use a low ISO number, such as ISO 100 or 200.  This will give you images of the highest quality, with very little grain (or noise).

Taken at ISO 100, the image does not show signs of noise

(even when looking at the 100% crop (right)

High ISO numbers
If shooting in low light ...

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