Let's begin with the first property: The ISO

It refers to the sensitivity  to light of a roll of film or the digital sensor inside a camera. 

The lower the ISO number (25-200), the less sensitive to light it isThis makes a low ISO the preferred choice for bright daylight conditions.

With an ISO of 800 or more, the sensitivity increases, meaning we are able to shoot in low-light conditions. Also, if we use a high ISO in bright conditions, this increased sensitivity means we can shoot with very fast Shutter-Speeds, making it perfect for sports events. Cameras feature a "Sports" setting which sets a high ISO for us.

A side effect to ISO is the grain (film) or noise (digital) found in an image. 

A low ISO will have a tight grain/noise structure, making it virtually unnoticeable to see. As we increase to a higher ISO, the structure starts loosening up, dramatically increasing the visibility of grain/noise.

Sometimes this is exactly  what we want, other times it's not. Analog film's grain is more pleasing to the eye, while digital's color noise is the worst, adding random color specks to our photograph.

The photographs used in this example cover ISO 100 to 12800. The images were magnified to 400% then cropped to really  show the effect of grain/noise.

Notice how grain/noise is not really visible at ISO 100, 200 and even 400, but at ISO 800, it starts to become visible and as we increase sensitivity, it becomes even more pronounced.

ISO 100

ISO 100

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