ISO sensitivity is a unit of measurement for the sensitivity of an image sensor to light.
High ISO sensitivities mean that you need less light to get a bright picture.
The common values for ISO sensitivity are built from the standard sensitivity of 100 by doubling the numbers:
|100 – 200 – 400 – 800 – 1600 – 3200 – 6400 – …|
Modern DSLR cameras go beyond the end of the row up to ISO 25600 and top models of the professional cameras even higher.
As for aperture and exposure time this series is built in steps of full exposure values. Each ISO sensitivity in the series needs half the amount of light as the one before and can be exposed with half the exposure time or the next smaller aperture (=next higher f-number) to keep the brightness of a photo unchanged.
And also here cameras use additional intermediate values that are not shown above.
On the left are a few samples how digital cameras display the ISO sensitivity. The first two ones try to tell you that they change the ISO sensitivity automatically, then there is the monitor of a DSLR camera set to ISO 200 and the last one is an LCD display that does not show the ISO sensitivity – that is also possible.
To give the sensitivity to light in ISO is a standard that digital photography has taken over from film photography. „ISO“ is an abbreviation for the International Organization for Standardization which has defined this standard, not a specific term from photography.
ISO sensitivities are usually given as e.g. „ISO 100“.
In digital photography a film’s sensitivity to light is simulated by amplifying the signals from the image sensor.
Therefore the ISO sensitivity is a setting that can vary with each picture – an advantage compared to shooting with films where you needed to exchange a roll of film to get different ISO sensitivity.
Depending on your camera settings it can happen that the camera varies the ISO sensitivity automatically if it finds it necessary or that you need to adjust it yourself.