In photography, an ISO setting on a camera is an indication of its relative sensitivity to light: ISO 200 indicating twice the sensitivity of ISO 100 and so on.
Whereas the ISO for a roll of film is fixed, a digital camera allows the ISO value to be varied, apparently changing the camera’s sensitivity to light.
We say "apparently" because nothing actually happens to the sensor itself – its sensitivity remains the same whatever ISO value is selected.
For more advice, read our guides on how to use a DSLR camera and DSLR astrophotography.
Using and changing a camera's ISO setting
What does change with ISO is the amplification or gain of the camera.
As data is read off the chip it is electronically multiplied by a factor used to simulate the ISO change.
The downside of using this technique is that the signal-to-noise ratio (SNR) is lowered with amplification, so noise becomes more obvious at higher ISO values.
This can be somewhat offset by using image stacking techniques.
In addition, the dynamic range of a high ISO image will also be less than that taken on a low ISO setting. Lower ISO gives you better tonal quality.
Think of it this way
The three main variables in taking a photograph are:
- Shutter speed
- Aperture
- Film/sensor sensitivity.
Imagine a film frame or CCD as a bucket to be filled with water (light).In this analogy:
- Aperture is represented by the diameter of a hose used to fill the bucket
- Shutter speed is the time the hose is running
- ISO is the water pressure
Double the pressure and the bucket will fill in half the time.
