ISO SETTING

What is ISO?

The ISO (International Standards Organisation) index is a system that was designed to calibrate the sensitivity to light of film emulsions.  This index is also used to calibrate the sensitivity of digital sensors to light. The ISO rating of digital sensors is based directly on film ISO, such that a 200 ISO setting for a digital camera has the same sensitivity to light as a 200 ISO film.

The ISO setting may also be called “film speed” and indicates the light gathering capacity of a digital sensor; the higher the ISO number the more light that is absorbed by the sensor. If the camera is set to an ISO setting of 100 the sensor will absorb little light, while at a setting of 1600 it will absorb a lot of light. The great advantage of the digital SLR camera is that the ISO setting can be changed for each photograph, whereas with a film camera the ISO setting is fixed for any roll of film.

The Olympus E520 has ISO numbers:

100     200     400     800     1600
        --->       increasing sensitivity to light

Each change in ISO setting will either increase or decrease the shutter speed by one full stop i.e. the amount of light reaching the sensor will either halve or double. A change in ISO from 200 to 400 will double the amount of light that will reach the sensor (and the shutter speed can be halved for the same exposure); a change to ISO 100 will halve the amount of light reaching the sensor (and the shutter speed will need to be doubled to achieve the same exposure).

Unfortunately as the ISO setting is increased so will the digital ‘noise’ in each photograph. This makes the photograph look speckled rather than smooth. With film this is called ‘graining’. Noise also disrupts the clean line of an image so that it may appear slightly unfocussed.

The 2 images below are examples of 'noise', both being taken at ISO 1600.

Notice the 'graininess' at the top of the photo

Notice the lack of clarity and graininess of the squirrel's fur.



References:
Collins Complete Photography Course. John Garrett and Graeme Harrris. 2008. Harper Collins Publishers. ISBN: 978-0-00-727992-0
Get the most from your digital SLR. The Digital SLR Guide. Chris Roberts. 2008. Downloaded from the Internet March 2009.