ISO on #WordyMonday

One Youth Dance rehearsals and production of Emerge, (c) Carole Edrich 2014

One Youth Dance Easter Production, Emerge, (c) Carole Edrich 2014
1/160 sec
Focal length 85mm
Centre Weighted metering
No Flash

One of my students at a recent City Academy photography workshop asked me what ISO was so I thought I’d talk about it here for #WordyMonday. To accompany it I’ve chosen this recent example of a shot taken at high ISO and shown clearly what settings enabled me to take it (EXIF data).

ISO used to be a number that represents film speed. By film speed I mean how sensitive the film is to light and therefore, all other things being equal, the amount of time needed to get a good exposure. Light insensitive film requires more light than light sensitive film and the more light sensitive the film is the less light is needed to make a good exposure. Highly sensitive films are known as fast films and have high ISOs. In general the faster the film is, the more film grain there is (this translates to noise in digital sensors) and we say that the image quality is reduced.

International Standards Organisation from which the scale gets its name expresses sensitivity in terms of the luminous exposure H (in lux seconds) but it’s not that simple. In fact ISO numbers in digital cameras are directly related to the camera’s ‘standard output sensitivity’ which itself can be calculated in different ways and Japanese camera manufacturers still have to say how the sensitivity rating has been derived.

You can find more details, including formulae from the wiki page, but ultimately each digital camera’s sensitivity is limited by its ability to convert the photon into a measurable electron. This is its Quantum Efficiency or QE.