ISO equivalence settings

For those of us growing up with film, the idea of an ISO number (or if I really show my age - ASA number) indicating how sensitive a particular film is to light is not new. These values, ranging from slow (50 ISO) through medium (200 ISO) to high speed (1600 ISO), have been one way that photographers have selected which film stock to choose for particular jobs. Each film type had its advantages and disadvantages and selecting which was best was often a compromise between image quality and film speed. If the day's shooting involved a variety of different subjects then the decision was always a difficult one, as once the choice was made you were stuck with the same stock for the whole of the roll.

In the digital era the restrictions of being locked into shooting with a single film with all its particular abilities and flaws has been lifted. The ISO idea still remains. Though strictly, we should refer to it as 'ISO equivalence' as the original ISO scale was designed specifically for film not CCDs. Most cameras have the ability to change the ISO equivalent setting for the sensor, with a growing number offering settings ranging from 100 to 1600. Each frame can be exposed at a different 'ISO' value, releasing the digital shooter from being stuck with a single sensitivity through the whole shooting session. Oh happy days!

Unlike changing film, where one photographic recording device was swapped with another that was either more or less sensitive, changing the ISO setting of a digital camera doesn't swap the old sensor with a new one. Instead, selecting a higher ISO amplifies the output signal from the sensor, which allows the photographer to capture images with less light. The downside of this freedom is that amplifying the signal has the undesirable effect of increasing the noise or grainy look of our digital files. So you should take care when choosing to work at high ISO values as such a decision does have a direct effect on the image quality of your captured files.

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