What is ISO?
ISO a non-standardized unit to measure the sensitivity of film or digital sensors to light*. Film and digital sensors both use numerical values to show how sensitive they are. ISO may be changed by swapping your film or you may select a new ISO setting on a digital camera. In digital sensors the lowest or base ISO setting is typically 100 ISO (although 50 is common too).
*There are actually a number of accepted standard measurement systems for ISO which is why I choose to refer to it as non-standardized.
How does it effect the Exposure?
Every time you double or halve the ISO number you gain or lose a stop of light. So if you raised your ISO from 100 to 200 you would gain 1 stop of light*. From ISO 100 to 800 you would gain 3 stops of light (100->200->400->800).
A stop of light refers to how much light hits the sensor. Every time the light hitting the sensor is halved or doubled (through ISO, Aperture, Shutter Speed or otherwise) the exposure will gain or lose a stop.
What else does it effect?
As well as effecting the overall exposure of an image, an additional side effect of ISO is that it adds noise or grain to an image. The higher the ISO the more grain or noise becomes apparent.
Noise or grain are those tiny specs that appear on image. When using film the effect is often refereed to as grain and can be considered as having a certain aesthetic quality. Digital noise on the other hand has a more uniform effect and generally regard as being undesirable.
What ISO should I choose?
Though there are no set rules for choosing your ISO there are a few guidelines which can help.
The first guideline is to use the lowest ISO that the light will allow. This will help keep your images clean and noise free. The trade off is that in low light your shutter speeds might become too long, increasing the chance of motion blur, or you’ll end up shooting with a very wide aperture, creating a shallower depth of field (which may be desirable).
Another rule of thumb is to set your ISO based on the recommendations below.
100 – Outdoors Sunny
200 – Outdoors Cloudy, Light Shade
400 – Heavy Shade, Indoors (near a window)
800 – Indoors
Again, these are only recommendations. Light changes and can be difficult to predict. Personally when I’m shooting I set my ISO based on the available light, set my aperture, all while keeping an eye my shutter speed. If the shutter speeds get to slow I then bump up my ISO.
Still haven’t had enough?
Here’s Mike Browne again all the way from the British Isles explaining ISO on location.