ISO is the number indicating a digital camera sensors sensitivity to light. The higher the sensitivity, the less light is needed to make an exposure.
Digital cameras automatically select the ISO but most have a setting to change it manually. Auto ISO generally works best for bright scenes.
ISO and the degree of light
Shooting at a lower ISO number requires more light than shooting at a higher number. Lower numbers result in images with the least visible noise, which is desirable. The higher the number, the more noise.
Effects of high ISO
The amount and degree of noise varies from camera to camera. Noise, when present, can be seen throughout a photo but usually appears most in dark areas.
Digital single lens reflex cameras produce images with the lowest noise at all ISO numbers compared to compact cameras. This is because they have larger sensors. However, some consumer digital cameras do better than others when it comes to noise.
Suggested ISO settings
AUTO ISO – digital camera automatically sets the ISO speed according the the brightness of the scene, increasing or decreasing the sensitivity. User has no control over which ISO number is used.
ISO 80 – for taking photos in bright light; excellent for close-ups, landscape, and portraits. Produces fine detail and image quality.
ISO 100 – for extra sensitivity with little, if any, reduced image quality.
ISO 200 – cloudy and overcast days. Acceptable image quality, with some visible noise.
ISO 400 – suitable for indoor photography whether or not a flash is used. Useful for “stop-action” and sports photographs. Most compact digital cameras produce high to very high image noise.
ISO 800, 1600 and above – useful for taking photos in very low light, or outside in good light when increased shutter speeds are required. Results can be disappointing when shooting at these high numbers with compact digital cameras, so take test photos before photographing an important event.
High Auto ISO mode
Some digital cameras have a High ISO scene mode, which uses a very high ISO number such as ISO 3200. If you use High Auto ISO mode, be aware that your camera may automatically reduce the size of images shot in this mode.
Use High Auto ISO mode with caution and only if you have no other options such as using a tripod or other camera support.