ISO (sensitivity)
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.
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. The amount and degree of noise varies from camera to camera.
Digital single reflex cameras (DSLR), because they have larger sensors, are best for producing noise-free images. However, some consumer digital cameras now have improved sensors that produce acceptable images at higher ISO.
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 50 - 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 ISO numbers with compact digital
cameras, so take test photos before photographing an important event.
Changing ISO can change the aperture and shutter speed, depening on what exposure mode is used.
ISO越高，曝光需要的时间越短。
就是说ISO值越低照片越细腻，但是如果你拍照环境很暗，而且又不可以打光，就需要高的iso来令曝光，iso高一倍，曝光时间缩短一半，就可以解决光线不足咯。
补充一句·~你平时看到那些相机说什么iso几千的，其实是不好的，因为我们很少会在黑暗中拍照的，而且黑暗中有闪光灯嘛，除非你做间谍的咯~哈哈 ISO越高，图片越粗糙。