Shooting at higher ISO numbers allows more light to enter the camera. In low light, high ISO helps reduce the effect of camera shake, which causes images to blur. It also allows photos to be taken without a flash, making photos appear more natural.
However, unlike digital single lens reflex cameras, most compact digital cameras have small sensors so image quality starts to degrade when sensitivity is set above ISO 100. The higher the ISO, the more noise is introduced in images.
Pitfalls of using high sensitivity mode
If your camera has a High Sensitivity Mode, use it cautiously! The mode automatically selects a very high ISO number according to the level of light in a scene: ISO 800, 1600, 3200 and above!
When set to very high ISO, a digital camera may automatically reduce the size, or resolution, of images. The reason the camera reduces the resolution is because noise is usually less visible when an image is reduced in size.
High ISO photos may not print well, since image detail is smeared due to noise. Depending on the degree of noise, it may not show in a 4X6″ print. But noise becomes readily apparent in large prints.
Noise reduction and high ISO
Digital cameras address noise with built-in noise reduction (NR). However, noise reduction can cause images to look soft. Loss of fine detail and smearing of colors occurs in the original image. The higher the ISO, the more noise reduction is used resulting in greater smearing and loss of detail. The detail is rarely recovered when editing.
Bottom line about using high ISO
Avoid using very high ISO number unless you have a digital camera that is capable of handling it well. For important low light shots, consider shooting at a lower ISO number and using a camera support such as a tripod.
Remember though, getting a shot, even at reduced size and image quality, is often more important than not getting the shot at all. So if there are no other options, shoot at high ISO numbers.