While using a higher ISO setting is often needed to capture images with reduced blur in lower light, it also increases the noise level of the image (In film this is often referred to as "grain"). A lower ISO setting is preferred whenever possible since it helps to reduce this noise or grain.
ISO settings can also be used to help control the shutter speed of a camera while in automatic mode. In order to "freeze" motion in a scene, a camera needs to be able to use a higher shutter speed. By selecting a higher ISO you are allowing the camera to gather more light, this automatically forces the camera to select a higher (faster) shutter speed, which helps to reduce motion blur.
With film, the ISO rating is considered a "speed" rating. ISO 100 would be considered a slow film, while ISO 400 would be considered a faster film. Digital cameras obviously don't use film thus the ISO number corresponds to the image sensor's light sensitivity.
|
ISO |
Effect on Picture |
|
100 |
Great for bright sunny days, at the beach or on the snow. Produces clean images that are great for enlargements. |
|
200 |
Great for overcast daylight pictures (noise levels may increase, but in most cases not noticeably) |
|
400 |
Great for lower lighting conditions (indoors, night time) or when you need to capture faster moving subjects in lower lighting conditions. In many consumer cameras, ISO 400 can make photos look very noisy in dark areas of the picture. The reason a higher speed ISO helps you capture fast moving subjects is because a higher ISO makes the image sensor of the camera more light sensitive. This forces the camera to use a higher shutter speed to compensate for the extra brightness, which in turn helps to "freeze" movement in the captured frame. |
This example demonstrates increased noise (and
depth-of-field, which by the way is the area in focus in your
picture, high dof, more area in focus, low dof, less area in focus)
in a photograph as you increase the ISO. This example was shot
using a Canon EOS-10D which is known to have very
little noise at higher ISO's when compared to
consumer level digital cameras. As you can see, the photograph on
the far right has increased noise (grain) but is also a bit more in
focus. I focused and zoomed the camera in on a tree (bottom left
corner) which was about 80 feet in front of the garage (window).
The higher ISO setting allows the image sensor of the camera to be
more light sensitive. It also allows the camera to set a smaller
aperture which helps it to achieve a greater depth of field. This
is why the Window is a bit more in focus in the photo on the
right.The most common ISO speed settings are: 100, 200, 400 and 800. Depending on your digital camera model you may also have them in the range of 64, 100, 160, 200, 400, 640, 800, 1600.
