ISO - What is it, and how does it affect my photos?

ISO, i.e. Film Speed - The measurement of sensitivity to light. The higher the ISO value, the more sensitive to light your camera will be.

Values of ISO common today are 50, 100, 200, 400, 800, 1600, and 3200. Some cameras will let you set your ISO in 1/2 or 1/3 increments, but most photographers think of ISO in the whole values listed here.

Digital cameras use ISO settings, even though there is no actual "film" in the camera. It's a way for the photographer to set an equivalent sensitivity for the CCD and define how the CCD will react to available light during the photo capturing process.

When taking pictures on a sunny day in an outdoor setting, you will generally want to use a low ISO setting, such as 100 or 200. When shooting indoors or in low light scenes, you will most likely want to raise your ISO setting, such as 800 or 1600. In effect, when you raise the ISO setting to higher values, you are able to use a faster shutter speed to obtain a proper exposure.

Like most aspects of photography, there is a trade-off for using higher ISO settings. You might have thought to yourself, why not just use the highest ISO setting possible so I can get the fastest shutter speeds? The downside to higher ISO values is the appearance of "grain" or "noise" in your photos. Also, color saturation is sacrificed to some degree with higher ISO settings.

For better quality colors and image clarity, use the smallest ISO value you can that still yields the minimum necessary shutter speed for the subject matter. Sometimes you can get away with slower shutter speeds (like still-life photography, landscapes, architecture, etc.), and so you won't be too concerned about fast shutter speeds. In these cases, use as low an ISO value as you can. Other times you will want the fastest shutter speed available to you, such as sports photos and low-light scenarios. In these situations, use higher ISO values.