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ISO

ISO (International Organization for Standardization - Standard 12232:200) is the measurement of how sensitive the digital sensor is. It corresponds to the notion of "film speed" in a film camera, and could be called "sensor speed" in a digital camera. Like shutter speed an aperture, ISO is part of the exposure triangle.

Sensor design has a lot to do with ISO speed, and generally the higher the ISO, the less quality you can expect. The degradation is primarily in the form of noise, which can be characterized as pixels having random colors, especially noticeable in the darker areas of the photo. High ISO Noise in a digital camera is not unlike high ISO Grain in a film camera.

ISO speeds typically range from a low of 100 to a high of 12,000 and beyond in the top performing cameras. Like Apertures and Shutter speeds, ISO uses the Inverse Square Law, and have standardized sensitivities that are spaced so that they are 1 EV (exposure value) apart. Each change in ISO value is twice as sensitive to light, or half as sensitive, depending if you are increasing or decreasing the sensitivity.
 

Standard 1 EV Scale
64
100
200
400
800
1600
3200
6400
12800

 

The effect of High ISO Values

Generally it is desireable to use the lowest ISO that you can, as the higher the ISO, the more noise is introduced in the photo. Some cameras do have ISO noise filters, and they work to some degree, but often at a cost of delay in the ability to take the next photo as there is some processing time involved.

Some post-processing software, such as Adobe Photoshop Lightroom V3 have outstanding noise filter reduction processes, and can often correct noise in photos. While there is a limit to what is achievable, you can often compensate for a slow lens with increasing the ISO, then noise correction in Lightroom.

 

Examples

 

This photo was shot with a 70-300mm f/4.5~5.6 lens (not exactly a good low-light lens), and an ISO of 6400. As a result, there is significant noise.
 
 

An enlarged section of the original photo shows the noise in the photo.
 

The same photo - processed in Adobe Photoshop Lightroom. The noise is about all but gone.
 
 

The closeup of the processed photo shows the noise has disappeared. There is a bit of softness in the photo, but totally acceptable.
 

Remember that unlike shutter speed and aperture where there are creative reasons to use both, ISO is usually always the best at the lowest end. The only real reason to adjust ISO higher is to allow the camera to obtain the proper exposure in low light conditions.