ISO is a standard of measurement by which the light sensitivity of your Film/Sensor is rated.
ISO numbers range from 25 to 400,000 (today anyway)
ISO numbers are measured in stops, and are as follows: 25, 50, 100, 200, 400, 800, 1600, 3200, 6400, 12500, 25000, 50000, 100000 200000 400000
Every ISO stop you move, either half’s, or doubles the previous number.
Example 1: ISO 400 is twice as bright as ISO 200. ISO 100 is half as bright as ISO 200.
ISO 25 is a very slow speed, generally only used in very bright daylight, or long exposures. ISO 400,000 is extremely crazy fast and I’m not sure what you would use it for, except maybe night vision. Most of the time anything between 100 and 400 gets the job done.
An ISO number of 25-800 will generally produce a Grain/Noise free image and can be used in fairly bright light.
An ISO number of 12500 and higher will usually have Grain/Noise, but are best for capturing images in low light situations. This will vary by camera, some cameras show visible noise at ISO 800.
On Film Cameras, the ISO may only be changed by changing out the actual roll of film. You can however change the ISO dial so that the camera thinks you are using a different ISO. This is called “Pushing Film”.
Example 2: If you are using ISO 100 film, but you would like your photos a little brighter than normal, you can change the ISO to 50 on the dial so that your camera thinks it has to expose longer due to a slower ISO speed.
On Digital Cameras, the ISO may be changed at any time manually, or automatically by your camera if set to “Auto”. In some cameras you can also set a range of ISO that the “Auto” mode is allowed to use.
Long story short:
Small ISO number = less light, less Noise.
Large ISO number = more light, more Noise.
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