In the beginning, the US had laid down certain standards which applied directly to the quality and graininess of photographic film. The grain was caused by the chemicals on the film’s surface that produced the image when subjected to light. Therefore they were “light sensitive“. The less sensitive to light they were, the lower the number given in the standard and the less graininess the film. Resulting in a clearer and sharper image.
You only relate grain with film, on digital cameras you get what is called digital noise. Like when you turn up the sound on your radio and the louder you go, the more hissing and buzzing you get, but it shows up on your prints as minute little coloured dots and the higher the ISO rating the more obvious it becomes. Which in some cases you may not even notice, until you have your prints enlarged.
The ASA (American Standards Association) numbers would range from say, ASA 25, ASA50, ASA100, ASA200, ASA400, ASA800, etc. These numbers also coincide with the camera’s shutter speed and aperture settings, but that’s another story.
The body now governing these standards is known worldwide as the International Standards Organisation - hence the ISO rating. ISO400 is accepted as a standard speed, below that are slow speed settings and above ISO400 are known as fast speed settings.
Some higher end of the scale cameras will go up to ISO3200. although the default setting on most domestic quality digital cameras is usually ISO100.
When you use your camera in auto mode, it will automatically select the correct ISO rating for the type of photography you are doing.
For instance, if you select “Sport’ on the mode dial, depending on the brightness of the available light source, it will set a higher ISO number, to enable a higher shutter speed, which is required to freeze the action of your fast moving subject.
Some photographers however, don’t really mind the grain, but the higher number can also flatten colours somewhat.
Now, if you were to select “Portrait” mode, it will set a smaller ISO number to produce better skin tones and saturated colours, and without the grain (noise).
If your camera allows, you can manually alter the ISO setting to fool your camera into thinking it has more or less light to play with, depending on how you wish to adjust the exposure. For more light choose a smaller number, for less, a higher number.
In an ideal situation, the smaller the ISO rating the better quality your photos will be. That is, after taking into consideration all other factors. Your digital camera‘s default setting is probably ISO100.
See also "The correlation between shutter speed and aperture settings"
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