Updated: Apr 2
Whether you shoot on a cheap cellphone or a high end DSLR, whether film or digital, every camera essentially relies on 3 variables: aperture size, shutter speed, and ISO.
"ISO" stands for "International Standards Organization". In the context of cameras, it is always followed by a number (e.g. ISO 100, ISO 200, ISO 400, ISO 800, etc).
You may also see certain kinds of film using "ASA" instead of "ISO". For all intents and purposes, ASA can be treated the same as ISO. "ASA" stands for "American Standards Association".
The short and sweet explanation of ISO is that it refers to the level of light sensitivity of the film or sensor.
The higher the number, the more sensitive the sensor is and the less light you need to expose the photo correctly. The trade-off is that higher ISO also results in more noise or grain in the photo - too much of this can be very unpleasant.
The noise is particularly visible in the dark parts of the photo.
Low ISO means the sensor is less sensitive to light, so you need more light to expose the photo correctly. However, the lower the ISO, the cleaner and less noisy or grainy the photos tend to be. If you want a really clean photo with minimal noise or grain, you should opt for a low-ISO film or set your digital camera to a low ISO number.
Low ISO, little or no noise
To summarize, here's a table of how ISO affects your photo-taking:
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