ISO explained
Hello, Lifetime Learners I found this post on F-stoppers website that explains ISO very well. It’s not a perfect post. The author claims that raising the ISO increases the brightness of the capture. It does not. It increases the sensitivity of the sensor and amplifies the ability of the sensor to respond to light, like turning up the volume on a radio. For a given aperture, raising the ISO requires a higher shutter speed to provide a correct exposure; or, for a given shutter speed, raising the ISO requires the aperture to be closed down (higher f/stop number). And, like a radio, the higher the volume the higher the “hiss” background noise. That’s the increase in grain-like artifacts in the image that degrades quality, as ISO is raised. After you wrap your head around this concept, the rest of the article is quite accurate. You might also want to glance at the comments that follow the story. I’m always amazed at how uncivil these discussions can be and the astonishing lack of knowledge and experience form many who post. fstoppers article on choosing ISO 
Comments
Jim Kahnweiler(non-registered)
Thanks, Tom, for clarifying my post. I'm no audiophile and I used the wrong analogy. So, I should change "radio" to "microphone."
"...It increases the sensitivity of the sensor and amplifies the ability of the sensor to respond to light, like turning up the volume on a radio."
Actually turning up the volume on a radio is the opposite of turning up the ISO on a camera. Cranking up the ISO is applying gain to the sensor signal, and as we know will introduce artifacts like visual noise. Conversely, turning up the volume on a radio is actually reducing the resistance to the outgoing audio signal. Without a volume knob your radio would play at full volume, and unless you're 18 years old you probably don't want that, so a volume attenuator is needed. This is why the volume knob on a Denon receiver, for example, has an infinity symbol at the position where NO sound would come out of the connected speakers. It's indicating infinite or absolute resistance to the audio signal. Applying gain to an audio signal is common when recording or capturing an audio signal (as opposed to playing it back). Much like ISO and capturing a still/video image, one might need to turn up the gain on the audio recorder or on the microphone itself if the microphone isn't getting the sound levels desired, and in doing so artifacts like hiss will probably show up.
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