Digital cameras forum: How does ISO work?

Ya, ho capito about the exposure thingamajiggy, but I still don't understand how ISO works. Ya haven't explained that yet.

I don't listen to radio that often, and when I do, I keep it low (I like to protect my hearing, you see), so I don't get your metaphor.

Mind 'splainin' some more for a fool? ;]

Thanks Joe.

- David

Instead of output level, think of ISO as input sensitivity (because that's what it really is). So, if we stick with an audio analogy, instead of speaker volume, think of it as microphone sensitivity.

In order to make an audio recording of a quiet sound (e.g. someone whispering), you need to turn up the mic sensitivity. Similarly, in order to make a video recording of something dim, you need to turn up the ISO sensitivity.

But when you turn up the mic sensitivity, EVERYTHING becomes louder, not just the whisper. You also hear more background noise (wind, crickets, etc.) as well as any electrical "hum" in the system. Similarly, when you turn up the ISO sensitivity, EVERYTHING becomes brighter, not just your subject. You also see any background lighting artifacts as well as any electrical "grain" in the system.

Another metaphor could be pressure. e.g. a (weight) scale. If you want to weigh something light, you need a sensitive scale. But the more sensitive the scale is, the more susceptible to fluctuations (due to wind, movement of the thing being weighed, a worn-out spring) it is. The camera's sensor corresponds to the scale, and the photons of light are the thing being weighed.

HTH - Mark

It doesn't really "come from" anywhere because it's always there, you just don't see it (or hear it, in the audio analogy) unless the sensitivity is high enough.

How about this: have you ever noticed that, during the day, when the TV is on, the kids are playing, the dogs are barking, etc., you don't hear your clock ticking? That's because your brain "sets" your ears to a relatively low sensitivity. But when you're laying in bed at night and it's quiet, and your brain sets your ears to be more sensitive, now you hear your clock ticking. Well, it's not like the sound of the clock suddenly "came from" somewhere; it was always there, you just weren't sensitive enough to notice it earlier.

As for ISO affecting blur, I am not aware of this. Well, since ISO affects exposure, and any changes in ISO will necessitate adjustments in shutter speed and/or aperture (both of which *do* affect blur) in order to keep the pic properly exposed, I guess you could say ISO affects blur. But that's not as accurate a statement as it could be.

HTH - Mark

Here is an example of noise from three different cameras:

http://www.dpreview.com/reviews/canonsx100is/page6.asp

...

"How does ISO reduce blur?"

It doesn't (by itself)

Assume that the camera has automatically selected an exposure of ISO 100, aperture of f/2.8 and shutter speed of 1/125 of a second.

If you raise the ISO setting from 100 to 200, that permits you or the camera to raise the shutter speed from 1/125 of a second to 1/250 of a second, while maintaining the present aperture setting.

Raise the ISO from 100 to 400, that permits you or the camera to raise the shutter speed from 1/125 to 1/500 of a second, while maintaining the present aperture setting.

A faster shutter speed will reduce motion blur, such as the blurred feet of a runner.

..
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But how does it all work? Why does a higher ISO result in more noise? How does a higher light sensitivity reduce blur?

lets steak to the idea of input and pixels...
before digital era, ASA or film sensitivity was the count of cells which expose to the photon! more sensitivity means bigger cells which expose with less light! now a day ... same idea.... when we adjust higher sensitivity (iso) on the camera means we order to a group of pixels to become one big pixel...like 4 to 1, 16 to1, 64 to 1...naturally when we adjust a very high iso then result will be distorted (grained) and that is because of the large size pixels... so if you print that photo in a large size will be completely distorted... in photography we call those distorted points grains...

hope it was helpful
regards