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How does ISO work?

by SX10 IS / October 14, 2009 9:51 AM PDT

After reading the title, you may be thinking, "Ooh boy, we got ourselves a newbie here!" Don't worry, it's not that bad.

I know what it does. I know what it stands for ("International Standards Organization"). I know that ISO is a measurement of light sensitivity. I know that upping the ISO setting is very useful in low-light situations (to create "light", or where the subject is moving very quickly (to reduce blur). I know that various other factors affect the effectiveness. I know, also, that factors such as light-gathering ability (e.g., bigger pixels/photodiodes) affects the degradation that results from too high an ISO setting. I know that ISO is a give-and-take, where the higher the setting, the more "noise" is introduced into the image.

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

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by snapshot2 Forum moderator / October 14, 2009 10:46 AM PDT
In reply to: How does ISO work?

When you raise ISO from 100 to 200, you double the output of the CCD sensor.
Raise it from 200 to 400 you double it again.

It is much like raising the volume on a small portable radio.
The higher you raise the volume, the more distortion you get.
Turn that small radio all the way up and the distortion is overwhelming.

ISO speed, shutter speed and aperture all work together to get the correct EXPOSURE.

So you need to understand Exposure.

Here is a link:

After you understand that, then read about ISO:


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Thanks, but
by SX10 IS / October 14, 2009 11:00 AM PDT
In reply to: ISO

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

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snapshot had it right
by kalel33-20416052469708587370302374692233 / October 14, 2009 1:41 PM PDT
In reply to: Thanks, but

You might just try your radio in the car. Turn it up loud and you will hear distortion, where the music does not sound good. There's a limit to the gain of each system. It's the same as with a camera, if you turn up the ISO(volume) too much then you start introducing noise(distortion). If you turn it up really high(loud) then the photo(music) will look(sound) extremely noisy(distorted) to the point that it's not useful.

Here's the definition from Wiki

In digital camera systems, an arbitrary relationship between exposure and sensor data values can be achieved by setting the signal gain of the sensor. The relationship between the sensor data values and the lightness of the finished image is also arbitrary, depending on the parameters chosen for the interpretation of the sensor data into an image color space such as sRGB.

For digital photo cameras ("digital still cameras"), an exposure index (EI) rating?commonly called ISO setting?is specified by the manufacturer such that the sRGB image files produced by the camera will have a lightness similar to what would be obtained with film of the same EI rating at the same exposure. The usual design is that the camera's parameters for interpreting the sensor data values into sRGB values are fixed, and a number of different EI choices are accommodated by varying the sensor's signal gain in the analog realm, prior to conversion to digital. Some camera designs provide at least some EI choices by adjusting the sensor's signal gain in the digital realm. A few camera designs also provide EI adjustment through a choice of lightness parameters for the interpretation of sensor data values into sRGB; this variation allows different tradeoffs between the range of highlights that can be captured and the amount of noise introduced into the shadow areas of the photo.

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Re: How does ISO work?
by MarkatNite / October 14, 2009 7:04 PM PDT
In reply to: How does ISO work?

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

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I get it - sort of
by SX10 IS / October 14, 2009 10:44 PM PDT
In reply to: Re: How does ISO work?

Thanks, Mark. I'm starting to get it.

But where does noise come from?

And how does ISO reduce blur?

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Re: where does noise come from?
by MarkatNite / October 15, 2009 1:16 PM PDT
In reply to: I get it - sort of

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

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Thanks. Sorry to bother, but...
by SX10 IS / October 16, 2009 3:22 AM PDT

Ya that makes sense, except one thing: The world doesn't have RGB dots floating around!

BTW, what's the "HTH" in your sign-off?

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Looking at Noise
by snapshot2 Forum moderator / October 16, 2009 8:24 AM PDT
In reply to: I get it - sort of

Here is an example of noise from three different cameras:


"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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by SX10 IS / October 17, 2009 10:19 AM PDT
In reply to: Looking at Noise
Ich verstehe. Vielen Dank.

P.S. I'm not German - I just like using various languages.
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by Anwish12 / August 20, 2012 7:08 PM PDT
In reply to: How does ISO work?

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

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by snapshot2 Forum moderator / August 21, 2012 6:13 AM PDT
In reply to: Question

This is not an easy subject.
I suggest you start with this tutorial:

For the complete subject visit this short course:

Regarding : "How does a higher light sensitivity reduce blur?".
Directly it does not reduce blur.
Indirectly it permits you to have enough light available so that you can use higher shutter speeds.

Blur is caused by movement.
Subject blur is caused by the subject moving during the capture process.
Camera blur is caused by any camera movement

You resolve subject blur by stopping all movement of people/things in front of the camera or
use a faster shutter speed. (i.e. if you shot at 1/500th of a second set the shutter speed to 1/1000th of a second.)

You resolve camera blur by making sure you are not moving the camera during the capture process.
The best way to do this is use a tripod or set the camera on a sturdy surface.
To keep from touching the camera during the capture process, put the camera on a tripod and use the 10 second shutter release feature.


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Why does a higher ISO result in more noise?
by earefi / October 4, 2012 1:16 AM PDT
In reply to: ISO

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 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

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