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If I understood correctly, base ISO is in full stop steps from the lowest possible ISO setting on my camera. For example if the lowest setting on my camera is ISO 100, than the following table would be base ISO:

  • 100, 200, 400, 800, 1600, 3200, 6400

While steps in between like

  • 125, 160, 250, 320, 500, 640, 1000, 1300, 2000, 2500, 4000, 5000

would be third-stop "pulled" values, which result more noise than perhaps higher base ISO values (for example 2500 is worse than 3200).

  • Did I get this right or missed by a mile, what did I miss?
  • Do non-base ISO values generate more noise than next higher base-value ISO stop?
  • How do I find what are my base-value ISO stops (is it the lowest ISO value multiplied by 2n)?
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2 Answers 2

up vote 6 down vote accepted

I think you might be confusing a few issues here. The terms "Base ISO" or "native ISO" are often used to refer to the unamplified sensitivity of the camera.

In addition to this digital camera sensors have built in amplifiers to amplify a weak signal (such as you get in low light) before it is digitised in order to reduce read noise and increase signal to noise ratio.

Some camera makers also use digital amplification (i.e. increasing the numerical values in a RAW file in software).

Canon for example use hardware gain for the whole stop sequence, and software gain for the in between stops. Software gain is generally inferior as it happens after readout and so you amplify the read noise, plus it doesn't do anything you couldn't do on your computer in post processing if required.

Some manufacturers use hardware amplification for all ISO settings so this isn't a problem.

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1  
In the case of Canon cameras (in my case 60D) is there a way to turn off the "software gain"? –  Randy K Jun 19 '12 at 22:23
    
@RandyK That is best asked as a separate question rather than posed as a comment to Matt's answer. –  Michael Kjörling Jun 21 '12 at 12:04

Yes, the base ISO steps is generally what you describle. Note that some cameras have extended ISO settings below the lowest base ISO, so the lowest base would be the lowest "normal" ISO setting.

The steps in between is created similarly to push processing film. The picture is simply slightly over- or under-exposed, and the developing process compensates for this.

This process generates slightly more noise than the base ISO steps, because the signal is amplified after the AD conversion instead of before. However, the inbetween steps would not be worse than the next higer base ISO setting, simply because you get more light in, so the total amplification is still less.

You would see the difference in noise if you for example use ISO 100 and underexpose three steps compared to using ISO 800. For the inbetween ISO steps the difference in noise is so small that you won't see it.

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