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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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Take a look at clarkvision.com/articles/iso/index.html , which is very interesting. He works on space probe imagers or something, and knows the technology from before it trickled down to consumer camera. –  jdlugosz Dec 8 at 6:17

2 Answers 2

up vote 8 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

Sorry, you're way off. :-) Your digital camera only has one base ISO value. The base ISO is the single ISO setting at which your sensor/processing pipeline produces its best signal-to-noise ratio.

The base ISO is typically the minimum ISO value which can be selected numerically. (ISO settings below the base ISO are usually marked PULL or LO to avoid any confusion.)

The 1/3 ISO stops available on your camera do not entail any compromise. The sequence 100, 200, 400, 800 isn't any more special than 125, 250, 500, 1000.

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Are you saying you'll get the same result when exposing an image with for example F2.0, 1/100s, ISO1600 and F2.0, 1/100s, ISO100, but pushed 4 stops up in post processing? –  Miljenko Barbir Dec 8 at 7:03
    
No. You want to read Matt's fantastic answer here. –  Philip Kendall Dec 8 at 8:59
    
-1: sorry, but this is wrong for at least some sensors. As Matt has noted, at least on some Canon sensors, things outside the whole stop sequence are produced in software, not hardware - so there is a fundamental difference between ISO 100 and ISO 125. –  Philip Kendall Dec 8 at 9:00

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