What is base ISO, and are intermediate ISO settings noisier than full-stop values?
Asked 6/19/2012
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I’m trying to understand what “base ISO” or “native ISO” means on a digital camera. Is base ISO the camera’s single lowest native sensitivity setting, or does it refer to all full-stop ISO values such as 100, 200, 400, 800, etc.? Also, are intermediate third-stop settings like 125, 160, 250, or 320 sometimes created differently, and can they produce more noise than the next higher full-stop ISO? How can I determine my camera’s true base ISO?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
14y ago
2 Answers
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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.
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
14y ago
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Generated from our catalog & community — verify before relying on it.
Base ISO usually means the camera’s single native, unamplified ISO setting where it delivers its best dynamic range and signal-to-noise ratio. It is not the whole series of full-stop values like 100, 200, 400, 800, etc.
On many cameras, higher ISO settings use analog amplification before readout. Some cameras also generate certain intermediate ISO settings with digital amplification after readout, which is generally less ideal because it also boosts read noise. In those designs, some in-between ISO values may be no better than shooting at a nearby native setting and adjusting exposure later.
So: no, you should not assume every full-stop value is a “base ISO,” and you also shouldn’t assume every third-stop value is always worse. It depends on how your camera implements ISO gain.
To find your camera’s base ISO, check the manual or reliable technical reviews/specs. It is typically the lowest normal ISO setting, while lower expanded settings are often labeled “Lo.” For many cameras this is ISO 100, but some models use ISO 200.
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