PetaPixel

Lower ISO Doesn’t Always Lead to Higher Quality Images

When learning about ISO, you’ve probably heard that the lower the number, the lower the noise and the higher the image quality, but did you know that this isn’t always the case? The reason is something called the base (or native) ISO of a camera — the ISO achieved without amplifying the data from the sensor. This is usually somewhere between ISO 100 and ISO 200. Why does this matter? Bob Andersson of Camera Labs explains:

We all know that using high ISO numbers results in more sensor noise. More surprising, perhaps, is that using an ISO number below the native ISO number also degrades the image.

An interesting example is that when shooting on a Canon EOS 1Ds MkII, ISO 50 has roughly the same signal to noise ratio as shooting at ISO 800. This explains why the lowest possible ISO numbers can only be accessed through custom functions on some cameras.

Know your Base (or Native) ISO (via Reddit)


Image credit: Photograph by Filya1


 
  • Anonymous

    interesting. All I know is that I love my D7000’s native iso range ;D

  • http://www.petapixel.com Michael Zhang
  • http://twitter.com/WeiseStudios Dana Weise

    Ah wow, I didn’t even know. Very interesting indeed.

  • http://twitter.com/TIBoine Tor Ivan Boine

    feel like this article is missing something.

  • Kyle Charbonneau

    Maybe this will help clarify so here is a simplified example. Think of each pixel on the CCD as a bucket. As photons start hitting it it starts to fill up. Once the shutter closes photons stop hitting the pixel and it stops filling. The photons create an electrical charge, which is read out, measured, converted to a digital value and eventually all of the pixels combined create a digital image.

    At the native ISO the the brightest bits should have completely full buckets, the darkest bits basically completely empty buckets and everything else is in between. In practice this is more complicated because there is usually more dynamic range in the real world than your camera can capture AND there are also limits on well you can measure the charge at each pixel and convert it to a digital value but don’t worry too much about that.

    If you’re shooting higher than the native ISO none of your buckets will be full. The camera has to amplify all the values to get the proper dynamic range. This leads to noise since you have less information to work with in the first place.

    If you’re shotting below the native ISO then the buckets where there are highlights start to get full before the shutter closes. What happens when you keep pouring water into a full bucket? It doesn’t get any fuller thats for sure. Therefore you start to lose information in the highlights.

    I believe the reason you can shoot below the native ISO at all without losing your highlights is because the CCD capacity for charge isn’t always the constraining factor. It can in fact get fuller than can be properly measured. Therefore, when you start going above the ideal highest charge you lose precision in reading out the values.

    Hope that makes sense and is all accurate. I have a decent understanding but I’m not an engineer.

  • Sherrie

    Um, Kyle, that was awesome.

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  • Guy J

    Interesting, but where do I find the “native” iso of my EOS 550D?

  • http://www.pixeltraining.net Armando

    I love these technical articles you post.

  • Amadeus Hellequin

    My brain is full of fuck.

  • http://elabua.myopenid.com/ Bua

    Kyle that was a super cool explanation yo!