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"Native" ISO... is it real and does it make a difference

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i keep my iso in 1 stop increments mainly so i don't have to twiddle the dial too much...


--- Quote from: Pinchers of Peril on November 22, 2012, 10:04:45 AM ---So my friend told me that it is better to use ISOs that are multiples of 160 since that is the camera's native ISO levels and the ISOs between those are just "pushed or pulled" digitally. Is this true?  Is there any real world difference in the native vs non native ISOs.  I've tried doing some research on it and have found conflicting information. I'm just trying to figure out if there are certain ISOs that I should use or avoid. -Thanks

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This is a mistaken notion based on the behavior of Canon ISO settings and their noise characteristics. Canon uses a 1/3rd stop push/pull approach to achieving non-full stop ISO settings, which can result in some third-stops being less noisy and others being more noisy than their full-stop neighbors. To explain:

Canon sensors use ISO 100 as base ISO, and all of the standard full-stop ISO settings are indeed native. Third-stop settings are actually achieved by futzing with the exposure a bit, rather than directly amplifying the signal to those levels. For example, ISO 125 is actually ISO 100 with a 1/3rd stop underexposure, which is then digitally corrected, or "pushed" up to the correct exposure. ISO 160 is similar, only that it is ISO 200 with a 1/3rd stop overexposure which is then digitally "pulled" down to the correct exposure. ISO 125, since it is a digital push of 1/3rd of a stop, tends to be noisier than either ISO 160 or ISO 200. Conversely, ISO 160, since it is a digital pull of 1/3rd of a stop, tends to be less noisy than ISO 100. The consequence of this approach is that you lose 1/3rd of a stop DR for those intermediate stops of ISO. A third of a stop change in DR is rarely ever an issue in the very vast majority of circumstances though, especially at the lower ISO settings where you have more DR to work with anyway (which is the only time it exhibits...higher ISO's above 800 use an alternative approach.)

People who have noticed this quirk in Canon's noise behavior have made the rather naive assumption that it means Canon sensors actually have a base ISO of 160. On the contrary, when the actual ISO mechanics are investigated, Canon sensors most definitely have a true or native ISO 100, as well as a native ISO 200, 400, 800, and 1600 at the very least, and potentially more depending on the model. All third-stop ISO settings are achieved via push/pull, and at lower ISO settings that results in oscillating noise characteristics. This quirky approach to third-stop ISO settings is actually something I hope Canon moves away from when they move to a new 180nm fabrication process. A true sensor-level analog amplification to all ISO settings would be a better approach.


--- Quote from: MarkII on November 22, 2012, 01:05:40 PM ---
--- Quote from: Marsu42 on November 22, 2012, 12:01:33 PM ---Luckily, the Magic Lantern devs have figured out what iso is "best" - and it's rather surprising and more complicated than one might think...

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The charts here are instructive:

If you look at the 5DIII, you can see the waving up and down of the DR with intermediate ISO steps - exactly what you would expect for a mix of analogue and digital gain setting. In contrast, the 1DX plot is smooth, perhaps because it is using analogue gain for the intermediate steps.

The plots suggest a peak DR at ISO 160 on the 5DIII, which is consistent with the ML folks conclusion that the native ISO is somewhere around 80ish and everything else is a push/pull of that.

The effect is small, however, and probably not worth the hassle of fretting about when shooting... however I wish Canon had a RAW capture mode and metering that operated only at native ISO values.

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What's insane (if its accurate) is the 5D Mark III/1Dx vs D600 .... God damn it :/


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