Warning with extended low ISO !

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Christof21 Veteran Member • Posts: 3,907
Warning with extended low ISO !


This issue happens with every Fuji cameras..

Here are the amounts of overexposure in any of the auto exposure modes::

  • ISO160 gives you 1/3ev more exposure
  • ISO125 : 2/3ev overexposure
  • ISO100 ev overexposure

The problem is it will shift the raw histogram 1ev to the right !!! For ISO100, this is expected, but you would not expect this for ISO160.

The 3 of them are in fact equally risky if you care about clipping highlights.

Switching from ISO200 to ISO160 has a much bigger impact than expected

I took 3 pictures of the same scene with these 3 different ISOs.

The x-axis at the top labelled in ev

  • ISO200 shot (blue channel with rawdigger)

  • ISO160 (1/3 ev more exposure)

  • ISO100 (1ev more exposure)

The histograms has shifted to the right from -3 => -2 for ISO100 and ISO160. It clips as much highlights !!

  • What is happening ?

At ISO160, you arer loosing quite a lot DR.


I thought this was an error.. In fact no.

So at ISO160, you just overexpose 1/3ev but you have a penalty of 2/3ev due to the reduced DR.

More precisely: when you use ISO160, which is not a real ISO, internally it uses ISO320, not ISO200. This is not an optimal implementation, this implementation wastes DR stupidly.

ISO100 and ISO200 have the highest DR

ISO125 and ISO250 have the same DR (ISO125 uses ISO250 internally)

ISO160 and ISO320 have the lowest DR, thus the highest penalty.

For all of the extended ISO, you must have 1ev margin to the right of the histogram.

  • Conclusion

If you have plenty of light and you want to optimise exposure, switch from ISO200 to ISO100 or don't switch. But do not use intermediate ISO, Fuji implementation is really weird.

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