D4 vs 1Dx (Imaging Resource), RAW, ACR, 6400-12800

Started Sep 4, 2012 | Discussions thread
bobn2 Forum Pro • Posts: 59,068
Re: What is ISO?

Jack Hogan wrote:

bobn2 wrote:

Jack Hogan wrote:

DxO uses ISO as a proxy for illuminance/luminance (as the formula you linked shows), not metering accuracy.

Nor processing tone mapping, which is the other bit of ISI.

Assuming we are looking at raw data, what tone mapping is there in ISO? Do you mean linear multiplication/division to get the correct 'brightness', like EC in PP? That's what I referred to as amplification/sensitivity (whether analog and/or digital).

Two levels of 'tone mapping'

i) Which number in the raw file relates to which number in the sRGB utput file is completely indefined within ISO, as is which lux seconds value relates to which raw number. ISO (in the 'hard' versions) maps only lux seconds on the sensor to a (single) value in the sRGB output file. Anything that goes on in between is outside the scope of 'ISO'.

ii) The 'hard' versions of ISO map sensor illuminance in lux seconds to a sRGB value after application of gamma correction (2.2).

iii) Even using those 'hard' definitions of ISO, there is a single fix point (100% or 18%) - what happens apart from that point is completely undefined.

If DSLRs were responding according to the ISO standard, for a given iilluminance/scene luminance, exposure and amplification (ISO)

'amplification' is not ISO. It has nothing to do with ISO, except that in some cameras the amount of analog gain ('amplification') varies with the ISO setting - but that is an engineering kludge, not what ISO is.

You are right, to be clearer that should state 'for a given iilluminance/scene luminance, exposure and amplification (both analog and digital as controlled by the in-camera ISO setting, which numerical value should conform to the relative standard)'

No, because scene luminance is already included in exposure and amplification is not defined under the ISO standards. The only thing they define is exposure (sensor time-illuminance) and (a single) output file value in sRGB. All the rest is implementation dependent.

all cameras would record raw values that represent the same percentage of saturation (Studio460 said it well in his post). But they don't.

No, they don't, and there is absolutely no reason why they should.

Yes they don't, but shouldn't they if they conformed to the standard?

No, the standard has nothing to say about the senor saturation level.

Isn't the ISO variable Ssat determined by observing at what luminous exposure the camera first reaches its maximum raw output (100%, saturation)?

No, it's determined by the maximum sRGB output, after all the processing that goes to make an sRGB file.

So for instance, both cameras at 'base ISO' and all other things being equal, Camera A reaches 100% at 1000 cd/m2 while Camera 2 reaches it at 500 cd/m2, so according to the formula you mentioned

SsatA=1/2 SsatB.

Using the wrong Hsat. Hsat is the sRGB saturation exposure, not the raw file saturation exposure.

And, aotbe and linear, should one make both cameras ISO as measured above the same by pushing Camera A's ISO in-camera by one stop, wouldn't both cameras now reach 100% output at a luminance of 500 cd/m2? And if we cut luminance to 250 cd/m2, wouldn't both cameras output raw values that read 50% of full scale?

Wouldn't this be true of any camera that conformed to this standard, all other things being equal?

No, because the standard does not apply to raw files.

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