Kibo informs me that Timo Autiokari stated
that:

On Mon, 22 Nov 2004 21:32:21 GMT, wrote:

I'm not so sure that ACR works in a totally linear domain.

It definitely doesn't.

Images
exposed with bracketing, and compensated to be the same with the
exposure slider, may have equal mid-tones, but the shadows and
highlights will display that a different gamma is used. If you drag the
ACR exposure slider to the left, after it runs out of "hidden
highlights", it stretches the highlights so that 4095 in the RAW data
stays anchored at 255 in the output, and never gets darker. That is not
linear exposure compensation.

Nor is it an exact analog of adjusting by F-stops (ie; non-linear),
which is what I'd like it to be. You should try C1 some time, its
exposure compensation adjustment control is *way* more like adjusting
the exposure compensation dial on a camera. Going from that to to ACR
weirds me out every time.

That editing operation is still applied over linear image data, even
if the operation itself is not linear. Exposure adjustment in fact is
a linear operation, multiplication by a factor,

Incorrect. It's scaled in F-stops, which are exponential, not linear.
You'll find the mathematical details in any good textbook on
photography.

So at the middle it will calmly snip one level away, the coding there
is: ...7FF8h 7FFAh 7FFCh 7FFEh 8000h 8001h 8003h 8005h ... Due to this
discontinuity 'd say that the 15th bit of Photoshop is quite un-usable
for applications that require accuracy.

*sigh*

You're talking about the LSB of a 15 bit value sometimes skipping a
value, which, (assuming that you're correct about it), is an inaccuracy
of around 0.003%. To put this hypothetical error into perspective, it'd
have to be at least *four times greater* to alter a 12 bit RAW image by
even a single step in value - a change that would be not only be
completely invisible to the human eye, but would be completely swamped
by the much, much greater errors contributed by the sensor noise in the
camera, *plus* the ADC error in the camera, *plus* the colour-space
rounding errors in the computer, *plus* the DAC inaccuracy in your video
card, *plus* the video amp inaccuracy in your monitor. The 'error'
you're talking about is about as significant as an ICBM missing the
targetted position by a couple of feeet.

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