Re: Linear-light RAW 12bit vs R'G'B' 8bit: how much better is it really?
Re: Linear-light RAW 12bit vs R'G'B' 8bit: how much better is it really?
- Subject: Re: Linear-light RAW 12bit vs R'G'B' 8bit: how much better is it really?
- From: Ray Maxwell <email@hidden>
- Date: Tue, 24 Jul 2007 14:40:40 -0700
Mark wrote:
Hello Ray, list,
There's still something I do not understand:
<SNIP>
Confusion starts with prints and film. Film gets projected through a
light bulb and prints reflect light falling in. Naively I would
suppose that both should be coded linearly to light intensity
(transmission or reflection), in order to mimic the real life subject
they represent - as with the example abobe with the R'G'B' image and
the CRT - the overall power function should be 1, so the image gets
interpreted correctly by our eyes.
In the study of sensitometery (the measure of the sensitivity of film to
light) we usually plot the typical transfer function on a Density vs
Log of Exposure. On this plot there is a linear portion. Note that
density is logarithmic and the log of Exposure is logarithmic. So the
reaction of film to light is not linear with respect to energy.
On the typical press you would get about 20% to 25% dot gain so that
the 50% dot on plate would print as a 70% to 75% dot on paper. If
you plot the input dot area specified and measure the L* value out on
press, you will find that it is visually linear which means that the
end to end gamma is about 2.2.
Well, that breaks my logic described above. I would expect 50% gray to
have L* value of 50 both on print as on film. Why are both encoded
with approx. end to end gamma of about 2.2? Shouldn't overall gamma
always be close to 1 so the light hitting our eyes gets interpreted
correctly.
For a print to appear visually linear to the human eye the density of
the dye or pigment must be non-linear. The amount of light reflected or
transmitted is non-linear. Remeber that L* values are a model of human
perception. L* is not linear with respect to energy.
The bottom line is...Perception is in your brain and the end to end
system is not linear. This allows you to see in very low light levels
and avoid being eaten by a tiger at night and also not go blind when you
are exposed to direct sun light.
Ray Maxwell
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