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: Mon, 23 Jul 2007 16:16:37 -0700
Hi Mark,
I will try to explain a little about the word "linear" that is thrown
around a lot in photographic and printing circles. If I am wrong, I am
sure that someone in this group will kindly correct me.
First, the data that comes out of the sensor of your digital camera is
"linear" with respect to energy. That means if twice the energy comes
into the sensor (increase of one f stop or double your shutter speed)
the number that comes out will be twice as big. It is correct that the
first stop at maximum exposure without clipping contains 2048 levels in
a 12 bit system. The next stop down contains 1024 etc.
Now, the only problem with this is that humans don't react the same way
as sensor chips. Most of the human senses are logarithmic. This is why
we usually covert to a color space with a gamma of 2.2 which is
approximately visually "linear". This means that if we make a change in
the highlights of five units and then make a change in the shadows or
mid tones of five units our eyes will perceive the same amount of change.
This is the reason that sRGB and Adobe RGB 98 are both gamma 2.2. This
makes these spaces visually linear for easy editing.
When everyone was using imagesetters to make film for offset printing
they would "linearize" the imagesetter. This meant that when you said
that you wanted a 50% screen in illustrator the imagesetter would output
a 50% dot on film. When you make a plate and print on paper you get dot
gain. 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. Do to people linearizing their imagesetters
they adopted the term with respect to inkjet proofers when they usually
mean "calibrating" to a known condition rather than making them linear.
The proofer has to emulate the dot gain on press and would not work if
it were "linear".
Hope this helps to understand the difference between energy linear vs.
visually linear.
Ray Maxwell
Mark wrote:
Hi all,
can someone please explain me why 12 bit linear light RAW images are
supposed to be much better than gamma corrected 8 bit images?
Some sources state that 8 bit R'G'B' could be coded linearly with
about 11 bits. So while a 12 bit RAW image does have finer coding
than a 8 bit R'G'B' image, it is not that much more (as one might
naively think at first).
Is that correct or have I gotten it all wrong?
If it is like that, what's the big point in shooting RAW?
Cheers
Mark
_______________________________________________
Do not post admin requests to the list. They will be ignored.
Colorsync-users mailing list (email@hidden)
Help/Unsubscribe/Update your Subscription:
This email sent to email@hidden
_______________________________________________
Do not post admin requests to the list. They will be ignored.
Colorsync-users mailing list (email@hidden)
Help/Unsubscribe/Update your Subscription:
This email sent to email@hidden