RE: 8 bit vs. more bits
RE: 8 bit vs. more bits
- Subject: RE: 8 bit vs. more bits
- From: "Mark Rice" <email@hidden>
- Date: Sun, 17 Apr 2005 20:46:33 -0400
- Organization: Zero One
There certainly have been a lot of specious and non-scientific arguments on
this subject lately.
I would like to establish several things:
1. Viewing a monitor to detect differences in 8 bit vs. more bits is a
thankless task. Have you ever asked yourself, how can I get the 16.7 million
colors promised by 8 bit (or 24 bit for 3 colors) from only 1024 lines of
video resolution? YOU CAN'T!!!. We simulate it with dithering.
2. Can a color printing device display 16.7 million colors? Not likely. The
Durst Lambda, at 400 dpi, would need to make a print 3,479 FEET long in
order to use all 16.7 million colors! So dithering is also used in printing
devices.
Both of the above statements are true, and I suspect that is where Dan M's
reasoning coming from. If one takes a perfectly exposed digital photos, or
makes a flawless scan utilizing the entire 256 bit range, than no higher bit
depths are necessary.
However, this is rarely the case for two reasons:
A. Almost every digital device -scanner or digital camera - has a
chip or detector with logarithmic response to linear light values. Some have
tried to produce logarithmic D to A converters to linearize this response,
but log amplifiers are notoriously unstable. So the most common function is
to use high bit depth output to linearize the output curve on 8 bit output.
(See the "Scanning Curve" graphic at
http://www.seafriends.org.nz/phgraph/darkroom.htm)
The result of the conversion, such as the 12 bit to 8 bit conversion from an
LVT film recorder looks something like this:
0-4096 0-255
112 0
115 10
119 20
245 30
... ...
1960 225
2567 235
3200 245
4056 255
This is if you are lucky, and the light sources are flawless, the modulators
are flawless, and film processing is precise.
More likely is a table something like this:
0-4096 0-255
1950 0
2100 10
2250 20
2450 30
... ...
3100 245
3700 255
4025 255
4096 255 CLIPPING!
Digital camera chips work in a similar fashion - if the exposure is perfect
(Ha!), then everything works fine. If not, then you have similar problems
with clipping and curve shape distortion. The greater the bit depth, the
more headroom there is for curve shape correction in conversion. This is
critical point number 1. Point number 2 to follow in separate post.
Mark Rice
email@hidden
www.zero1inc.com
Today's Topics:
1. RE: 16 bits = 15 bits in Photoshop? (michael shaffer)
2. Re: 16 bits = 15 bits in Photoshop? (email@hidden)
3. Re: 16 bits = 15 bits in Photoshop? (Robert L Krawitz)
4. Re: 16 bits = 15 bits in Photoshop? (Marco Ugolini)
5. Re: 16 bits = 15 bits in Photoshop? (Jim Rich)
6. Re: 16 bits = 15 bits in Photoshop? (email@hidden)
7. Re: 16 bits = 15 bits in Photoshop? (email@hidden)
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