[svlug] Monitor calibration, photo processing

John Conover conover at rahul.net
Tue Apr 8 09:26:49 PDT 2008


Larry Colen writes:
> Last night I was sorting more photos from this weekend, I used Monica
> to set the gamma, which gives me a gamma of 0.46 - 0.50 (or about
> 1/2.2). However, anything but photographs looks crappy, watching
> MPEGs, they look way too dark. Even the text in my termial windows
> looks a bit weird.
>

Some LCDs are set to about 1.1 out of the box-which is good for HDTV
and DVDs. Some have settings in the monitor setup menu to customize
the monitor itself to a specific gamma without using xgamma. Others
are detected by X-Windows and gamma set appropriately in the drivers.
Yet others, (particularly in TV sets,) set the gamma dynamically,
depending on picture intensity data.

The 2.2 gamma spec is a legacy from the image orthicon camera tube
used in the original TV system of the 1940's, which had a square law
output vs. light intensity. The CRT had a square root light output
intensity vs. input, so the two canceled for a system gamma of
1.0. With a 1% accuracy in intensity, this gives about a 10,000:1
contrast ratio, which most LCDs can not meet. (The original spec, from
the 1930's, was 2.0-but provided inadequate intensity ratio, and was
changed to 2.2 in the US spec; the European PAL, which was implemented
later, spec'ed a gamma of 2.9, which is the default for most CRT
computer monitors from the factory.) The overall orthicon/crt system
provided 5500 degrees K colors, which was the spec, (and most CRT
monitors have a configuration button for that-most LCDs don't.)

The www consortium RGB standard for jpegs, for legacy compatibility,
(and many cameras use this spec,) is 2.2 at 5500K. (The raw format, if
you can decode it, is usually 1.0, which is the "native" output of a
CCD.)

Adding a printer in the loop further complicates things, since it has
a gamma, too, (usually 1.8, which is a legacy provided by the original
Apple/LaserJet, and is a defacto standard in the industry,) and those
that produce camera ready artwork do not use RGB colors, (they use
four colors, CMYK,) for the palette, (and RGB does not match the gamma
for pinks, under a 5500K source light.)

Probably accounts for why corporate communications departments favor
Apple computers, since all this is taken care of out of the box, (and
explains why they like PhotoShop, with its endless twiddles on gamma,
etc.)

	John

-- 

John Conover, conover at rahul.net, http://www.johncon.com/




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