wrote in message
...
In message ,
"David J Taylor"

wrote:

wrote:

In therms of luminance, it is actually (255^2.2):1 or 196,965:1.

Thanks, John. Our understanding is the same, then. I only quoted
200,000:1

Sorry; I didn't even notice that you wrote that. I read it fast, and
somehow thought you were writing about 255:1.

simply because if I quoted the exact number I expected someone
to think I was being pedantic! I can appreciate that JPEG might be
optimised in a slightly different way to TIFF so that errors are masked.

By the way (for anyone else who's still reading), this 2.2 gamma figure
is
what some cameras call the "contrast" setting - use a smaller number
when
converting from linear to JPEG and you'll get more contrast, a larger
number gives less contrast.

Most JPEG conversions, I think, vary the gamma across the histogram.
This is important if you want to keep middle grey standard, but expand
or compress contrast relative to it.

I really wish I had a greyscale wedge the quality of the Gretag-MacBeth
color checker, but with about 10 stops of grey rectangles with steps of
about 1/6 stop, or even a pie wedge to discount light roll-off.
--


John P Sheehy


Re the grayscale you desire ... get a Kodak Q-13 "Kodak Color Separation
Guide and Gray Scale-Small," costs something like US$20 at places such as
B&H.

http://www.bhphotovideo.com/bnh/cont...=1&sq=desc&Ini
tialSearch=yes&O=SearchBar&A=search&Q=*&bhs=t&shs= Kodak+Q-13&image.x=10&ima
ge.y=5

They are literally an "industry standard," and have been used as such for
over fifty years.