[opendtv] Re: RGB mania
- From: Doug McDonald <mcdonald@xxxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Mon, 17 Jan 2005 09:55:34 -0600
Craig Birkmaier wrote:
> At 9:01 PM +0000 1/16/05, Alan Roberts wrote:
>
>>Doug, I'm with you 100% here. Just take a look at any of the published data
>>curves for film stocks, like 5279 or 5277 and you'll see a clear linear
>>slope in the D/LogE curve, which means a power law. And we happen to call
>>that "gamma".
>
>
> Apple and oranges. Film gamma and video gamma are entirely different
> things. What i was describing in terms of film density is what Doug
> is calling film gamma. Turns out that this is a common terms used to
> describe the differences between low and high contrast film stocks.
>
> Gamma for video is the way we take the range of samples from any
> source and map them to the limited range of the encoding system so as
> to adjust for the characteristics of the display system.
Craig, you lose! Negative film gamma is EXACTLY "the way we take the
range of samples from any
source and map them to the limited range of the encoding system".
And that's the reason that it is typical, in garden commercial
film systems, 0.6. It squeezes a wide range of brightnesses
into a narrower range of densities. This means, among other
things, that a wider range of light values can be easily
accomodated on teh film stock. It also means that spillover stray
light in an enlarger has less effect on the print. The gamma
of print paper (or print film stock) is typically greater than
1.66 ... the product of 0.6 and something greater than 1.66 is
greater than one, which means the print has an overall power
law greater than one, which makes a "peppier" print.
The idea is exactly the same. It is true that the "display system"
for B&W film has a huge variability, in that there are different
contrast grade (print gamma) papers. Color film and print paper
NOMINALLY has no such range .... that's what the books and data sheets
say, but they are wrong ... you can play with negative color film
just like negative B&W if you want to be "arty". People
stopped doing that with the advent of Photoshop, however.
Doug McDonald
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