## [opendtv] Re: RGB mania

• From: Doug McDonald <mcdonald@xxxxxxxxxxxx>
• To: opendtv@xxxxxxxxxxxxx
• Date: Sun, 16 Jan 2005 12:33:06 -0600

```Craig Birkmaier wrote:

>
>>You set the blackest black well up on the toe if
>>you want details in the blacks. If you don't, by definition
>>you lose detail in the blacks! I'm not saying you have to set
>>it up above the toe into the linear gamma region, of course.
>>But if you want detail in the blacks (Zone 1) you MUST get them
>>well up onto the toe. As I said, I have never seen a movie with
>>detail in Zone 1. Stills, in photo art galleries, yes, they are
>>a "dime a dozen". Movies, no.
>>
>
>
> I think you are out of your league here Doug.
>
> What gamma are you talking about? There is no gamma in film negative,

There most certainly is. Most certainly, most emphatically,

> although there are both linear and non-linear ranges for negative
> density in the normal response curve. If you set the blackest black
> at the point where the density response turns linear, you have thrown
> away all of the potential for detail in the blacks.

A negative film curve, density versus log exposure,
starts as a horizontal line at the bottom left (below the toe),
then rises up with upwards curvature (toe) into a linear
region (the slope of which is gamma) and then at high
exposures it has a downward curvature region (shoulder)
then a flat region and, perhaps, goes down again (solarization.)

You say "If you set the blackest black
> at the point where the density response turns linear, you have thrown
> away all of the potential for detail in the blacks" . This
is implying the part that is "below the toe". You actually mean
where it is flat, not linear. Linear is above the toe, into
the region where gamma is pretty much constant in most film.
When I say "If you set the blackest black
> at the point where the density response turns linear" I mean
into the linear region, at an exposure just above the toe.

>
> Gamma is a uniquely video/display concept.

Uh, no. Gamma is just the third letter of the Greek alphabet.

It is a symbol for many things. And one of those is film gamma.
Typical pictorial camera negative film gamma is 1/2, though you can vary
it from 1/6 to  over one with different development, and some film types
have gamma as high as 5 or occasionally more. B&W print paper comes in
feature of Photoshop is a convenient way to simulate the effects
of changing print gamma.

Doug McDonald

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