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Film is not linear ?

 
 
Alfred Molon
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      10-06-2004
I got this impression when processing the scans of my brother's slides.
Very strange colours and brightness levels. While CCDs are linear
devices, where the output (before gamma) is a linear function of the
light level, I get the impression that film does not perform like that
and that the response curve is far from linear. Is this the case?
--

Alfred Molon
------------------------------
http://groups.yahoo.com/group/Olympus_405080/
Olympus 5060 resource - http://myolympus.org/5060/
Olympus 8080 resource - http://myolympus.org/8080/
 
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Joseph Meehan
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      10-06-2004
Alfred Molon wrote:
> I got this impression when processing the scans of my brother's slides.
> Very strange colours and brightness levels. While CCDs are linear
> devices, where the output (before gamma) is a linear function of the
> light level, I get the impression that film does not perform like that
> and that the response curve is far from linear. Is this the case?



I'll take you word that CCD's are linear. Film I know is not linear.
Just look at the response curves.
--
Joseph E. Meehan

26 + 6 = 1 It's Irish Math



 
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Jeremy Nixon
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      10-06-2004
Alfred Molon <(E-Mail Removed)> wrote:

> I got this impression when processing the scans of my brother's slides.
> Very strange colours and brightness levels. While CCDs are linear
> devices, where the output (before gamma) is a linear function of the
> light level, I get the impression that film does not perform like that
> and that the response curve is far from linear. Is this the case?


Not linear at all. Kodak used to publish little pamphlets with all the
information about each of their types of film; they probably still do.
One of the things you get there is the response curve, which is far from
linear, even taking gamma into account.

--
Jeremy | http://www.velocityreviews.com/forums/(E-Mail Removed)
 
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James Silverton
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      10-06-2004

"Alfred Molon" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) ...
>I got this impression when processing the scans of my brother's
>slides.
> Very strange colours and brightness levels. While CCDs are linear
> devices, where the output (before gamma) is a linear function of the
> light level, I get the impression that film does not perform like
> that
> and that the response curve is far from linear. Is this the case?
> --
>

Isn't that what the gamma curve's all about? The non-linear response
of film!

Jim.

 
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Jim
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      10-07-2004

"Alfred Molon" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) ...
> I got this impression when processing the scans of my brother's slides.
> Very strange colours and brightness levels. While CCDs are linear
> devices, where the output (before gamma) is a linear function of the
> light level, I get the impression that film does not perform like that
> and that the response curve is far from linear. Is this the case?
> --

No film is not linear. Reciprocity failure is just another description of
non-linearity.
Jim


 
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Dave Martindale
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      10-07-2004
Alfred Molon <(E-Mail Removed)> writes:
>I got this impression when processing the scans of my brother's slides.
>Very strange colours and brightness levels. While CCDs are linear
>devices, where the output (before gamma) is a linear function of the
>light level, I get the impression that film does not perform like that
>and that the response curve is far from linear. Is this the case?


Definitely not.

To a first approximation, over the main portion of its response curve,
film behaves approximately like

transmitted_light = some_constant * (exposure)^(gamma)

where "^" means exponentiation. This is a power function
whose effect depends greatly on the value of gamma.

For B&W negative film, gamma is about -0.6. The negative gamma says
that the process is a negative one: brighter light gives a darker image
on the film. The 0.6, being less than 1, means that a certain contrast
range in the scene is compressed to a smaller range on the film. For
example, a 5-stop range (32:1) in the scene becomes only a 3-stop range
(8:1) on film.

Colour negative film has a gamma of about 0.5 or 0.55. The three layers
are slightly different in gamma, and the "constant" is very different
for the 3 layers because of the orange mask.

Transparency film has a gamma of about 1.5. The positive value means
it's a positive process, and 1.5 says the slide has a higher contrast
than the original scene.

There is special internegative film which has a gamma of -1.0 or close
to it. This neither increases nor decreases contrast, but it still
gives a negative image. Copying onto internegative film *twice* gives a
reasonable copy of the original.

A linear film would have a gamma of 1.0, but I know of no such film.

Also, all the above applies only to the middle of the response curve.
There are usually "shoulder" and "toe" regions with lower contrast for
the shadows and extreme highlights respectively. And the centre section
really isn't exactly straight either.

Dave
 
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Alfred Molon
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      10-07-2004
In article <ck2c6g$kre$(E-Mail Removed)>, (E-Mail Removed) says...

> Definitely not.
>
> To a first approximation, over the main portion of its response curve,
> film behaves approximately like
>
> transmitted_light = some_constant * (exposure)^(gamma)
>
> where "^" means exponentiation. This is a power function
> whose effect depends greatly on the value of gamma.
>
> For B&W negative film, gamma is about -0.6. The negative gamma says
> that the process is a negative one: brighter light gives a darker image
> on the film. The 0.6, being less than 1, means that a certain contrast
> range in the scene is compressed to a smaller range on the film. For
> example, a 5-stop range (32:1) in the scene becomes only a 3-stop range
> (8:1) on film.
>
> Colour negative film has a gamma of about 0.5 or 0.55. The three layers
> are slightly different in gamma, and the "constant" is very different
> for the 3 layers because of the orange mask.
>
> Transparency film has a gamma of about 1.5. The positive value means
> it's a positive process, and 1.5 says the slide has a higher contrast
> than the original scene.
>
> There is special internegative film which has a gamma of -1.0 or close
> to it. This neither increases nor decreases contrast, but it still
> gives a negative image. Copying onto internegative film *twice* gives a
> reasonable copy of the original.
>
> A linear film would have a gamma of 1.0, but I know of no such film.
>
> Also, all the above applies only to the middle of the response curve.
> There are usually "shoulder" and "toe" regions with lower contrast for
> the shadows and extreme highlights respectively. And the centre section
> really isn't exactly straight either.


Thanks for the reply. Is this then the reason that images from digital
cameras look different from images from film cameras ? I get the
impression that digital photos better reproduce reality, while film is a
bit surreal.
--

Alfred Molon
------------------------------
http://groups.yahoo.com/group/Olympus_405080/
Olympus 5060 resource - http://myolympus.org/5060/
Olympus 8080 resource - http://myolympus.org/8080/
 
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Alfred Molon
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      10-07-2004
In article <E9_8d.2975$(E-Mail Removed)>, sligojoeS_PAM_2
@hotmail.com says...

> I'll take you word that CCD's are linear.


It's basically light which charges a capacitor. A linear process until
the capacitor is full.
--

Alfred Molon
------------------------------
http://groups.yahoo.com/group/Olympus_405080/
Olympus 5060 resource - http://myolympus.org/5060/
Olympus 8080 resource - http://myolympus.org/8080/
 
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David J Taylor
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      10-07-2004
Alfred Molon wrote:
[]
> Thanks for the reply. Is this then the reason that images from digital
> cameras look different from images from film cameras ? I get the
> impression that digital photos better reproduce reality, while film
> is a bit surreal.


Yes, and the way that white level "clips" quite sharply - anything over
white is lost whereas it can be recovered on film.

Having worked with television (and basically linear sensors with gamma
correction) for about 40 years I much prefer the "digital" to the "film"
look, although I could imagine a cinema-goer might prefer the opposite!

Cheers,
David


 
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Joseph Meehan
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      10-07-2004
Alfred Molon wrote:
....
>
> Thanks for the reply. Is this then the reason that images from digital
> cameras look different from images from film cameras ? I get the
> impression that digital photos better reproduce reality, while film is a
> bit surreal.


It is a reason not the reason. There are many differences, but I would
guess that it is a primary reason.


--
Joseph E. Meehan

26 + 6 = 1 It's Irish Math



 
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