What makes film react logarithmically?

Hi,

according to the Weber-Fechner-law our visual perception reacts in a
logarithmic way to lightness/brightness. That is the intensity has to
increase by steps of equal ratio (1,2,4,8, etc) in order to be
perceived as double as bright. Does anybody know what makes silver film
react in that same logarithmic way to exposure? Is it a natural
characteristic of the silver or is ist due to some special
ingrediences?

Thanks for your input!
Tony Adams

Tony Adams a écrit :

Hi,

according to the Weber-Fechner-law our visual perception reacts in a
logarithmic way to lightness/brightness. That is the intensity has to
increase by steps of equal ratio (1,2,4,8, etc) in order to be
perceived as double as bright. Does anybody know what makes silver film
react in that same logarithmic way to exposure? Is it a natural
characteristic of the silver or is ist due to some special
ingrediences?

Thanks for your input!
Tony Adams

I don't think film reacts logarithmically ...
If you check the sensitometric curve, by example the TMX in D-76:
http://www.kodak.com/global/en/profe...009_0438ac.gif
You'll see the manufacturer tries to render it as a straight line (both
X and Y axis are logarithmic).

Once you print the negative onto paper, this one too tries to keep this
linearity, as in the real scene.

Then, your eyes apply their non-linearities.

Claudio Bonavolta
http://www.bonavolta.ch

Claudio Bonavolta wrote:

I don't think film reacts logarithmically ...
If you check the sensitometric curve, by example the TMX in D-76:
http://www.kodak.com/global/en/profe...009_0438ac.gif
You'll see the manufacturer tries to render it as a straight line (both
X and Y axis are logarithmic).

Once you print the negative onto paper, this one too tries to keep this
linearity, as in the real scene.

Then, your eyes apply their non-linearities.

So film is manufactured (and developed) to react linear to exposure and
we do only apply logarithmic scales to descripe this reaction?

Tony Adams

Tony Adams wrote:
Hi,

according to the Weber-Fechner-law our visual perception reacts in a
logarithmic way to lightness/brightness. That is the intensity has to
increase by steps of equal ratio (1,2,4,8, etc) in order to be
perceived as double as bright. Does anybody know what makes silver film
react in that same logarithmic way to exposure? Is it a natural
characteristic of the silver or is ist due to some special
ingrediences?

Thanks for your input!
Tony Adams

Howls of derisive laughter.....

Rolling on the floor puking my guts out at how ****ing stupid anyone
would have to be to write this question...

Are heartbeats always some fraction of 60 per minute?

"Tony Adams" wrote in message
oups.com...
Claudio Bonavolta wrote:

I don't think film reacts logarithmically ...
If you check the sensitometric curve, by example the TMX
in D-76:
http://www.kodak.com/global/en/profe...009_0438ac.gif
You'll see the manufacturer tries to render it as a
straight line (both
X and Y axis are logarithmic).

Once you print the negative onto paper, this one too
tries to keep this
linearity, as in the real scene.

Then, your eyes apply their non-linearities.

So film is manufactured (and developed) to react linear to
exposure and
we do only apply logarithmic scales to descripe this
reaction?

Tony Adams


Log scales are used because of the very long range of
film. Density is proportional to exposure for pictorial
films. Some other films, notably lithographic films, have a
very sharp threshold between a developable exposure and one
that is not so that the density goes from minimum to maximum
for a very small difference in exposure.
The scale of the film (or paper) depends on the
distribution of sensitivity of the silver halide crystals.
Each crystal has a threshold below which it is not
developable and above which it is. The gray values depend on
the number of developed crystals in a given area. By
controlling the range of sensitivities of the crystals in
the emulsion the contrast of the emulsion can be varied over
a very wide range. This distribution is controlled by the
exact method by which the emulsion is made, the rate of
solution of the components of the halide, nature of the
gelatin, additives, etc. Also, emulsions are often made from
two or more components and many pictorial films have
multiple coatings of emulsions of differing characteristics.
Since a pictorial scene may have a brightness ratio of
1000:1 or more, and since the density range ratio is around
1:20 log scales are convenient.


--
---
Richard Knoppow
Los Angeles, CA, USA

Tony Adams wrote:

according to the Weber-Fechner-law our visual perception reacts in a
logarithmic way to lightness/brightness. That is the intensity has to
increase by steps of equal ratio (1,2,4,8, etc) in order to be
perceived as double as bright. Does anybody know what makes silver
film react in that same logarithmic way to exposure? Is it a natural
characteristic of the silver or is ist due to some special
ingrediences?

The eye does not respond in a logarithmic way. Some people choose to
display/portray it that way for metric convenience. Film is about the same.
It would be rather difficult to display response curves in a linear manner;
they would run off the chart.

j wrote:
Tony Adams wrote:

according to the Weber-Fechner-law our visual perception reacts in a
logarithmic way to lightness/brightness. That is the intensity has to
increase by steps of equal ratio (1,2,4,8, etc) in order to be
perceived as double as bright. Does anybody know what makes silver
film react in that same logarithmic way to exposure? Is it a natural
characteristic of the silver or is ist due to some special
ingrediences?

The eye does not respond in a logarithmic way. Some people choose to
display/portray it that way for metric convenience. Film is about the same.
It would be rather difficult to display response curves in a linear manner;
they would run off the chart.

Whether they run off the chart (on a linear chart) would have a lot to
do with the scale used to make the chart. One small reason for the log
exp. vs. log density is simply historical. A slightly deeper interest
in the reason is that the film speed is set taking into account the
behavior near the toe of the curve. On most linear scales, where a
range of a thousand to one needs to be displayed, it would either need
to (as you said) run the highlights off the chart or if it needed to be
a complete chart, not much detail could be shown near the toe.

Tony Adams wrote:

Claudio Bonavolta wrote:

I don't think film reacts logarithmically ...
If you check the sensitometric curve, by example the TMX in D-76:
http://www.kodak.com/global/en/profe...009_0438ac.gif
You'll see the manufacturer tries to render it as a straight line (both
X and Y axis are logarithmic).

Once you print the negative onto paper, this one too tries to keep this
linearity, as in the real scene.

Then, your eyes apply their non-linearities.

So film is manufactured (and developed) to react linear to exposure and
we do only apply logarithmic scales to descripe this reaction?

Tony Adams


Film is fundamentally non-linear. The curve in the
example provided above is simply a Tmax curve. An
inherent characteristic of Tmax is that it exhibits
a very long straight line section above the toe and
never really shoulders off. Other films exhibit
different curves depending on the emulsion.

Likewise, different papers will exhibit different paper
curves...