Agfa Library: Technical Terms - M




















 




M

Magnetic coating (Advanced
Photo System)

The transparent magnetic coating applied to the complete area of the
film back. It is used for storing information on the film, shot and
process, and permits inter-communication between the system components
( IX-240).

 

Main color density

The term is used to describe the color density of a single layer in the
spectral region in which the absorption maximum lies. The main color
density of the yellow dye lies in the blue spectral range, that of the
magenta dye in the green and that of the cyan dye in the red spectral
range (masking, secondary color density).

 

Masking  

In ideal circumstances (which are unattainable under practical
conditions), a developed color negative film should only form, in each
individual color-forming layer, the component dye which has a spectral
density in that respective blue, green or red spectral third of visible
light. In other words, in theory, only one particular dye should,
through oxidative color coupling, form without any secondary color densities in
each of the color-forming layers.

However, the emulsion dyes have secondary absorptions in the other
spectral thirds which can combine to make the colors greyer.

For example, the emulsion dye of the developed magenta layer has its main color density in the
green spectral range as well as additional undesirable secondary color
densities in the blue and red regions of the spectrum.

Color negative films are nowadays masked to correct the saturation
losses and color impurities caused by these secondary absorptions.
During color development, color positive masks are formed in addition
to the emulsion dyes. Conventional color negative films are provided
with two color masks, but most AGFACOLOR negative films have three. The
color of these masks corresponds to the particular undesirable
absorptions of the image dyes. This means, for example, that the yellow
secondary density of the magenta dye is compensated by a yellow dye
image of low density that has an opposite gradation to the magenta
image. Both yellow densities add up to generate a uniform yellow color
cast, which is then filtered out during the subsequent printing
process. The coloring of the mask and the emulsion dyes jointly
determine the film-specific color rendition.The secondary densities
that are undesirable in photographic practice are:


the yellow secondary density of magenta (in the blue spectral region),

the magenta secondary density of cyan (in the green spectral region),

and the yellow secondary density of cyan (in the blue spectral region).


With conventional color negative films, only the yellow secondary
density of the magenta and the magenta secondary density of the cyan
are compensated. Modern AGFACOLOR films contain an extra third mask to
correct the yellow secondary density of the cyan.


Fig. 18: The extra third color mask of the
AGFACOLOR Professional films ensures cleaner reds and more brilliant
yellows.

Masking is carried out automatically during the processing (without any
additional operation) through the incorporation of mask couplers into
the emulsion layers. The colors of the mask couplers correspond to the
colors of the secondary color densities to be corrected. The mask
couplers react with the oxidation product of the developer, releasing
their original colors. If we take the example of the yellow secondary
density of the magenta image, this means that a yellow image of low
density is created opposite the magenta image. The yellow secondary
density and the opposite yellow secondary image add up to create a
homogeneous color haze, which can be filtered out during subsequent
printing.

 

Micrometer (µm)

A physical unit of length equivalent to 10-6 meters, i.e. one
thousandth of a millimeter. The average thickness of a human hair is 23
µm. The carrier material used for the Advanced Photo System has a
thickness of only 90 µm (35 mm film has 125 µm).

 

MTF (modulation transfer
function)

Producing an image on a photographic emulsion always involves a loss of
sharpness, which is attributable to, among other things, the effects of
scattered light in the emulsion layer. The modulation transfer function
is used to give a graphic representation of this transfer loss. It
characterizes the image sharpness of a given emulsion layer.

In order to determine the MTF, a line screen consisting of thinner and
thinner black lines is exposed under defined testing conditions onto
the film. After processing the film - also under defined conditions
-and then measuring the line screen in the microdensitometer, we obtain
the image modulation, which, because of transfer losses, is smaller
than the subject modulation of the original screen. The image
modulation decreases constantly with increasing spatial frequency, i.e.
with an increasing number of line pairs per millimeter, until the limit
of the film has been reached. The image sharpness, which declines as
the lines become finer, is expressed visually in the graph of the transfer factor (see also edge effect). The transfer factor
forms a curve which falls with increasing spatial frequency and ends at
the resolution limit of the film.

In the case of color films, all the color-forming layers are examined
to establish their MTF. The curves are determined by taking
measurements behind separation filters and are then evaluated jointly.
The individually processed color layers make different contributions to
the sharpness of the film. Yellow contributes least to the visual
impression of sharpness, while magenta contributes most.

MTF data relating to a film are dependent on the measuring equipment,
because the MTF of the lens system and the measuring system are
included in the measurement as additional links in the transfer chain.
MTF curves of films are therefore only comparable with one another if
they have been corrected to take account of the MTF of the
microphotometer.

 

Multiple layers

The number of individual layers in modern color films has risen
appreciably compared with earlier color films. Films with twelve or
more layers are nowadays rather the rule than the exception. At the
same time, the overall thickness of the emulsion has actually been
reduced with the development of new coating techniques (e.g. cascade
coating).

It is nowadays normal to apply two of each of the three color-forming
emulsions, and in some cases even three of each. Yellow, magenta and
cyan double layers consist of a faster top layer and a slower bottom
one. This "bi-pack" system gives less graininess in low and
medium color densities, a factor which benefits, for example, skin
tones because it produces a homogeneous surface effect.


Fig. 19: Layer structure of the AGFACOLOR
PORTRAIT 160 PROFESSIONAL with double and triple layers.


The light-sensitive emulsion layers of the several AGFACOLOR
Professional films and of the AGFACOLOR HDC 100, HDC 200 and HDC 400
films comprise double and triple emulsion layers. The triple layers
have a fast layer with a rather coarse grain at the top, a medium-fast
layer in the middle and a slow layer of particularly fine grain at the
bottom.
A number of black-and-white films also have double layers. The AGFAPAN
APX 400, for example, is a double-layer film with a high-speed and a
low-speed emulsion layer. The top, high-speed grainier layer is exposed
at full intensity and thus ensures a high practical speed. The lower,
slower layer, provides a fine grain in the low and medium densities
which have a major influence on the picture.



 



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