"In The Trenches" <no.RemoveThis@no.domain.com> wrote in message
news:BqMsc.588797$Ig.273737@pd7tw2no...
> Can someone explain to me contrast indexes. Are they
published for each B&W
> film, or do I have to calculate it by myself? TIA.
>
Contrast Index, or CI, is a method of specifying contrast
devised by Kodak to be applicable to a large variety of
materials.
Contrast is the slope of the characteristic curve. This
curve shows the relationship between exposure and density.
It varies with development: the more the film is developed
the steeper the curve will be and the higher the contrast.
At one time contrast was defined as the slop of this curve
at a point on the straight line portion. Actual film curves
vary in slope from low to high densities. The difference in
slope is determined by the way the emulsion is made and
represents the distribution of sensitivity of the silver
halide particals which make up the emulsion. The shape of
the curve is decided on by the manufacturer to yield a
certain tonal rendition. The curve has three parts: the toe,
the straight line, and the shoulder. The toe is the low
density part. In general this has a lower contrast than the
straight line portion. The shadows are recoreded in this
region. The straight line portion represents most of the
film's recording range. It records differences in brightness
as a linear change in density. This is the region used for
most of the brightness information in the photo. The
shoulder is the high exposure- high density part of the
curve. Again, the contrast is lower than in the straight
line portion. Most modern films have such a long range of
densities that they never reach the shoulder in normal use
but many older films did resulting in "blocked" highlights.
Until about the 1940's contrast was usually measured as
"gamma" Gamma is the contrast of the straight line portion
of the curve. However, most films record part of the image
in the toe region and many films have relatively long toe
sections. Some, like Tri-X sheet film and the discontinued
Plus-X sheet film, have no straight line portion, the
contrast increasing with density at all values. For films
like this measuring contrast as gamma can be very misleading
about the printing quality of the negative. For instance, a
long toe film, like Tri-X Pan sheet film, when compared to a
short toe film, like T-Max 400, will have much lower
highlight density when developed to the same gamma. One
solution to this was the development of Average Contrast, or
bar-G (i.e., the letter with a dash over it to indicate its
an averaged value). Average contrast is, essentially, a
straight line drawn from the minimum density to some maximum
density. For the ISO method of measuring film speed bar-G is
defined indirectly by the specified exposure range and
expected resulting density range. For this method the range
of exposure is log 1.30. The film is developed so that the
density resulting from the maximum exposure is log 0.9 above
base density plus fog. The speed is measured between this
point and a minimum of logd 0.1 above base density and fog
for a density range of log 0.8.
Contrast Index is similar to bar-G but it specifies a
minimum density of log 0.2 above base and fog and is the
slope measured from this point to a high density point
corresponding to a range of LogE 2.0 from the exposure
needed to produce the low density reference. This is a more
realistic range of exposure in normal photography and the
reference point of logD 0.2 above base and fog is far enough
up the toe to eliminate problems with very low contrast toe
regions (i.e., no shadow detail). Contrast Index is
applicable to all sorts of film, including special purpose
films, like high contrast copy or graphic arts films.
While CI can be determined from a film curve it is easier
to measure using a graphic overlay designed by Kodak Labs.
These used to be available from Kodak but have probably been
discontinued or, perhaps, are available from Silver Pixel
Press.
CI was originally described in a paper published by the
Kodak Research Labs.
"Contrast Index" C.N.Nelson and J.A.C.Yule, _Photographic
Science and Engineering_ V. 10, No. 1, January-Febuary 1966
It is also described in detail in several books on
photographic science.
I should clarify that two negatives on different films
developed to the same contrast index will not look the same.
The curve of the film has a strong influence on tonal
rendition, however, at the same CI both negatives will print
on the same grade of paper and yield reasonably good prints.
Where they are developed to the same _gamma_ they will not
print on the same paper grade and the long toe film may have
inadequate shadow detail on any paper.
Kodak uses CI for its contrast specifications. Agfa seems to
still use gamma, and Ilford and others use bar-G.
--
---
Richard Knoppow
Los Angeles, CA, USA
dickburk.RemoveThis@ix.netcom.com<!-- ~MESSAGE_AFTER~ -->
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