On Mon, 2 Dec 2002 15:17:02 +1100, Marc Lawrence
<mlawrence@xxxxxxxxxx> wrote:
>Sorry for asking a generic photography question, but, what do
>you mean when you say "crossed curves"? Also, is this a good
>general practice for such negative films, regardless of light
>conditions (for instance, shooting daylight sports with 400 to get
>a fast shutter speed on a slow and better-shut-down lens)?
When a colour negative film is correctly exposed in the light for
which it is balanced (usually daylight) the images in the blue, green
and red sensitive layers have the same contrast. Plotting curves of
density against relative exposure for each layer would give parallel
(basically straight) lines although they would be displaced from one
another vertically, so to speak. The filtration in the enlarger
compensates for this displacement and yields a neutral print. If the
lighting is off balance (excessively blue or red for example) the
relative vertical displacement of the curves will change but, properly
exposed, they will remain straight and parallel and you can change the
enlarger filtration to yield a neutral print. Stage lighting has a
colour temperature of about 3000K (less when dimmed) compared with
daylight at around 5500K. This means that the balance is skewed away
from blue towards red and the danger is that the blue sensitive layer
can be under exposed while the green and red sensitive layers are OK.
Then, the curves are no longer parallel (blue sensitive layer has too
low a slope or is even no longer a straight line) and it is impossible
to adjust the filtration in the enlarger to compensate. The result is
that you can get neutral mid-tones and highlights but the shadow areas
will look awful.
Colour negative films are very tolerant of over exposure (even
benefitting from it) and increasing exposure in tungsten lighting will
ensure that the blue sensitive layer is properly exposed without
detriment to the green and red layers.
Although a blue (A-D) colour balance filter can be used on the camera,
its filter factor is very large (about 2 and a half stops) and means
that the effective speed of (say) a 400ASA film is reduced to about
80ASA. By using the film's tolerance and compensating during printing
(whether during enlarging or with computer software folowing scanning
from the negative), speed is preserved.
Unlike conventional B&W films (in which graininess is increased by
over exposure) chromogenic B&W films (like Ilford XP2) and colour
negative films exhibit reduced graininess with increased exposure.
Many years ago someone brought me some colour negatives that were so
grossly over exposed that they were virtually black and no commercial
outlet could print them. I succeeded and the results were quite
remarkably good. The only serious problem was halation in the
highlights.
Hope this helps.
Regards
John Gruffydd (Mold, Wales, UK)
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