Technical data

Does a film scanner convert
negatives to positives automatically?
The scan process for scanning positives
(slides) is as follows: A lamp screens the film
and a sensor measures the transmitted light.
The brightness of the individual shades is
used to compose the finished photo. But
what about negatives? Do you get a "negati-
ve" image when scanning a "negative" photo
which then must be inverted and edited with
an image editing programme to eventually
obtain a normal positive? Or does the scan-
ner perform this time-consuming job automa-
tically?
To start with: A negative is not simply
converted to a positive by inverting the
colours, i.e. converting black to white, etc.
Colour inversion indeed is required to obtain
a normal photo from a negative. In addition,
the orange coloured mask must be subtrac-
ted or extrapolated. Since carrier material
and film emulsion vary from manufacturer to
manufacturer and partly from film type to film
type, different computation procedures are
necessary for a best conversion of a negative
to a positive.
With modern film scanners, users do not
have to care about this negative/ positive
conversion: The scanner or the scan software
will realize this automatically. The basic set-
tings of each scan software require to set the
type of film being scanned: positive, negative
or negative black and white. If the type is set
to negative, a scan will automatically produce
a positive image on the monitor or in the out-
put file. Due to the more complex scan pro-
cess and the necessary inversion and film
carrier correction, the scanning of a negative
takes up to three times longer than scanning
a positive.
Why does the scanning of negatives take
so much longer than scanning slides?
When studying data sheets and test reports
about film scanners, you will realize that the
time to scan a negative is distinctly longer
than to scan a positive. Users who digitise
both negatives and positives will confirm that
the time difference between the two methods
amounts to a factor of 2 to 4. Why does the
scanning of negatives take so much longer
than scanning positives?
With positive scans, the photo is simply
screened and the transmitted light is conver-
ted directly to pixels. For negative scans,
the necessary colour inversion and the extra-
polating of the yellow-orange coloured mask
from the measured image signal is performed
in addition. Thus, a negative scan requires
more steps than a positive scan to create a
finished image.
If you once scanned a negative as normal
positive unintentionally or on purpose, you
may have realized a distinctly longer scan
time than would have been the case for a nor-
mal slide. What is the reason for the longer
duration although a normal positive scan was
performed? The reason is brightness. A cor-
rectly exposed negative is significantly darker
than a correctly exposed positive. Film scan-
ners require more time for dark images than
for bright ones. If you have ever compared the
scan time for a photo taken at night (slide) to a
beach motive (slide), you will confirm the con-
siderable time difference.
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