But even panchromatic film doesn't see the world the same way your eye
does. First of all, the film is still sensitive to ultraviolet light, which
is invisible to you.
(And there's quite a lot of UV light around,
especially in daylight.) Second, the film's relative sensitivity to
different colors isn't the same as your eye's. For example, the human eye
is most sensitive to green, but pan film is more sensitive to red.
back at Figure 2-6 to see how the eye responds to light. Even if the film's
relative sensitivity actually matched your eye's, there would still be a
problem. You see a red apple against the green leaves of the tree, and you
have no problem distinguishing between the two because you see their colors
and they look very different.
But pan film might see those particular
shades of red and green almost equally and convert them into similar shades
of gray; the apple is now much harder to pick out. If there were only a way
to darken or lighten the apple without doing anything to the shade of the
foliage, then the apple would again stand out as clearly in the black and
white print, much as you saw it in real life. Fortunately, there is a way,
and that's by using filters.
Learning Event 3:
DESCRIBE THE CONSTRUCTION AND EFFECTS OF FILTERS
A filter is nothing more than a device to block some of the color
wavelengths while letting others pass unobstructed. They are commonly made
of two types of materials, and each has its advantages and disadvantages.
Gelatin (gels) filters are just that; very thin sheets of a special
type of gelatin to which dyes have been added to give them their color.
They are very fragile, easily scratched, and almost impossible to clean once
they have gotten dust or fingerprints on them. But they are cheap, so if
you have a special need for a filter and don't expect to use it again, it
could be a good economic move to buy a gelatin filter for one-time use and
then throw it away when it wears out.
Another advantage of gels is that
they come in the widest selection of colors and densities. So again, if you
have a special need, you may not be able to find the filter you want in
anything other than gelatin.
Glass filters are really gelatin filters, but to make them more
durable, the gel is sandwiched between two thin pieces of optical glass.
These filters are more expensive, of course, but they are much more durable.
If you plan to use the filter frequently for a long time, the extra expense
of glass will pay off.
They can be cleaned just like a lens and come
mounted in metal rings so they can be easily attached and removed from the
Another type of glass filter is solid glass which has been dyed to the
desired color. These are extremely expensive because of the difficulty of
coloring molten glass to precise color shades and densities.
They are by
far the most durable since they resist fading. Both gelatin and sandwiched
glass filters will fade over time.
The expense of solid glass filters
limits their use to critical technical tasks and for calibration.