Misunderstandings about VC
paper
As the popularity of VC (variable
contrast) papers has increased, a layer of folklore has
accumulated surrounding them. Although the actual workings of VC
papers are straightforward (and Phil Davis has discussed the
subject in his excellent articles) they are often poorly
understood.
As a result, over the years
various myths, rules of thumb, and misunderstandings have cropped
up. Some of these start out as factual observations but soon
accumulate a certain level of marketing hype beyond what the
facts merit. Some of them are just superstition, with no basis in
fact; often these are assumptions that have simply gone untested.
Often these are repeated by instructors, respected darkroom
workers, and even writers of articles and soon become a sort of
religious doctrine, never questioned because of the authority of
those making the statement.
However, the time has come to sort
the wheat from the chaff. The result will be a better
understanding of how VC papers work and can be used, and less
time spent on methods that simply don’t work. Before we can
begin reviewing the conventional wisdom on VC papers and printing
techniques, however, it’s worth taking the time to make a
short review of color theory and of how VC papers work.
Color Theory
The human eye can detect light
with a wavelength between roughly 400 and 700 nm (nanometers).
For our purposes, we can break this range into three sub-ranges,
corresponding roughly to the wavelength ranges of the Wratten
filters used to for color separation.
A Wratten #25 filter passes
wavelengths from roughly 600nm to 700nm, and we’ll call that
range of wavelengths red.
A Wratten #58 filter passes
wavelengths from roughly 500nm to 600nm, and we’ll call that
range of wavelengths green.
A Wratten #47B filter passes
wavelengths from roughly 400nm to 500nm, and we’ll call that
range of wavelengths blue.
Additive mixing
Blue, red and green are also, of
course, the three additive primaries. That is, we can reproduce
all the colors that the human eye can see by stimulating the human
eye with some combination of those three colors, varying only the
intensity of the three colors of light.
Note that this is not quite the
same as saying that all colors are composed of some amount of
blue, green, and red. In the real world, the spectral content of
light reflected from an object does not consist of three
wavelengths, but a continuum of wavelengths at various
intensities. However, we can reproduce the response of the
human visual system to any color by using light of only three
wavelengths (which correspond to blue, green, and red).
Additive mixing uses three light
sources; one for each additive primary color. You create blue,
green, and red by using a single light source emitting that
primary color. To create all other colors, you mix the primaries
in varying amounts; for example, if you mix equal amounts of
green and blue (but no red) you get cyan. Likewise, when you mix
equal amounts of red and blue, you get magenta, and when you mix
red and green you get yellow. Mixing all three additive primaries
in equal amounts produces light which appears white.
This gives us the following
pseudo-equations, which we can use as a convenient notation to
help us get a handle on the relationships of colors:
W = R + G + B (white is the
same as red, green and blue in equal proportions)
C = G + B (cyan is equal
proportions of green and blue)
Y = R + G (yellow is equal
proportions of red and green)
M = R + B (magenta is equal
proportions of red and blue)
In an additive color head, there
are three lamps. Each lamp produces light in one of the primary
colors. With an additive head, we control the intensity of the
three primary colors by adjusting the intensity of the light from
each of the three lamps.
Subtractive
mixing
In subtractive mixing, we start
with white light (which contains all colors) and proceed by
subtracting colors by filtering the light.
The three subtractive filter
colors are magenta, yellow, and cyan. Note that above, we defined
cyan as the color you get when you mix green and blue light. We
can also get cyan by subtracting (filtering out) red light from a
white light source, leaving the green and blue. To put it another
way, cyan is white light with the red light subtracted, and a
cyan filter is one which blocks red light but passes blue and
green light unimpeded.
Likewise, magenta is white light
with the green light subtracted.
Finally, yellow is white light
with blue subtracted.
Again, we can use pseudo-equations
to get a handle on these relationships:
C = W – R
M = W – G
Y = W – B
Note that these are nothing more
than the pseudo-equations for additive mixing rearranged.
We can see that a cyan filter
blocks red light but passes green and blue. Likewise, a magenta
filter blocks green light but passes red and blue, and a yellow
filter blocks blue light but passes red and green.
In a subtractive color head,
there’s a single lamp, which produces white light. We
control the intensity of each of the three primary colors by subtracting
them from the white light, using filters that are the
complementary color to the primary. We reduce the level of red
light by introducing cyan filtration; we reduce the level of
green light by introducing magenta filtration, and we reduce the
level of blue light by introducing yellow filtration.
VC Cold Light Heads
It’s worth noting that some
cold light heads are equipped with two tubes; one that produces
blue light, and one that produces green light. These heads are
additive heads, essentially the same as a three-color additive
head but without a red light source.
Equivalence of subtractive and
additive mixing
Note that with both additive and
subtractive mixing, we’re always adjusting the intensity of
the primary colors: red, green, and blue.
In the additive case, we can
adjust the intensity of the three lamps and adjust the intensity
of the three primaries. We start out with no light at all, and we increase the intensity of the primaries by increasing the
amount of light produced by the colored light sources.
In the subtractive case, all of
the three primaries are present in the white light source that we
start with, and we can reduce the intensity of the
primaries by introducing a filter that subtracts just one of the
colors. Recall that cyan is white light minus red light; a cyan
filter blocks red light, but passes blue and green. All that
remains after white light passes through a cyan filter is blue
and green light; this is exactly the same result as adding blue
and green light (but no red) and getting cyan.
As an example, suppose we want to
use subtractive mixing to produce blue light. We start with white
light, which contains equal amounts of blue, green, and red
light. To get just blue, we need to subtract the green and red
light. We can subtract red by introducing cyan filtration; this
leaves us with equal amounts of green and blue light (and we call
this combination cyan; this should be no surprise!). Now, we can
subtract the green light by also introducing magenta filtration;
this leaves us with blue light alone.
To get just one primary color with
additive mixing, we use only that color. To get just one
primary color in subtractive mixing, we use two filters –
each subtractive filter subtracts a different primary color, so
any combination of two subtractive filters will result in a
single primary color remaining.
Additive and subtractive mixing
produce identical results. Additive mixing works by starting with
no light, and adding in the primaries. Subtractive mixing works
by starting with white light (consisting of all three primaries)
and subtracting the primaries. In both cases, however, the result
is the same – a light source consisting of controlled levels
of blue, green, and red light.
How VC papers work
The emulsion of a variable
contrast black and white paper consists of two or more emulsion
components. Some of these emulsion components are sensitive to
both blue and green light, and the rest are sensitive to only
blue light. None of the emulsion components are sensitive to red
light; this is why we can work with VC papers under a red
safelight.
By changing the ratio of blue
light to green light, you change the relative exposure of the two
components, and thus change how the densities of the two
components add together to produce density in the final print. If
the exposure consists of mostly blue light, the two emulsion
components receive about the same exposure; in this case, the
result is a high contrast print. If the exposure consists of only
green light, one emulsion component will receive much more
exposure than the other; the result is a low contrast print. The
contrast of the final print is controlled by the ratio of
blue and green light; more blue light produces higher contrast,
more green light produces lower contrast.
Conventional Wisdom Reexamined
Now that we understand how the
filtration in a color head works, and understand how VC paper
works, it’s time to review some of the conventional wisdom
regarding VC printing.
Differences between results with
additive and subtractive heads
The usual way this you find this
worded is that you’ll get different (and perhaps better)
results if you print with blue and green light instead of magenta
and yellow light. Often this is an argument for a two-color light
cold light head.
However, as far as the VC paper is
concerned, blue light is the same as magenta light. The only
difference between blue light and magenta light is that magenta
light contains red, and blue light does not. VC paper is
insensitive to red light, so this makes no difference at all.
Likewise, the difference between yellow light and green light is
yellow light contains red, whereas green light does not.
The only reason why an additive
head might produce different results from a subtractive head
would be that it might be possible that one or the other could
produce a more extreme ratio of blue light to green light. This
more extreme ratio might allow a higher or lower contrast.
Cyan filtration
In the discussion on color theory,
we saw that cyan filtration controls the amount of red light
present in the light path. As a result, cyan filtration
doesn’t have any effect on VC printing at all, because VC
paper just isn’t sensitive to red light.
I’ve seen three claims
regarding cyan filtration and VC printing:
- Cyan filtration acts as Neutral
Density for VC printing. This claim has been made in so many
places that it’s not possible to actually list them all.
However, all cyan filtration does is reduce the amount of red
light. While this might dim the image on your easel, it
doesn’t make any difference to the paper at all.
- Cyan filtration can produce
changes in tonality that can’t be achieved any other way.
Generally the claims are along the lines of "cyan filtration
adds some magical something to your prints that ‘makes them
sing’ – luminous shadows, better mid tone separation,
sparkly highlights". Sorry, it’s just not true. If
you’re seeing a difference, it’s either process
variation or wishful looking.
- Adding cyan filtration to maximum
magenta increases contrast. The rationale behind this claim is
that adding cyan makes the light ‘more blue’, and thus
increases the contrast. The only problem is that the only thing
adding cyan filtration does is make the light less red – and
the paper isn’t sensitive to red. The only way to increase
the contrast of the paper is to reduce the amount of green
light. Cyan filtration doesn’t affect the amount of green
light.
The amazing thing is that
it’s so easy to test these claims– just make a print
without using cyan filtration. Make another print, using exactly
the same exposure. If the two prints are the same, then you have
your process under control. (If they aren’t the same, you
had better work on exposure and processing consistency). Now,
make another print in exactly the same way, except use the
maximum cyan filtration. If the prints are identical, cyan
filtration makes no difference.
Mixed magenta and yellow filtration
There’s a lot of confusion on
what happens when you use both magenta and yellow filtration at
the same time.
It’s easier to understand
once you realize that there are two things that matter when
making a print on VC paper: the amount of blue and green light
that falls on the paper, and the ratio of the amount of blue
light to the amount of green light.
If we dial in equal amounts of
magenta and yellow, the result is that the magenta filtration
reduces the amount of green light, and the yellow filtration
reduces the amount of blue light. If we dial in equal amounts of
magenta and yellow, we reduce both blue and green light by the
same factor. The ratio of blue light to green light will not have
changed.
The net effect is equal amounts of
magenta and yellow constitute neutral density – that is, it
decreases the amount of light but doesn’t change the color
balance. For color printing, of course, neutral density consists
of equal amounts of magenta, yellow, and cyan. However, cyan has
no effect on VC paper, so we only need equal amounts of magenta
and yellow.
The bottom line is that we can use
two simple rules to understand the effect we’ll get when we
use both magenta and yellow simultaneously:
If both magenta and yellow are
used, part of the effect is neutral density. The amount will be
whatever part of the filtration is present in both
channels. For example, if we have 65M and 50Y, we have 50CC of
ND. Likewise, if we have 85M and 120Y, we have 85CC of ND.
If the amounts of magenta and
yellow are not equal, it’s as if we’ve both added
neutral density and some other filtration. That is, 65M and 50Y
is the same as 50CC ND and 15M. In our second example, 85M and
120Y, we have 85CC ND and 35Y.
There are two common
misconceptions about mixing magenta and yellow filtration that
I’ve read:
Mixed magenta and yellow
filtration can produce changes in tonality that can't be achieved
with just magenta or yellow alone. We’ve already
demonstrated that when you mix magenta and yellow, you get the
effect of some amount of neutral density, and the effect of
either the magenta or the yellow remainder. There’s no other
effect.
Mixed magenta and yellow
filtration can allow better control of contrast (when using
gelatin filters) since the effect of yellow filtration is less
than the effect of magenta filtration - e.g. adding 5cc yellow to
20cc magenta will produce something between 15 and 20 cc magenta.
The base assumption here is that because the rate of change of
the contrast for yellow filtration is generally lower than that
for magenta, that the rates apply when you mix the two. If this
were the case, we’d need far more yellow than magenta to
make up neutral density, and that’s just not the case. The
effect of adding 5cc of yellow to 20cc of magenta will be exactly
the same as 15cc of magenta (and 5cc of neutral density).
Two exposure printing
As we’ve seen already, the
exposure of a VC print is determined by the amount of blue
and green light that falls on the paper. The contrast of the
print is determined by the ratio of blue light to green
light.
It makes no difference whether
this light falls on the paper in a single exposure, or in several
exposures. For example (excepting reciprocity departure) five 5
second exposures will have the same effect as one 25 second
exposure. Likewise, it makes no difference if all of the green
light is delivered in one exposure, and all of the blue light in
another exposure; the effect will be the same as if the blue and
green light exposures were made simultaneously. When you make a
print with either an additive or subtractive color head,
that’s exactly what you’re doing: delivering both the
blue and the green light simultaneously.
It seems there’s a lot of
confusion about what the effect is of combining two VC filters
(such as Kodak’s PolyMax filter set or Ilford’s MG
filter set) by using them both simultaneously. Generally, people
try this because they want to get an effect that lies between two
of the filters. For example, they want to produce a grade 2.25
print, so they put both the #2 and the #2.5 filter in the
enlarger. This doesn’t work, but it’s interesting to
see why.
The filter sets sold by the paper
manufacturers consist of magenta and yellow filters in varying
densities chosen to produce the appropriate contrasts. In
addition, each filter contains some amount of ND to even out the
speed changes that occur as the magenta or yellow filtration is
varied; this is called ‘speed matching’. Each filter in
the set consists of some amount of yellow and magenta filtration.
Suppose we use both a #4 and a
#4.5 filter. Both of those filters are going to consist of lots
of magenta, with a small amount of yellow and magenta as neutral
density. When we use both of them, we’re getting LOTS of
magenta, and about twice the neutral density of either. The huge
amount of magenta is going to produce more contrast than either
filter would alone, rather than some contrast between #4 and
#4.5. And, the result will not be speed matched, since it’s
extremely unlikely that the larger amount of ND will be
appropriate for the very strong magenta filtration.
A similar result occurs when we
use low contrast filters (e.g. below grade 2 or so. These
filters consist of primarily yellow, and when combined, will
produce a softer result than either filter will alone, and the
result will not be speed-matched.
The only case where we’ll get
a result that falls between the results of either filter used
independently is when one of the filters is a higher contrast
(than the unfiltered contrast of the paper) and one is a lower
contrast. This is because high contrast filters are primarily
magenta, and low contrast ones are primarily yellow. When we
combine them, we get a large amount of neutral density and a
small amount of either magenta or yellow. As a result, the effect
is that we’ve made a non-speed matched filter for some
indeterminate grade between the two filters.
The way to get an effect between
two filters with a filter set like this is to make part of the
exposure with one filter, and part with the other.