The Zone System - Part Four
Calibrating Exposure
(by Lars Kjellberg)
Film speed and developing times are not sacred
cows. On the contrary, they often need to be adjusted for your
pictures to be full in shadow detail and to control contrast.
The speeds and times provided by the manufacturer should be regarded
as starting points for you to make your own adjustments by. In
this part of the Zone System course you will learn several different
ways of calibrating for your personal needs. If you use one of
these methods while going through your routines, your black and
white negatives will be easier to print and the technical quality
of your pictures will be better.
Why Calibrate?
Exposure index (ISO, or film sensitivity setting) is calibrated
because a camera's shutter and aperture aren't always correct,
the exposure setting may not be correctly adjusted, and the film
developed in your developing time may not hold the speed the manufacturer
claims it does. If your equipment fails in a negative way (your
shutter speed are too short, your aperture is too small and your
meter shows values that are too high), your negatives will come
out underexposed, which always results in a lack of detail in
the dark parts of the picture. A simple calibration will reveal
such defects and will provide a proper exposure index for your
equipment, resulting in pictures with full shadow detail.
The contrast of the negative depends on several differents factors.
The most important ones are, of course, what film and developer
you choose to work with. Every combination of film and developer
demands it's own developing time. But contrast is also affected
by developing time, agitation, concentration and temperature.
Usually the contrast of the negative is regulated by varying the
time of developing, while other variables are kept constant. The
ideal contrast is not clear. It depends on the source of light
in your enlarger, your choice of photographic paper and your personal
taste. Besides that, it can be to your advantage to vary the time
of developing with the contrast of the photography light in order
to always keep the contrast of the negative unchanged. An enlarger
with a diffuse light (cold light or a color head with light mixing
box) demands more contrasty negatives than enlargers with condensor
light. If you choose to print on a high contrast photographic
paper your developing time has to be shortened so as to not make
the picture's contrast too high. All this might seem difficult,
with all the different variables to keep track of, but it really
isn't that complicated.
The Principle of Calibration
The scale of tones start with black and end in white. Between
black and white are a large number of grays. These gray shades
relate to black and white in a predictable manner, they are always
between black and white. If we calibrate black and white, we will
also control all the gray shades. Black and white are at the far
ends of the spectrum. Normally, the scale consists of seven stops
of exposure and the point of metering is at the middle of the
scale, 3 1/2 stops away from black and 3 1/2 stops away from white.
When calibrating, we first check where the black and white points
are located at standard exposure and standard developing. If we
have to, we adjust these two points in order to control them perfectly.
We will find out how to expose to hit the black point exactly,
and how to develop to hit the the white point exactly. When we
know how to hit these two points, we can also figure out how the
grays in between will fall.
Correct Exposure Index
Let's begin with a few methods of calibrating the exposure index
(EI). Normally, you set the exposure meter on the camera (or a
detached exposure meter) at the value recommended by the manufacturer.
If you use a 35 mm camera you may even let the camera read the
DX-code on the cassette. In most cases this is all that is needed,
but we still recommend that you check if the shadow detail is
good enough when using the standard setting.
1) The first method is the simplest and maybe the
best. Place your negatives on a light table and look at them through
a magnifying glass to see if there is a slight shade of gray in
the lightest part of the negative (the part of the negative that
turns almost black when printed). If large parts of the negative
is almost completely or completely transparent, the EI should
be adjusted somewhat lower. If, for example, you have used Tri-X
set at 400, and the shadow part of the negative is too light,
the EI should be adjusted to 250. This simple method of calibration
is most accurate if you shoot an entire roll of film of a motive
that has been placed in Zone 1 1/2, that is: a motive that has
been exposed 3 1/2 stops below the meter reading. Expose the dark
parts of the motive in Zone 1 1/2. Develope as usual and take
a close look at the negatives on a light table. If the EI is correct,
the dark parts of the negative should show a slight, but distinct,
shade of gray.
2) Calibrating with a densitometer. Very few photographers
have access to a densitometer. A densitometer is an instrument
that measures blackness. All graphic art companies and photographic
labs have one. Call them up and ask if you may use their densitometer
to measure some negatives. Densitometers measures blackness in
a logarithmic number that describes the proportions between the
light that has been absorbed and the light that has been let through.
If all light was let through the negative (which is an impossibility
in reality) the densitometer would show the value of 0. If half
the light was let through, the densitometer's value would be 0.30.
A forth would be 0.60, and an eighth would be 0.90. All negatives
consist of a film base with fog that absorbs some of the light.
We call this base+fog (b+f). B+f absorbs light evenly across the
entire negative and therefore does not affect the finished picture.
To be able to reproduce on paper the darkest part of a negative,
it has to be exposed so that the developed negative shows a densitometer
value of 0.10 above b+f. This is what we have to measure with
the densitometer.
Set the exposure meter to the films suggested speed.
Photograph an evenly lit surface (a gray card for example) and
expose it 3 1/2 stops below the meter reading. If the meter suggest
you set the camera to 1/60 at aperture 8, you may choose to set
it at 1/500 (3 stops below) and adjust the aperture to between
8 and 11 (another 1/2 stop below). Develop as you'd normally do
and measure the negative with a densitometer. Let's start by measuring
the b+f which is usually at about 0.30 for 35 mm, and 0.15 for
120 and sheet film. The value of the exposed negative should be
at about 0.10 above the b+f. If the negative is too thin, less
than 0.08 (not exposed enough) the EI should be lowered. Try 1/3
below for 0.07 and 2/3 below for 0.04. If the suggested speed
of the film is 400, 1/3 below will result in an EI of 320 and
2/3 will result in an EI of 250. In a similar way the EI should
be increased if the negative is too dense. At 0.14 above b+f the
EI should be adjusted by approximately 1/3 and at 0.18 above the
b+f by 2/3. Increased by 1/3, an ISO of 400 becomes 500, and increased
by 2/3 it becomes 640.
3) Calibration using a 0.10 ND-filter (a gray filter
with a blackness measured to 0.10). Kodak sells ND-filters of
different blackness values. This filter can be used as a reference
when we evaluate the correct EI.
The ideal blackness for a negative that has been exposed 3 1/2
stops below normal, is 0.10 above the b+f. Therefore you need
to use an unexposed and developed negative (filmbase+fog) with
an ND-filter placed on top of it as a reference when the correct
EI is evaluated. Start by photographing a detail of a motive that
has an even tone and is evenly lit, for example a gray card. Expose
it at 3 1/2 stops below the meter reading. Develop as you normally
would. Compare this on a light table to the gray shade of the
b+f+ND 0.10. If the developed gray is lighter than the reference,
the EI should be decreased. Your equipment is underexposing, which
can be compensated by a decrease of EI. Do another shoot with
a decreased EI and compare it again to the reference. If you find
it difficult to compare them just by looking at the negatives,
they can be printed on a high contrast photographic paper. A high
contrast paper provides the most visible result and is easier
to evaluate. Print the b+f+ND 0.10 to a middle gray shade on the
hard paper. Then print the exposed and developed negative at the
same exposure time and compare the developed prints. If the b+f+ND
0.10 turns a lighter shade of gray on the print, your EI is too
high and your negative is too thin (underexposed).
Calibrating the Normal Developing
Time
To balance the contrast of the negative, the developing time needs
to be calibrated. Here are some different methods for calibration.
1) The visual method. It's rather hard to judge
whether the developing time is correct or not just by looking
at the negatives. It's essential to first make prints and then
evaluate if the contrast in the prints is correct. Print on a
paper of average grade. Print more than one negative and compare
the dry prints in a normal light (not too bright). Do you experience
the contrast as beeing correct? Or are the prints too low or high
in contrast? If the prints are too contrasty, the developing time
should be shortened. If the prints are too soft and gray, the
developing time needs to be longer. You may use a more sophisticated
way of deciding your developing time, but this visual method should
be the definitive check. If you think that the prints are generally
too harsh, the developing time should be shortened no matter what
the densitometer says!
2) Contrast can easily be measured with a densitometer.
Expose two negatives, one black-point negative and one white point-negative.
One should be exposed 3 1/2 stops below the meter reading and
the other 3 1/2 stops above the meter reading. The white-point
negative is, after developing, the dark negative, and the black-point
negative is the light one. Measure the blackness in both of them
and calculate the difference. If, for example, the white point
negative has a blackness of 1.42 and the black point negative
has a blackness of 0.32, the difference in blackness is 1.10.
The contrast is 1.10. As mentioned earlier, enlargers have different
sources of lights and they demand negatives of different contrast.
The approximate negative contrast desired when printing in an
enlarger with diffuse lights is 1.20. A print made in an enlarger
with a condensor light should measure between 1.00 and 1.05 in
contrast. These values are approximates and should be regarded
as starting points for your own printing. Personal taste may require
negatives of deviating contrast.
3) Print the white point and the black point negatives
simultaneously in the enlarger. Place them in the negative carrier
in a way that ensures that half of each negative is printed. Between
the negatives is a small slice consisting of the b+f alone. In
the finished print, this slice should come out completely black,
the black point should come out almost black and the whitepoint
almost white. If the white point is too dark, then the contrast
is too low. You need to increase your developing time. If the
white point is too bright, the contrast is too high, and this
can be adjusted by shortening the developing time.
4) Use a fact sheet. The contrast of a negative
can be described by another number: contrast index (CI). The contrast
index describes the inclination of the film curve. An ideal CI
for a negative that is to be printed in a condensor enlarger is
0.50. The corresponding number for a print that is to be printed
in a diffusor enlarger is 0.57. Some manufacturers provide fact
sheets with charts showing the developing time appropriate for
a certain CI. Along the y-axis of the chart are different developing
times and along the x-axis are the different CI values. The chart
has a number of lines and every line represents a developer. Suppose
you are going to shoot T-MAX 100 and develope it using D-76. You
want the negative contrast most suitable for a diffusor enlarger,
this means a CI of 0.57. Draw a line parallell to the x-axis at
0.57 until that line crosses the D-76 line. Draw another line
down to the x-axis and read the developing time, which according
to the Kodak fact sheet is 8 minutes. This method is very simple
and doesn't demand any darkroom work. The drawback is that not
all manufacturers provide fact sheets like Kodak do.
Calibrating Plus- and Minus
Times
For calibrating plus- and minus, we can use a similar method as
when calibrating the developing time. Normal developing assumes
that the contrast of the motive spans seven stops. That is why
you expose at 3 1/2 stops below and 3 1/2 stops above. You get
a difference of seven stops. At minus 2 developing the contrast
is nine stops. The two frames exposed are minus 3 1/2 and plus
5 1/2. At minus one, developing it is minus 3 1/2 and plus 4 1/2.
The corresponding frames for plus 1 developing is minus 3 1/2
and plus 2 1/2. The contrast should be the same in all examples,
and this is done by varying the developing time. Start from the
following approximate times: For minus 2 developing time should
be shortened by 30 %. For minus 1 by 15 %. For plus 1 time should
be increased by 20 % and for plus 2 by 40 %.
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by Lars Kjellberg on the Photodo.com website - a great resource
for photographers around the world!