Now that you know your minimum exposure time, we can find out
what values each of our digital % values print at. For example, if the value 70%
prints as white in our gum prints, we want to be able to tell the computer to
print whites in our images as 70%. These tests will allow us to define each %
value and what it prints as in a gum print. This seems like it might be a
terribly complicated task. But if you are patient enough to make gum prints in
the first place, you can handle it, I know you can!
We will print a single 0% - 100% scale at three different exposure
times and use the information from those to create a custom Photoshop curve,
enabling us to accurately print any digital image file we wish in gum bichromate.
>> First you will need to make a 0%-100% scale
using Photoshop that you will use to print with. Here's how.
In Photoshop,
create a new document just smaller then the 8.5x11 inch transparency size,
say at 7x10.5 inches at 300ppi. Select an area about an inch or two wide and
as tall as your file with the selection tool.
(It's best to build your scale off to the side so the rest of the transparency can be used for other tests or test
negatives and not go to waste.)
>> Use the gradient tool and make a linear
gradient from 100% (black) to 0% (white) within your selection. Hold down the SHIFT key
while making the gradient to be sure it is absolutely vertical.
>> Then go to Image>Adjust>Equalize...
and
equalize the selected area only. This assures that there are equal amounts
of each % value in your scale.
The effect is
subtle...but it matters!
>> Then Go to Image>Adjust>Posterize...
This
step reduces the 256 values of gray to any number you specify, equally. If I entered 3 for
example, it would show my scale as white, 50% gray, and black.
In this
example I enter 51 which gives a scale in 2% increments. This image to the right is a
detailed look after posterization.
(entering 11 would give you the familiar 10 zones of the Zone system from zone 0-10, but may not be precise enough for these tests)
>> Next you will need to add a 100% black line
into the center of the darker values, and a 0% white line into the lighter
values.
Then draw mark lines at the edges of each value change on the side of the scale. Also include
dots specifying 20%, 40%, 50%, 60%, and 80%.
It will turn out something like the
scale below.
>> Print your scale using glossy paper settings,
or whichever settings you plan on printing with from for your negatives. If
you are not sure, just use a glossy paper setting like I have. This setting
deposits a thick coating of inks for a richer print, and thus a denser
negative. The setting choice is up to you and your printer. Also Make sure the
detail is at its highest setting.
(There is sometimes a setting for
transparency film, but don't use it! The transparency setting seems to print
with less ink, and that will give us a less dense negative, not to great for
printing gums with, unless you adore over exposure.)
>> Continuing on, once the inkjet
scale is printed and dry, locate the rest of that emulsion coated paper you cut near the
beginning of step 2. Cut another small strip just over the proportions of your
digital scale. Lay the strip and negative scale under glass and expose it at
your determined minimum exposure time. My time was 4 minutes as you shall recall, so that's what I used.
Then cut and expose another strip at double your minimum time and another at triple the
time. Develop all 3 together normally and carefully. Let them dry, and scan
them together into Photoshop.
I exposed these strips below at 4min, 8min,
12min
>> These next steps will allow us to extract information from our test strips and use them to create our custom negative curve!
In Photoshop, slightly blur the image going to Filter>Blur>Blur More. This evens out the texture of the paper to a consistent value.
We then adjust the levels under Image>Adjust>Levels so the black and white
points are at the ends of the histogram. I use the value pickers just above
where it says Preview and set a fully exposed area to black and a
clean paper value to white.
Doing this enables us to see our scale RELATIVELY to the digital 100% marks. We are now able to compare our original scale to our printed scales. The
resulting relative scan is shown below.
>> Now we need to find the relative values our
gum layer has printed at under various exposure times. It really REALLY helps
to make a data table to keep track of everything. You will need input and
output values.
The table to the right shows the values we already know. The 60, 50, and 40 are the dots on our printed scale, and 0 and 100% are the White and Black points. We
need to find the corresponding values and fill in our chart.
Remember
always that Input values are those from the negative. Output values are the
relative values on our gum print scan.
4 min
8 min
12 min
input
gum output
input
gum output
input
gum output
0
0
0
60
60
60
50
50
50
40
40
40
100
100
100
>> In our 4 minute test, I look to see where the
white line is no longer distinguishable from its surroundings. This is where
our Gum output is at 0% or white. In mine it looks like it happens at value level 74%. It is easy to think "74% became 0%"
I follow the black line along until it is no longer distinguishable. This
looks like it happens at around value 24. I fill this data into my table. 24% ont he input became 100% on the gum output.
>> I then use the color picker to find the
relative values my gum layer has printed in the mid tones. Looking at the 50% input mark
from my 4 minute exposure test, under the information tab in Photoshop, it tells me the
gum output value corresponding to the input value of 50% is 80%. 50% became 80%.
To say it easier...50% in a digital file prints as 80% in a gum print.
40% becomes 91%, and 60% becomes 48%. I fill this out in my table.
Repeat
this for the other two exposure tests.
4 min
8 min
12 min
input
gum output
input
gum output
input
gum output
74
0
80
0
86
0
60
48
60
80
60
90
50
80
50
92
50
96
40
91
40
95
40
100
24
100
36
100
40
100
>> Here is my completed table of input and
output values.
>> I graphed the data from the table above resulting
in the chart on the right.
For our custom scale we
want to combine each of these scales into one Photoshop curve.
By combining these scales we save some of the intermittent value data lost by the compression of values by printing a slightly longer or contrasty average scale, while retaining the response of the gum bichromate to your light source and negative material and ink.
>> To do this we need another small table of
information called the Average Scale as seen to the left and is color coded.
We get the data from our previous chart. 86% is the 0% value of our 12 minute scale and is the darkest we will need to
print our inkjet transparency. 24% is
the 100% value of our 4 minute scale, and is the thinnest value of our
transparency.
For our 60%, 50%, and 40% values, we
will average the gum outputs of the 3 scales, highlighted in orange yellow
and red. The orange 73%, for example, is the average of 48, 80, and 90.
Average Scale
4 min
8 min
12 min
input
gum output
input
gum output
input
gum output
input
gum output
86
0
74
0
80
0
86
0
60
73
60
48
60
80
60
90
50
89
50
80
50
92
50
96
40
95
40
91
40
95
40
100
24
100
24
100
36
100
40
100
>> The average scale looks like this super
imposed on our three scales. As you can see it is a combination of the
three.
>> This Average scale gives us more available values in our
transparency to compress an image's tonal scale into. Our values on the
average scale range from 24% to 86% rather then only 24% to 74% as in the 4
minute exposure test.
>> Printing with this average scale will be covered in the NEXT Tutorial, Printing with a custom Negative. But before we get too far ahead of
ourselves, we need to enter this data into a Photoshop curve, and tweak the data a bit.
