EZ
Adjustment Curves for Better Inkset Control
&
“Linearization”
By Paul Roark
10-14-05
If the Epson driver sliders do not
achieve a smooth, evenly-spaced grayscale, simple grayscale curves in Photoshop
or Picture Window can be used to fine-tune or further “linearize” the
printer. I recommend the sliders be used
first to get the image as close as possible to the ideal, and then use a
printing curve only to fine tune the image for those settings. The curve can be saved and applied just
before printing, or it can be on an adjustment layer and activated only for
printing. With Photoshop, the curves can
also be used as Transfer Functions and loaded in the “Print with Preview”
(Print with Preview>Output>Transfer>Load>File type >Curves
(*.ACV) ). Once the curves are
completed, using them in this method is probably the easiest workflow.
The driver controls and these curves can
also be used to match the print to the monitor.
However, I use the curves primarily to smooth out the curve and match my
standard printing scale. I then use
another Photoshop monitor view preview system to get the monitor to match the
print. See http://home1.gte.net/res09aij/Monitor-Profiling.htm
I am using Photoshop CS2 on a Windows XP
platform. I assume Mac is similar. One potential problem for some photographers
is the high cost of Photoshop. It is, however, the industry standard and most
serious printers will want to take the plunge at some point. Fortunately,
Digital Light and Color has an alternative image editor for PC’s called Picture
Window that is compatible with Photoshop curves.
21-Step Test Files and
Target Densities
Before getting into the details of how a
curve is built or adjusted, it helps to understand the test files and target
densities that are indicated on them.
I use 21-step test files to adjust
curves. These files are sometimes referred to as “step wedges” and can be
downloaded from my website. (See http://home1.gte.net/res09aij/Test-files.htm) The test strips will have a set of
increasingly dense steps in 5% increments from 0% (white) to 100% (black). I
also label the steps with the equivalent 0 – 255, 8-bit luminance code values.
On this scale, 255 represents white and 0 is black.
A typical 21-step test file.
Papers print with different maximum black
densities. As such, the target densities vary depending on what paper is used.
I’ve made a series of test files that have different target densities for the
papers I work with. When printed, the 21 density steps can be read with a
spectrophotometer or visually judged to see if they are evenly spaced.
To get an evenly appearing set of values
between the paper white and dmax, the Lab Luminance scale is used. The CIE (the Commission Internationale
d’Eclairage) is an international body that did extensive research on how the
eye responds to color and different light values. That scale uses an absolute black point of L =
0 and perfect reflectance of L = 100.
Since no actual print has these characteristics, the visual midpoint is
not L = 50. With Epson Enhanced Matte,
it is visual density 0.61. (The EEM
paper white density of 0.03 equals Lab Luminance 97. The dmax of 1.68 equals
Luminance 16. If one divides this range in half, the middle is Luminance value
56.5, which equals density 0.61. That is where the 50% patch is often placed.
The other steps basically follow the Lab luminance spacing.)
I currently deviate from the
“linearization” approach that places the 50% density “half” way between the
dmax (black) and dmin (paper white) because I want to print on any paper with
the same file and have the prints look like they are the same density and
contrast. This seems to work best if the same 50% point is used for all papers.
No matter what the Dmax and paper white are, I have stayed with the same 50%
density of 0.61 for all papers.
Photoshop Image Adjustment
Curves
To see how these curves work, open a
21-step grayscale test file.
In Photoshop, navigate to
Image>Adjust>Curves. A curve panel will appear.
Curves are used to edit the distribution
of information in an image file. They change the relationship between input and
output values. The horizontal axis in
the curve panel is the input value of the file; the vertical axis is output
value. The default 45-degree line indicates the default relationship each input
value has to a corresponding output value.
With the curve above, there would be no change to the values at all.
The curves boxes have 2 alternative axes
layouts and calibrations. I use the 0 –
255 scale with the dark ends of the curves at the bottom left because it allows
more control than the 0 – 100 step scales.
The arrows in the middle of the horizontal axis switch between the 2
scales.
Click on the center of the curve and the
Input/Output values appear. In the example, these show an input of 127 and an
output of 127, corresponding to the edit point.
This curve uses the 0 (black) to 255 (white)
scale, where 127 is the midpoint in Adobe RGB & Gray Gamma 2.2.
If the curve point is moved up the image
becomes lighter. The program smoothes the curve between points. The end points
have not changed (black is 0, 0 and white is 255, 255) but the new midpoint
position has altered the other input and output relationships along the
curve. To set the end points, levels is
a better tool than these curves.
This curve uses the 0% ink (white) to 100%
ink (black) scale, where 50% is the midpoint.
If the midpoint is moved up, the image
becomes darker.
Although I use the 0-255 scale form my
curves, they can be changed to the 0% to 100% with a click of the center
arrows. The Photoshop Print with Preview
system uses Transfer Functions that have a curves box that uses the 0% - 100%
axes and will accept the Image Adjustment Curves no matter which format they
were made in.
Making or Adjusting a
Grayscale Curve
The basic process involves printing
21-step test strips, evaluating the print densities of the steps, making
adjustments to the curve used for the test (or a new one if no curve was used
for the first print), saving the new curve, and printing a new test strip with
this curve. After several iterations the test print will have density steps
that are close enough to the target densities to be called a final curve.
Keeping track of what printer and paper
the curve is for can be accomplished in the name of the curve. Having different folders for the curves or
different printers and inksets also helps.
The driver settings also need to be noted. I sometimes put this
information in the name of the curve as a reminder. It is also important that
the test file has not been previously altered by a curve or profile. Save the
files before applying a curve that alters their values.
Step-By-Step Procedure
1.
Open
a 21-step grayscale test file. For EEM and Photo Rag, the K168 version is
appropriate. If you are doing this
visually, it doesn’t matte which file is used.
Select all and copy the test file onto the clipboard.
2.
For
efficient use of paper, create a new, blank (white background) 8x10 file. These
are the steps I take: File>New, OK; Double click Hand to get the width to
fill the screen. Increase canvas size
Height to 10 inches.
3.
Place
the first test strip on a layer at the top of the paper by grabbing the bottom
of the 8x10 and pushing it up as far as possible. Paste the test strip into the
new image (Edit>Paste). Grab the bottom of the file and pull it down so that
all the numbers are visible, but very little white paper is showing below them.
21-Step test
file as the top layer in an 8x10 image
4.
Notice
three sets of numbers at the bottom of the test strip. The top numbers are
percentages that appear in the Photoshop Information palette when the cursor is
on that step and the file is a grayscale. The second row lists the numbers
corresponding to these percentages when the file mode has been changed to RGB
and the Color Settings have made Adobe RGB (1998) the RGB working space. This second row of numbers is also what the
curves box uses when it’s in the 0 – 255 mode that I recommend. The numbers in the bottom row are the target
densities.
5. Open the curves dialogue box (Image>Adjust>Curves). If one were to start from scratch, this could be the starting curve. It is easier to start from an existing curve if one is already close to what is needed.
6. To make things easier, I’ve also made what I call the “EZ_starting_curve.” This has all the points that are usually needed. The points correspond to the 5% to 75% steps in the shadows (the usual place where problems show up) and to the 50% midpoint and 25% middle highlight point. With this starter curve one needs simply to move from point to point, and raise or lower the points as indicated by how the printed test strip looks. This curve is on my web page. Load it if there is no existing curve that is close. When you hit the Load button, Photoshop looks first in the Photoshop folder, but you can set up a folder anywhere for these, and Photoshop will look next time in the folder last used for the curves.
EZ starting curve
7. The keyboard is the easiest tool to navigate to points, as opposed to the cursor: Ctrl + Tab highlights the points from left to right, and. Ctrl + Shift + Tab highlights the points from right to left on a PC. Then one can click in the Output box and use the arrow keys to move the point up or down. To darken a step that is too light on the printed test strip, move the corresponding point down. I have put the curve’s points on the steps’ input values. Compare the numbers in the Input box with the 0 to 255 values used to label the 21-step test file.
8. Whether to move points up or down depends on how the printed and dry test strip compares to whatever standard one is following. I use a spectrophotometer to match the values indicated on the test strips for all my printing systems. Some match their monitors or just look at the evenness of the steps.
9. Go from point to point and move the points up or down as needed to match the desired standard.
10. Save the curve with the Save button, give
it a unique name that includes the paper and settings, and perhaps a version
number, and then apply it to the file by clicking OK.
11. Print the test strip on the target paper
(File>Print with Preview). The settings used to print this must be noted and
be consistent. I have a number of Readme files for different printers and ink
sets listed at http://home1.gte.net/res09aij/index.htm.
12. Go to the print driver’s Advanced panel
(Printer > Properties > Custom > Advanced). The settings in this box must also be
consistent. I have these settings also
in the Readme files.
13. I always print a Nozzle Check (on the
Properties Maintenance tab) on plain paper before I do any serious printing.
Print the test strip.
14. While the test strip is being printed
move the existing image of the test strip up with the hand tool until the
bottom has just disappeared from sight. Then delete that layer. Paste in a new
un-altered copy of the 21-step test strip to prepare for the next step.
15. Evaluate the test strip after it is dry.
I use a hair drier to speed the process. After the test strip has been dried it
may still change a bit by the next day. However, they will be close enough to
use after just 10 seconds under a hair drier. k.
Do another iteration
of adjusting the curve points up or down as needed. With the fresh test file pasted in place,
open the curves box and load the curve used to print the last test strip. Move from point to point making the
adjustments indicated needed, save the new curve with a new version number, and
print another test strip.
16. Repeat these steps until the print
densities match the target or desired densities.
Note on 16 bit Grayscale
Files
I recommend scanning and editing in
16-bit grayscale mode. However, even with 8-bit files, the print may be
smoother if the file is converted to 16 bit before the curve is applied. This
is also true if the file is then converted back to 8-bit after the curves are
applied.
Note on Monitor Profiling
Most photographers doing serious color
printing prefer to use profiles that calibrate their displays and printers to
achieve consistent results. These profiling systems, however, are not made to
work with grayscale files and inksets.
(Or, vice versa – the traditional grayscale approaches have not been
made to be consistent with the standard monitor calibration systems.)
Photographers who are accustomed to the
sophisticated color display profiling systems will often say that, at a
minimum, one must have a display calibrated with special software using a
sensor. I have one of those systems and have compared it to using Adobe Gamma,
which is a simple visual system included with all Mac and many Windows computer
systems. My conclusion is that the free
visual systems can be quite adequate for B&W monitor profiling. My workflow does not attempt to match the
tones (hues) of the prints. I approach
B&W as being a system that does not worry about this in the context of
working up a print. It is a separate
decision that is based on actual print, hard copy viewing in the context of
their display conditions.
Enjoy the journey.
Paul