EZ Adjustment Curves for Better Inkset Control
By Paul Roark
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.
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.