Halftone Calibration

1st June 2012

How to Calibrate a Halftone Image.

This is Little Tuttle, my two-year-old eastern painted tuttle. She has agreed to volunteer for our experimentation.

Images that wind up on a printing press should be calibrated for the best results. Calibrating a one-color image includes adjusting shadows and highlights for ideal ink coverage with consideration to the type of paper and printing process. You may also want to putz with the midtones to achieve more attractive contrast.

 A Note About Paper

Uncoated paper absorbs more ink than coated paper — think paper towel commercial. Without getting into the details of dot-gain and press fingerprinting, trust me when I say these percentages will produce lovely results on our offset sheet fed presses.

Finish
Shadow
Highlight
Coated
92%
3%
Uncoated
87%
3%

 

Download Little Tuttle and play along.
You don’t really notice calibration until it’s wrong: way too dark, way too light, whacked out contrast, muddy midtones

As you can see, this picture is pretty flat and the tonal range stops short of either black or white, this makes it hard to see her sweet little features. Clearly, Little Tuttle needs some help.

 

 


Using image manipulation software—I’m running Photoshop CS5.1—take the following steps to calibrate your grayscale image:


Step 1.

Confirm the image is grayscale and 300dpi.


Step 2.

Find and mark the darkest pixel in the image.


Step 3.

Find and mark the lightest pixel in the image.


Step 4.

Adjust output for ideal ink density based on the type of paper. For this example, I’ll be preparing Little Tuttle for a coated stock and aiming for maximum shadows of 92% and highlights of 3%.


Step 5.

Tweak the contrast for more punch. Try ramping up midtones by manipulating the curves tool (which spaces or compresses a tonal range) instead of the brightness and contrast tool (which erases tonal ranges with every manipulation).


Before
After

In Conclusion

That’s basically it: find the darkest and lightest parts of the photo and make them as dark and as light as they should be for the paper and the process. If your composition has adjustment layers, you’ll want to flatten your press-ready image before saving using the .tif format.

If this sounds as tedious as vacuuming the baseboards — and when your publication has 100+ images, it can be — I recommend batch processing. Save time by creating a desktop droplet in Photoshop, based on a set of pre-recorded actions, that you can use over and over again. Now there’s plenty of time to vacuum the baseboards. yay.



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