Black-and-white scanning made simple - tips for making high-quality half-tones

These days publishers are increasingly involved with production and prepress in the course of their projects. They are now responsible for tasks that used to be carried out at the printing plant, and need to produce files that are ready for Ripping to composite film. But in the area of continuous-tone imaging, many are having hit-or-miss success getting consistent quality when creating their own halftones. This is generally not because the hardware and software aren't up to the job; it's that the "warmware" - the the people using the stuff - don't really know what they're doing. Yet, by following a few simple rules, almost anyone can create great grayscale scans for their projects.

Calibrate your monitor. You have to do this first because your monitor is your window on the data. It's nice to spend $500 to $800 on calibration hardware, but the gamma control panel that comes with Adobe Photoshop or the Color Composer software in Radius products can do the basic job. The critical move is to set your screen's gamma (the relationship of grays to blacks and whites) so that a 50 percent tone on paper looks like a 50 percent tone on your screen. Without adequate calibration, no tonal corrections that you make on your image will be worthwhile.

Scan at the highest resolution. The relationship of points-per-inch of data capture to dots-per-inch of image halftone output is confusing. Suffice it to say that if you have enough memory and a fast machine, it pays to scan all your images at resolutions of at least 300 dpi. Two advantages to scanning at high resolutions are maximum flexibility and elimination of artifacts. The phrase "garbage in/garbage-out" holds particularly true when you make a poor, under-sampled scan. You can always resample smaller, but you can't gain detail that was never there, despite the best interpolation algorithms. If you have a photo that needs retouching because of dust or emulsion damage or whatever, the higher the resolution, the more finely detailed the repair.

The only major disadvantage to high resolution is reduced working speed. But if you're on a properly configured computer (say, 24MB of memory, 100MB free hard-drive space and a Quadra or 486-based processor) and Photoshop 3.01, the file sizes work fine.

Don't overwork the image. Capturing data effectively relies on the subtleties of the original image being retained in the final output. The slight variations in an image's adjacent pixels are the keys to retaining the look of the original. Every successive filter that is applied moves the data away from those original relationships. Try to make only three corrections, and do them only once: Correct the gamma; correct the sharpness; and compensate for dot gain. Anything else you do constitutes a "special effect."

Learn how halftones work in the printing process. A halftone is a kind of dither that allows grayscale information to be depicted in a black-or-white environment. A halftone screen is technically an amplitude-modulated dither shaped in an ellipse. What this means is that each halftone dot represents a level of gray based on the dot's size. If the level of gray is very light, the dot is small; if it's very dark, the dot is big. When adjacent dots are printing at 100 percent, the area is printing solid ink coverage.

The traditional rules of making halftones demand that there always be some dot in every part of an image. Old-time prepress folks realized that if there is a little information at the ends of the spectrum, there will be a good distribution in the middle. This principle is exactly the same in the digital domain as well. judicious setting of the black and white points in an image is the most crucial decision that needs to be made. In many instances it can be made only by examining the image and letting experience be your guide.

Traditional camera operators always knew which press would be running the film they were generating, and customized their halftones accordingly to compensate for both the press and paper. Many designers today forget that the whole purpose of their documents is to create film that prints on a printing press. That's the reason making negatives is called "prepress" instead of printing being called "post-negs."

The kind of printing press and the type of paper used can have a non-linear effect on the final halftones. The size of the ink dot that the plate wants the press to put on the paper and the size of the dot that actually gets laid down can vary dramatically. In most cases, paper and presses will enlarge the dot, making a 50 percent-sized dot into something much bigger. This effect is called "press gain" and is more noticeable as the quality of the paper decreases and the speed of the press increases.

Correcting for dot gain should be the last step in preparing a halftone for production. Open the Curves menu item to pull the curve straight down at the 50 percent level to the desired compensation point. Pulling the 50 percent level down to 40 percent is a 20 percent change that compensates for a 20 percent dot gain. Press compensation is the hardest part of g a proper halftone. Anyone who can't understand the ramifications of dot gain can't reliably produce excellent images.

Quick tricks of the trade

These simple tricks can eliminate the most common - and unsightly - scanning problems;

* Eliminate glare introduced by curling photos or bound-book pages by putting a big pieces of foam and a couple of heavy books on top of them to flatten the image on the bed.

* Moire can be avoided by scanning at high resolutions, using the Dust&Scratches filter in Photoshop to blur the screen, then resampling down to the proper image size. It sounds a little weird, but try it - it works.

* Show-trough of information on the back of an original can be almost totally eliminated by placing a black card behind the original. It's surprising how effective this can be.

* Dust is always a problem. Using canned-air to clean the surface whenever you scan will eliminate most of the problems.

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