Many factors affect the quality of an image’s final reproduction, including the quality of original material and scanner output. This article provides practical techniques that can be applied to help achieve the best possible image reproduction quality.

David Hinson

Image creation

The content of editorial photos can take precedence over their image quality but much can be done to achieve both goals. The latitude for electronically correcting a poor quality original is limited by the characteristics of its digital image file so it is important to start with quality originals. With a properly calibrated and controlled image reproduction system, final image reproduction will be directly related to the original photographic or graphics quality.

Using the lowest possible film speed such as ISO 200 over ISO 400 for a given scene will reduce the effects of grain. All attempts to should be made to either match the film to the lighting conditions or match the lighting conditions to the film, as dictated by the situation.

Digital cameras respond better with sufficient light levels to properly excite each pixel in the charged couple device chip. The dreaded green cast caused by florescent lighting can be minimized by not using film designed for daylight and by using magenta filters over the lens or flash.

By identifying common places where editorial photos are taken, such as courtrooms, at sporting events or in office environments, an optimum combination of camera format, film, lenses, filters and flash devices can be identified. The goal is to achieve the best possible color balance, minimized grain, image brightness and contrast.

The color selection, contrast and detail of physical artwork should be appropriate for the filter colors, bit depth and resolution of the image scanner being used. If a certain color is desired in the final printing, a slightly different color might be required so that when that color is interpreted by the scanner’s filters, the resulting RGB and ultimately CMYK dot values produce the desired printed color. The goal is to use colors that require the least amount of post scanning tone and color correction in order to achieve the desired result.

When selecting colors for digitally created artwork in RGB mode, the exclamation mark in the Adobe Photoshop Info window indicates colors that are out of range for the current CMYK separation setup. These colors will not match when printed. Working in CMYK mode for graphics and flat tints will improve predictability of results.

Image scanner calibration

Typically, a scanner is purchased, plugged in and used with the default calibration. The default settings must be checked for two basic issues: density range and color balance. The scanner calibration utility is used to adjust the scanner’s density capture range (dynamic range) to the density range of a given type of original. The exposure (gamma) setting is used to set the capture endpoints and the correct tone levels in-between, which relates to capturing image detail and color shades. Individual settings are required for color negative versus transparency film and for various brands but they remain constant once determined. Only when an original is very light, dark or it contains a major colorcast should scanner calibration settings be changed in order to help the capture for that original.

An IT8 target or another form of grayscale is used for reflection and transparency original materials. For color negative material, a photograph can be taken of a Gretag Macbeth Color Checker under controlled lighting conditions. A step in the scale is selected that best represents the density of a highlight area containing detail in a normal contrast original. The exposure is adjusted to achieve a 240 gray level for black and white or a 240 value for the one of the RGB channels for color.

The same is done for a step that represents a typical shadow density where detail occurs and set the exposure so a value of 10 is achieved. The goal is to have the highlight and shadow dot percentages close to the correct endpoint values for normal contrast copy even before the eyedroppers are used to make the final highlight and shadow settings. The values of 240 and 10 are provided as starting points.

Scanner calibration that allows the exposure to be set with a curve can be used to obtain the best capture of densities for steps between the highlight and shadow points. Curves also allow setting the mid-tone placement to minimize post-scanning tone adjustment to lighten faces and other mid-tones in images.

Scanner color balance calibration is a critical but often overlooked adjustment. Once the exposure has been adjusted using the green channel, the red and blue values are then adjusted to match the green values. A grayscale is scanned and the resulting red and blue values in each step are compared to the green values.

Adobe Photoshop’s Info window can be used to evaluate each test scan. The scanner’s red and blue color balance settings are adjusted so all three channels read nearly the same (within 3 units) for each step in the scale. The goal is to minimize scanner-caused colorcasts for various original types.

Certain scanner calibration utilities are limited to providing one setting for exposure and one setting per channel for RGB color balance. In these cases, the best possible highlight and shadow density captures are set and the color balance is set for the mid-tone step in a grayscale. At this point, the color separation software is used to apply tone curve adjustments in RGB mode to optimize the density range and color balance for the remaining steps in the grayscale. These corrections are saved to disk and named to identify types of originals and/or brands of film. When an original of a given type is scanned, the appropriate curve is applied before any other tone corrections to correct the scanner’s tone and color balance deficiencies. This approach will reduce the amount of time necessary to tone correct most images.

Applying these methods will reduce the amount of time required to process images and will improve image reproduction quality. The next article will discuss converting continuous tone images to screened images and imagesetter calibration.

David Hinson is president of Prismatic Inc. providing consulting services and printing process control software products He can be reached at 336.644.2003 or by e-mail at dhinson@prismaticsystem.com.