Prepress - Scanning Contrast

One major factor to consider in an original image before scanning is the image "keyness". Thus, it is important to set the contrast of an image correctly before continuing with any other image editing steps.

Keyness describes the tonal values in a picture. That is the variation from the lightest highlight in an image to the darkest shadow. A high-key image is composed mostly of highlights, such as a scoop of vanilla ice cream in a light colored bowl set on a white tablecloth. A low-key image is composed mostly of shadows, such as the silhouette of a horse rancher on the horizon at sunset. A normal key has equal amounts of highlights, middletones, and shadows distributed throughout the picture.

First, it is important to know the maximum print density range. That is the difference between the lightest and darkest tones achievable. A glossy coated paper will allow for greater contrast than an uncoated paper. You also need to know what the highlight, middletone, and shadow dot sizes are to produce neutral grays on a given substrate. These values should be entered as the desired diffuse highlight, middletone, and shadow values respectively.

To maximize the keyness of an image it is important to set the appropriate contrast points before any other settings are made to the image. When setting the highlight dot placement, it is important to select the whitest part of the image that still has detail. This is called a diffuse highlight and is considered a neutral white. A specular highlight does not have any dots or detail, such as the reflection on the chrome or glass of a car. Setting the highlight dot of an image in the wrong location will affect the keyness or contrast variation of the overall image.

Naturally the shadow dot placement should be set in darkest part of image. Ideally this point should be a neutral black with no color cast. If there are no appropriate shadows in the image, it may be possible to use the black border of the image as with transparencies. Again, setting the shadow dot of an image in the wrong location will affect the keyness or contrast variation of the overall image.

Adjusting the middletone dot placement will either clean and brighten the image or make it more dull and gray. The middletone dot may also be set to focus more contrast in a certain areas, open up the shadows, or clean up the colors. However, it is important to note that improperly setting the middletone may negatively affect the final print. Darkening the middletones will add more contrast in the highlights, but will flatten the shadows. Likewise, lightening the middletones will add more contrast to the shadows, but will flatten the highlights. To much middletone dot tends to put too much cyan in reds, magenta in greens, and yellow in blues, so that the colors look dirty, darker, or muddy.

Setting the proper highlight, middletone, and shadow for an image is a step often overlooked when scanning an image. Contrast is the most important factor for achieving optimum color. Assuming the scanner has captured the original properly is a misconception. It is therefore critical to take the time to set the highlight, middletone, and shadow values correctly from the start.

For more information on this issue, please contact: info@teamflexo.com

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