|
When preparing material for commercial printing, one of the first terms you as a customer will hear from the customer service representative is “camera-ready.” The rep will make it clear to you that you can save yourself or your company money and time if you present the printer with artwork that is suitable for the camera to photograph without any changes or manipulations. Though this simple explanation seems to make the situation sound quite straightforward, in reality very little is exactly as it appears in the printing industry.
Look closely at a commercially printed grayscale photograph under mild magnification and you will see that it is composed entirely of small dots. The dots vary in size, giving the illusion of different shades of gray. Actually, there is no gray; the only color is black. What happens is that your brain mixes the black dots and the white areas between them and sees a given shade of gray. The distance between the dots is determined by the line screen, which is set by the printer's camera. To prepare a photograph for print, you must create a halftone. Film is so finely grained that even under mild magnification, the tones appear continuous. For the camera to shoot the photograph successfully, the photo must be screened, which is just like viewing the picture through a piece of window screen.
The halftone method I will describe here utilizes a flatbed scanner and Adobe's Photoshop. It is a quick and simple way to achieve reasonable quality halftones for low to medium resolution printing and is particularly useful for handling large numbers of photographs quickly. The original photos may be color or grayscale. If you are scanning several photos simultaneously, try to pick ones with similar tonal range. I will assume the reader has a working knowledge of Photoshop and scanning.
Your first task is to determine your scanning resolution. As with most things in the print world, this depends upon what you plan to do with the final product. Consult your printer to find what the final line screen of your printed product will be. Most low-cost commercial offset printing is at 85 lpi (lines per inch). You should scan at 1.5 to 2.5 times this figure. For 85 lpi, a 200 dpi (dots per inch) scan would be satisfactory. Magazine quality printing is normally run at 121 lpi, so your scan should be done at 300 dpi. Artbook quality printing is run at 175 lpi, so the scan resolution should be set at 350-425 dpi.
So preview your scan and select the area to be saved. Set the approximate resolution and set the mode to color CMYK (process), if that option is available, or color RGB (display) if it is not. You might want to brighten up the shadows a bit unless the photos are already quite light, because flatbed scanners tend to lose detail in the darker areas. Do not sharpen at this time.
|
When you have the image displayed in Photoshop, adjust the window to give the largest possible view. If you acquired the image as an RGB file, convert it to Lab color, which is native Photoshop, then to CMYK. Show the channels palette, select all, and copy. Deselect, click on the black channel in the channels palette, then paste. Convert the image to grayscale. What you have done here is eliminate the black channel, and converted the other colors to grays. This will lighten the appearance of the photograph, particularly in the shadows, without sacrificing detail.
The next step depends upon the dot gain of the press, ink and paper on which the final product will be produced. Dot gain is a measure of how much the tiny dots, which make up the image, will grow as the piece is being printed. It is a result of the ink soaking into the paper. You will need to consult your printer for accurate figures. For this example, I am assuming a dot gain of 12 percent in the shadows and a dot loss of 8 percent in the highlights. Keep in mind that dot gain is greatest in the mid-tones. Dot loss happens when the camera fails to photograph the very small dots in the lightest areas of the picture. Again, your printer will have to furnish you with this figure.
Display the levels dialogue box. Set the output levels at 35 for the low end and 235 on the high end. Since the total output level range is 255, we arrive at these figures by taking 12 percent of 255 and adding the result to zero for the shadows, and taking 8 percent of 255 and subtracting the result from 255 for the highlights. Turning our attention to the input level, move the leftmost slider to the right to a point where the histogram shows significant data is present. Move the rightmost slider to the left in the same manner. Move the middle slider left until the image appears light and flat, with a washed-out look, but don't exceed 1.40. The image now appears to have lost most of its contrast and is mostly a rather middle gray. That is the look we are trying to achieve, because dot gain will restore the shadows at the low end and the contrasting in the mid-tones, while dot loss will restore the highlights.
Now it is time to size and save. You wish to resize the image to the exact size that it will be printed, keeping the same resolution we determined earlier. Save the file as a TIFF (tagged image file format) and designate a bit order appropriate to the computer platform your printer uses. Print a hard copy as a proof using the highest quality paper at the proper line screen. To select the line screen in Photoshop, go to Page Setup/ Photoshop options/Line Screen and deselect Use Printer's Default. Type in the figure your printer gave you and select Round Dot. If you are submitting camera-ready hard copy, select a line screen about 115 percent of the printer's line screen to avoid generating patterns.
|
