A general rule states that the preferred resolution in dots per inch (dpi) should be twice the frequency of lines per inch (lpi) of the final halftone screen to be used. Therefore, the resolution of a typical image to be reproduced by the lithographic process using 150lpi screen ruling should be 300dpi. In practice, the ratio can be somewhat lower and still achieve satisfactory results. This has nothing to do with the resolution of the output device provided that the maximum shades of color can be generated by the device (see below). The choice of halftone screen ruling is determined by the final printing process and the paper to be used. A coarse screen would be suitable for newspaper reproduction, finer screens for high quality lithographic reproduction.

Rules for imagesetting

The final quality of image reproduction is determined by the number of shades that it is possible to generate from each ink color being used to print the image. The number of shades ("levels") is determined by a combination of halftone screen ruling and imagesetter resolution. To achieve the PostScript maximum of 256 shades for an ink color, each halftone dot in the image must be generated from a matrix measuring 16 x 16 pixels. Thus if your chosen print method requires that you apply a150lpi screen ruling to your images, the imagesetter must cram 16 (dots) x 150 (halftone lines) into an inch ­ requiring an output resolution of 2,400dpi. In short, to calculate the optimum imagesetter resolution (in dots per inch) for outputting halftone images:

Multiply halftone screen resolution (in lines per inch) by 16.

Enlarging images

Although optimized to be reproduced at more or less the size that you find them on disc, BodyShots images can withstand a considerable degree of enlargement ­ in some circumstances by as much as 200% ­ without serious loss of quality. However, if you enlarge an image substantially without changing its dot frequency, it may degrade to a point where individual pixels become visible. For example, a 300dpi image which is enlarged to twice its original size will reproduce with a resolution of only 150dpi ­ close to the optimum halftone screen rulings of 133­150lpi for four-color printing. To minimize this degradation use an image-manipulation application such as Photoshop, which allows you to enlarge an image without changing its resolution. It does this by adding pixels through a process called "interpolation." Changing image size by adding pixels is known as "resampling" ­ enlargement is "resampling up" whereas reduction is "resampling down." Further enhancements are possible (and in some circumstances, very necessary ­ especially if you enlarge an image by a huge amount) by using "sharpen" filters, which also employ interpolation techniques.

Adjusting color for process printing

While BodyShots images are optimized for both screen display and for printing, what looks good on screen may not necessarily be the same when printed ­ colors of images for display are often more saturated than in images intended for 4-color printing. You may find that a little fine tuning of images enhances BodyShots images when they are printed by 4-color process methods. Try the following (although this may not give the same result with all the images, nor with all printing processes, so it's best to do a few tests first):

1. With your selected BodyShots image open, choose "Adjust" > "Levels" from the "Image" menu. In the "Input Levels:" fields, enter values of 0 - 1.20 - 256.

   

   Click "OK." This makes the image a little lighter in color, but desaturates it.

2. Choose "Adjust" > "Hue/Saturation" from the "Image" menu. Move the "Saturation" slider to the right until its value reads "+10."

   

   This puts a little color back into the image - it may need even more, up to a value of +20.

   Preparing images for color separation

   The BodyShots images are stored on disc in RGB mode. Whilst some applications can separate color photographs directly from RGB mode, it is best to convert them to CMYK mode first. To do this in Photoshop, just select "CMYK color" from the "Mode" menu.

   
   

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