Prepress and digital file preparation
The prepress domain tests whether you can take a customer file or an original image and turn it into a predictable, printable, imposition-ready PDF. Questions cluster around resolution arithmetic, ICC colour management, imposition schemes and the PDF/X output specification.
Resolution and image capture
- Capture or scan at the effective resolution needed at final size: about 300 ppi for continuous-tone images at 1:1, 600 ppi for bitmap line art, 1200 ppi for fine line work going to screen printing.
- Resolution scales inversely with reproduction size: double the printed width and you halve the ppi.
- Avoid repeated JPEG saves; each generation adds artefacts, and up-sampling adds pixels but never information.
Colour management
- Convert to the press profile once, at the end of retouching: perceptual intent for photographs, relative colorimetric with black point compensation for logos and solid builds.
- The exported PDF must carry an OutputIntent, for example FOGRA39L for coated offset or FOGRA47 for uncoated, so the RIP knows the intended printing condition.
- Cap total area coverage at the limit of that condition, often 300-330% for coated sheetfed and 240-280% for uncoated, using GCR rather than hand-flattening shadows.
Imposition and output
- Pages up = (usable sheet length / page length) x (usable sheet width / page width); subtract gripper allowance (10-12 mm) and colour bar before dividing.
- Work-and-turn prints both sides from one plate set and keeps the same gripper edge, so both sides carry the same head; work-and-tumble changes the gripper edge and puts the back head-to-foot.
- PDF/X-1a allows CMYK and spot only, with no live transparency; PDF/X-4 permits transparency and layers but still needs TrimBox, BleedBox (usually 3 mm) and no RGB.
Worked example
A 1200 x 1600 px photograph is requested at 200 x 267 mm. At 300 ppi the file can only fill 1200/300 = 4 in = 101.6 mm across, so placed at 200 mm it would run at roughly 152 ppi, half the requirement. Either recapture at 2400 x 3200 px or reduce the placed size to 101.6 x 135.5 mm, keeping the proportions identical.
Traps that cost marks
- Confusing ppi with dpi: an imagesetter running at 2400 dpi says nothing about the resolution of the image.
- Exporting RGB images or omitting the OutputIntent, so the RIP applies an unknown conversion.
- Assuming a 16pp A4 section needs 16 pages per side of an A1 sheet; it needs 8, giving 16pp for the two sides.
- Adding bleed as a white margin instead of extending image and background past the trim.