Prepress and Image Preparation
Prepress and Image Preparation is the bridge between a design file and a printable image carrier.
What the exam tests
Questions cover resolution, colour conversion, file preparation, imposition and proofing.
Core rules
- Raster images are pixel grids; vector art is mathematical paths. Logos and type should remain vector until output.
- Effective resolution = pixel dimensions ÷ final physical size. A 1200 × 1800 px image placed at 4 × 6 inches is 300 ppi; at 8 × 12 inches it is 150 ppi.
- CMYK is a subtractive process-ink model; RGB is additive screen light. Convert to CMYK with an ICC profile and soft-proof before output. Spot colours are separate plates, not CMYK builds.
- Bit depth: 1-bit line art, 8-bit per channel gives 256 levels; 16-bit holds more editing headroom.
- Bleed is extra image beyond the trim, commonly 3 mm; trim marks and registration marks sit outside the trim.
- Imposition arranges pages on a large sheet so that folding produces the correct sequence. Trapping slightly overlaps adjacent colours to hide register variation.
- CTP exposes plates directly from digital data, removing film and reducing dot gain.
Worked example
A customer supplies a 600 × 900 px photo for a 6 × 9 inch poster at 300 ppi. Required pixels = 6 × 300 = 1800 wide and 9 × 300 = 2700 deep, so the file is one-third of the needed resolution in each direction. At 300 ppi it would print only 2 × 3 inches. Either obtain a larger original or reduce the placed size; interpolation adds pixels but not real detail.
Traps
Confusing ppi with dpi; judging resolution on screen at 100% zoom; forgetting that scaling up lowers effective ppi; sending RGB files to a CMYK press without proofing; setting bleed after trimming.