Resolution, DPI, and color spaces — the three levers of image quality
7 min read
Most image-quality trouble comes from conflating three ideas: pixel count, print density (DPI), and color space. Separate them and most requirements decode themselves.
Pixels — the total information an image has
A digital image is a grid of colored dots. A 3000×4000 photo is twelve million dots, and that is all the information it will ever have. Enlarging doesn't add pixels — which is why blowing up a small image gives you blur.
Shrinking, on the other hand, just discards information, and you can always do it. Hence the rule: archive the large version, derive smaller ones per use.
DPI — how densely you lay that information on paper
DPI (dots per inch) is not a property of the image but of its printing. The same 3000-pixel-wide image printed 10 inches wide is 300dpi; printed 30 inches wide it's 100dpi. Same image, different DPI, depending on use.
“Send it at 300dpi” really means “send an image large enough that, at the size it will print, there are 300 pixels per inch”. For a full A4 page that works out to roughly 2480×3508 pixels.
Working values by purpose
The values that circulate in practice:
- Screen/web display: DPI irrelevant; only pixels matter (1–2× the display area is plenty)
- Ordinary document printing and scanning: 300dpi — the standard for document work
- Scans where small print must be magnified: 400–600dpi
- Commercial print (flyers, booklets): 300–350dpi, the print shop's spec wins
- Large banners: viewed from a distance, 100–150dpi is enough
Color spaces — why RGB and CMYK disagree
Screens mix light (RGB); presses mix ink (CMYK). The two produce different ranges of color, and the screen's neon brights and vivid blues often have no ink equivalent. That gap is the root cause of “the print doesn't match my monitor”.
For ordinary document work, RGB is fine. CMYK conversion matters only for commercial printing, where the print shop supplies the spec. What matters is knowing the gap exists — and requesting a printed proof when color accuracy genuinely matters.
Compression quality — the fourth variable
Identical pixels, DPI, and color space can still produce very different files depending on JPEG quality. JPEG is lossy — every save discards information — and the artifacts (smudges around sharp edges) show most on text.
Repeatedly editing and re-saving a document photo accumulates that damage. Work in a lossless format (PNG) during editing and export to JPEG once, at the end.
How this plays out inside a PDF
An image placed in a PDF keeps its original pixel data, with its display size on the page recorded separately. What a PDF compressor does, then, is find images with far more pixels than their display size needs, downsample, and re-encode them.
That's why scans compress so well — and why text can smear afterwards. Compression discards information; verifying that the smallest text is still readable is the non-negotiable last step.