Choose resolution from character size and source condition
A DPI setting is useful only in relation to the physical size of the original type and the scanner's real optical detail.
You should leave with a defensible starting resolution, a small representative test, and a clear reason to stop increasing DPI when no additional character detail appears.
Apply the decision with Image to Text. Then test the tradeoff against Color, Grayscale or Bitonal: Which Scan Mode Helps OCR? and How to Photograph a Document for Accurate OCR.
Resolution starting points
| Source | Test range | Review focus |
|---|---|---|
| Clean ordinary print | 300 ppi | Punctuation and small type |
| Faded or stained text | 300 to 400 ppi grayscale | Weak strokes and background |
| Small or fine detail | 400 to 600 ppi test | Real detail versus larger files |
| Software-upscaled image | Original effective resolution | Do not count invented pixels |
Decide whether higher resolution captures new evidence
Scan a representative region at the intended starting resolution and inspect the smallest meaningful characters at actual pixels. Look for closed counters, separated punctuation, clear accents, and distinct narrow strokes. Then repeat at the next real optical setting without changing contrast or sharpening.
Keep the higher setting only if it reveals detail and reduces transcription errors. A larger file that contains the same softened stroke shapes adds storage and processing time without improving recognition. Software enlargement should not be described as a higher-quality scan because it creates more pixels from the same captured evidence.
- Judge the smallest important type
- Separate optical capture from interpolation
- Test damaged or faded pages as their own source class
Test whether more real detail changes the result
Use 300 dpi as a test point
Ordinary printed text often has enough character detail at a genuine 300 dpi scan. Test small type and degraded originals rather than applying the number as a guarantee.
Raise resolution deliberately
Use 400 or 600 dpi for small print, weak ink, historical type, or characters with fine marks. Higher settings do not fix motion blur or an out-of-focus camera photo.
Avoid false precision
A file tagged 600 dpi after software enlargement contains no more source detail than the original. Inspect pixel dimensions and character height.
Resolution decision checklist
- Measure against original size
- Use optical rather than interpolated resolution
- Test the smallest important type
- Keep color mode appropriate to the source
- Compare errors before scaling an entire batch
Why one DPI number is not a guarantee
Nominal DPI does not prove optical detail. Original character size, scanner focus, interpolation, paper condition, and compression can matter more than the number stored in file metadata.
What to verify beyond the references
The sources provide digitization and imaging guidance. Confirm the advice with native pixel dimensions and a recognition test on the smallest important type in your collection.