Read the clues before you define the capture.
Preserve the control clue already present.
Check how one record begins and ends.
Use representative material when setting review criteria.
Two common widths. Many recording layouts.
Often document-oriented collections
Often larger originals or drawings
PPI samples pixels; DPI describes printer dots
Pixels per inch, or PPI, is the accurate technical term for raster-image sampling density. Dots per inch, or DPI, properly describes printer dots, although scanner interfaces often use DPI as a conventional label. FADGI uses PPI because a raster image is made of pixels.
The next question is “PPI relative to what?” A setting can describe sampling at the film plane, which is different from the dimensions of the historical paper document represented on the film. State the reference plane rather than assuming that a number is self-explanatory.
Film-plane sampling and original scale differ
FADGI explains that microfilm is digitized at a known, fixed PPI relative to the microfilm itself regardless of original reduction ratio. If the historical reduction is known, it can be recorded in image metadata and used with imaging resolution to help establish original-document size. If neither a ratio nor scale is available, that relationship may not survive digitization.
A statement such as “600 PPI” therefore needs a reference plane. It may mean 600 PPI measured on the film, not 600 PPI at the source document’s original size after enlargement. This distinction is especially relevant when comparing different reductions or explaining why a file’s apparent document dimensions vary.
| Term | Plain-language meaning | What it does not prove |
|---|---|---|
| PPI | How densely pixels sample the item at a stated plane | That all source detail is usable |
| DPI | Printer-dot density; commonly used on scanner menus | A complete capture specification |
| Reduction ratio | Original document size relative to film image | A digital scan setting |
| Effective sampling | How much fine detail/contrast the system conveys | That pixels can recover missing film detail |
More pixels are not the same as more information
Effective detail depends on the source film, focus, density, scanner or camera spatial-frequency response, and processing. FADGI uses spatial-frequency response to explain how actual rendition of fine detail differs from a nominal sampling number. Sharpening can make edges appear crisper without restoring information absent from the film, so it should not be confused with recovered detail.
Older or poorly filmed material may not yield what would normally be called reproduction-quality files. The practical goal may be reasonable legibility for access, with capture choices documented. Standard targets and representative samples can make the discussion more evidence-based than a universal resolution promise.
Ask resolution questions that lead to a specification
Ask what reference plane is being used, whether reduction is known, what fine detail users must read, whether targets exist, how polarity and density vary, and which output role is required. Specify whether the request is for a full-frame image, access derivative, OCR support, or a package for a repository. Each outcome can require a different balance of file size, usability, and metadata.
Fictional example — no PII: “The project needs users to read small account headers on a 16 mm roll; reduction is not marked; some frames are faint.” The next step is a representative review, not a claim that one PPI setting will resolve every frame equally.
Common questions
Is DPI wrong when discussing scanning?
PPI is more technically precise for raster-image sampling, though scanner interfaces often label their settings DPI.
Will a higher setting make faint film readable?
Not necessarily. Sampling cannot restore information lost through poor focus, density, damage, or the original filming process.
Does a resolution setting guarantee OCR results?
No. OCR also depends on text size, layout, contrast, fonts, source condition, and processing decisions.
Sources & further reading
These references explain technical formats, standards or destination requirements. They do not imply endorsement of National Microfilm.