Convert Lines per millimetre to Pixels per inch

Convert Lines per millimetre (lp/mm) to Pixels per inch (PPI) instantly and accurately.

Conversion Formula

PPI = lp/mm × 25.4

About Lines per millimetre

The line pair per millimetre (lp/mm) is the fundamental unit of spatial resolution in classical optics, characterising the resolving power of a lens, film emulsion, sensor, or microscope objective. A resolving power of N lp/mm means adjacent line pairs separated by 1/(2N) mm can be distinguished - derived from the Abbe diffraction limit (Ernst Abbe, 1873) and Rayleigh criterion (Lord Rayleigh, 1879). Film benchmarks at MTF50: Kodak Tri-X 400: 63 lp/mm; Fujifilm Velvia 50: 160 lp/mm; Kodak Technical Pan: 320 lp/mm. Lens benchmarks: Leica APO-Summicron-M 50 mm f/2 ASPH: ≈ 75 lp/mm at centre. CMOS sensor Nyquist frequency: for pixel pitch p mm, f_Nyquist = 1/(2p) lp/mm - a 4.63 µm pixel limits at ≈ 108 lp/mm. As a dimensional unit, 1 lp/mm = 25.4 LPI = 25.4 PPI exactly. 1 lp/mm = 25.4 PPI.

About Pixels per inch

The pixel per inch (PPI) is the universal standard for digital image resolution, defining how many pixels occupy one linear inch in a digital image, sensor, or display. Formally referenced in ISO 12232 (digital still cameras) and EXIF/TIFF (tags 0x011A/0x011B), PPI is both a capture parameter (scanner/sensor density) and a display characteristic. Display milestones: Macintosh 128K (1984): 72 PPI; Apple iPhone 4 Retina (2010): 326 PPI - above the human perceptual threshold at 25 cm; iPhone 15 Pro (2023): 460 PPI. Print benchmarks: 150 PPI (large-format, 1 m viewing); 300 PPI (standard offset, ISO 12647-2); 400-600 PPI (fine-art Giclée). EXIF stores PPI as unsigned RATIONAL; the historical default is 72/1, a carry-over from the original Macintosh screen standard. 1 PPI = 1/0.0254 px/m ≈ 39.37 px/m = 2.54 px/cm.

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