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M-unit (print size)

Also called M unit, M notation

The M-unit specifies print size by the distance, in meters, at which the print matches a 20/20 optotype in angular size.1 By definition, the detail of 1M print subtends 1 minute of arc at 1 m, and the whole letter subtends 5 minutes of arc at the same distance. The detail of 2M print subtends 2 minutes of arc at 1 m.2

Acuity in M notation is a true Snellen fraction, with the test distance in meters over the M size read. A patient who reads 4M at 40 cm is recorded as .40/4M, which equals 20/200.1 The M system is preferred for near acuity in visually impaired patients, because the fraction compares easily with distance acuity and simplifies the calculation of reading additions.1

More detail

The M size a patient reads sets the magnification needed for a task. A patient who reads 4M at 20 cm needs 4x angular magnification to read a 1M label at that distance.2 A +20 D lens supplies that magnification when the label sits at the lens's focal point, 5 cm away, and the patient looks through it with the distance correction and no accommodation.2

Sources (12)
  1. AOA Optometric Clinical Practice Guideline, Care of the Patient with Visual Impairment (Low Vision Rehabilitation), sec. II.A
  2. Schwartz, Geometrical and Visual Optics, ch. 13
  3. Colenbrander, A. (2002). Dimensions of visual performance. Transactions: Low Vision Rehabilitation, 2, pp.1-33.
  4. Ferris III, F.L., Kassov, K., Bresnick, G.H. and Bailey, I. (1982). New visual acuity charts for clinical research. American journal of ophthalmology, 94(1), pp.91-96.
  5. Schwartz, S.H. (2013). Geometrical and Visual Optics, 2nd Edition. New York: McGraw-Hill Professional Publishing.
  6. Visual Acuity - Webvision secondary
  7. New visual acuity charts for clinical research
  8. Psychophysics of reading. Clinical predictors of low-vision reading speed
  9. Dimensions of visual performance secondary
  10. Standardized ETDRS charts for estimating visual acuity at four viewing distances
  11. Standardized ETDRS charts for estimating visual acuity at four viewing distances (www.sciencedirect.com/science/article/abs/pii/S0042698916300263)
  12. New visual acuity charts for clinical research (www.aaojournal.org/article/S0161-6420(82)34825-5/fulltext)

Reviewed October 2026

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