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"Normal" vision is a specific definition, and a clinical definition:
Snellen acuity of 20/20 or lower (20/16 is "perfect" acuity, 20/200 is SSA disabled)
Peli-Robson contrast sensitivity of 1.95 or higher (2.25 is "best")
Farnsworth Munsell Hue Color TES of 60 or less (TES 0 is perfect)
100% Visual field with MD no lower than -2db (0 is best, -20db is SSA disabled)
The age-related baseline normal is ages 20 thru 40.
Below 20, contrast sensitivity is still developing, so young normal includes a lower contrast sensitivity.
Above 40, presbyopia is a normal development, so mature normal includes a lack of near-distance acuity.**
Depending on the study, "normal" often includes "with refractive correction" if the correction can acheive the above scores. An example: "normal with correction needed for presbyopia". Presbyopia being "normal" for over age 40 for instance.
"Normal" is not "most perfect"
Because normal vision is specifically defined, it also provides a useful baseline relating to user needs. Normal vision has natural limitations that define minimum requirements for readability. From this foundation of normal vision user needs, we can then indicate the degree of additional accommodation needed for varying degrees of impairment(s).
Thank you,
Andy
Footnote
** Presbyopia is not due to disease or degeneration, but due to the enlarging of the eye's lens to a point that prevents near focusing due to the reduced available distance for the ciliary muscles to work and pull the flexible lens into focus. Because it is cartilage, the lens grows throughout our lifetime (as do the ears and nose) increasing size and reducing the flexibility, resulting in presbyopia by the early 40s.
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Reposting this here for reference.
"Normal" vision is a specific definition, and a clinical definition:
Depending on the study, "normal" often includes "with refractive correction" if the correction can acheive the above scores. An example: "normal with correction needed for presbyopia". Presbyopia being "normal" for over age 40 for instance.
"Normal" is not "most perfect"
Because normal vision is specifically defined, it also provides a useful baseline relating to user needs. Normal vision has natural limitations that define minimum requirements for readability. From this foundation of normal vision user needs, we can then indicate the degree of additional accommodation needed for varying degrees of impairment(s).
Thank you,
Andy
Footnote
** Presbyopia is not due to disease or degeneration, but due to the enlarging of the eye's lens to a point that prevents near focusing due to the reduced available distance for the ciliary muscles to work and pull the flexible lens into focus. Because it is cartilage, the lens grows throughout our lifetime (as do the ears and nose) increasing size and reducing the flexibility, resulting in presbyopia by the early 40s.
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