Visual acuity

Normal visual acuity

In the sense of undisturbed vision.
It is inaccurate to refer to 6/6 visual acuity as "perfect" vision.
6/6 is the visual acuity needed to discriminate two contours separated by 1 arc minute.
When used as a screening test, subjects that reach this level need no further investigation, even though the average visual acuity with a healthy visual system is typically better.

| 6/6 (meters) | 20/20 (feet) | 1 arcminute gap | 2 arcminutes per line pair | cycles per degree |

Snellen (m/ft)
Cycles per degree
Arcminute gap
PPD
Arcminutes per line pair
Snellen chart
Visual acuity
6/0.6
20/2





Hawk - Veronica Seider
6/0.78
20/2.6

0.13 / 8
450


Vernier acuity limit (visual cortex)
6/1
20/3.3
~150




Maximal photoreceptor resolution (bypassing lenses)
6/2
20/6.6






6/2.4
20/8

0.4
150


Maximal / US Navy fighter pilot / Diameter of a foveal cone
6/2.7
20/9





visual
6/3
20/10
50 - 60

120
1.2
P E Z O L C F T D
acuity
6/3.9
20/13




F D P L T C E O

6/4
20/13.3





Average / Normal vernier acuity
6/4.5
20/15
40
0.6
80

L E F O D P C T
visual / Worse US Navy fighter pilot
6/
20/16
37
0.8

1.6

visual
6/5
20/16.6





acuity
6/6
20/20
18 - 30
1
60
2
D E F P O T E C
Normal visual acuity
6/7.5
20/25




F E L O P Z D

6/9
20/30




E D F C Z P

6/12
20/40
15



P E C F D

6/15
20/50




L P E D

6/18
20/60






6/21
20/70




T O Z

6/24
20/80
7.5 - 10





6/30
20/100
7.5



F P

6/36
20/120
5





6/48
20/160
3.75





6/60
20/200
3



E
Blindness

http://webvision.med.utah.edu/book/part-viii-gabac-receptors/visual-acuity/

They found that the maximum resolution was about 60 cycles per degree

More recent work on photoreceptor density and spatial resolution has shown that the receptor array in the human visual system can resolve in the order of 6/1 (20/3) or ~150 cycles/degree

Based on cone spacing, a maximum of about 60 cycles per degree is possible, which is well above conventional clinical measures as this does not compensate for the optics of the eye and post-receptoral neural processing.

http://www.swift.ac.uk/about/files/vision.pdf
Highest resolution perceivable pixels: 28 seconds of arc
This high resolution, however, applies only to the central 2° of vision

https://www.student.cs.uwaterloo.ca/~cs349/f14/files/07.colour.1up.pdf
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1043188/pdf/brjopthal00220-0003.pdf
http://webvision.med.utah.edu/book/part-viii-gabac-receptors/visual-acuity/
http://biomed.science.ulster.ac.uk/vision/-Snellen-fraction-conversion-.html

https://books.google.fr/books?id=aCHaBwAAQBAJ&pg=PA57
Cycles per degree = 600 / Snellen line number (600/20 = 30). Accurate only for the letter E (L 20/20 => 18 CPD)

https://www.smpte.org/sites/default/files/23-1615-TS7-2-IProc01-McCarthy.pdf
According to Nyquist sampling theory, the maximum theoretically achievable resolving power of the retina would thus be about 60 cycles per degree (1/2 the number of sampling points). It turns out that the eye’s maximum resolving power matches the mathematical limit8 – 60 cycles per degree, at least under ideal conditions.

20/20 vision corresponds to acuity of 30 cycles per degree

https://webcache.googleusercontent.com/search?q=cache:G7uFrehVnU8J:https://www.mcgill.ca/mvr/files/mvr/visual_acuity.ppt+&cd=14&hl=en&ct=clnk&gl=fr
Foveal cones spaced about 0.5 arcmin
Nyquist Limit - 2x the cone spacing => 1 minute = 60 cpd

http://jov.arvojournals.org/article.aspx?articleid=2485412
http://www.faculty.virginia.edu/ASTR5110/lectures/humaneye/humaneye.html
http://www.ilocis.org/documents/chpt11e.htm
http://www.telescope-optics.net/eye.htm
http://www.iloencyclopaedia.org/part-i-47946/sensory-systems/22-11-sensory-systems/vision-and-work


Average visual acuity

Of a healthy, emmetropic eye (or ametropic eye with correction).

| 6/5 - 6/4 | 20/16.6 - 20/13.3 | 0.6 arcminute gap | 2 arcminutes per line pair | ? cycles per degree |

From Foveated 3D Graphics :

A figure of 48 cycles per degree (20/12.5) is a good estimate of average foveal acuity for adults below 50 years of age [Colenbrander 2001].

Maximal visual acuity


Visual Acuity and Reaction Time in Navy Fighter Pilots (1986) :
The average high contrast acuity score was 0.40 minutes of visual angle, or 20/8 Snellen. => 150 pixels/degree

Baseball vision: When 20-20 eyesight just wont cut it (2012) :
The limit of human vision is around 20-8 - meaning a person can see from 20 feet what the average person can see from 8. Laby has never seen anyone in his Massachusetts practice at 20-8, but he sees a few baseball players in spring training who measure at 20-8. => 150 pixels/degree

Sports Vision Testing: An Innovative Approach to Increase Revenues (2006) :
The acuities closely follow a normal distribution with 20/8.9 being the best vision recorded to date. => ~135 pixels/degree

Visual acuity :
0.4 arc minute diameter of a foveal cone => 150 pixels/degree

Fovea centralis :
From these values, one can calculate the average angle of view of a single sensor (cone cell), which is approximately 31.46 arc seconds => 114 pixels/degree
Assuming average focal lengths, this suggests that individuals with both high cone densities and perfect optics may resolve pixels with an angular size of 21.2 arc seconds => 170 pixels/degree

Normal vernier acuity

Measures the ability to align two line segments.

| 6/4 | 0.6 arc minutes |

Limit to vernier acuity

Because the limit of vernier acuity is well below that imposed on regular visual acuity by the "retinal grain" or size of the foveal cones, it is thought to be a process of the visual cortex rather than the retina.

| 8 arc seconds | 0.13 arc minutes |

Veronica Seider


http://sapienplus.com/super-human-number-21-super-vision-veronica-seider/

Veronica Seider has an acuity of around 20/2

She is able to distinguish people from that far away (1 mile) and can also gauge their relative distance from her position.

Diameter of a foveal cone


| 0.4 arc minute |

Stereoscopic acuity

Ability to detect differences in depth with the two eyes.

|Two eyes are better than one: binocular enhancement in the contrast domain - Jeff Rabin (1995)

*"The purpose of this study was to compare the improvement in visual acuity (VA) with improvement in contrast sensitivity (CS) with two eyes as compared with one."*
*"Computer generated letter charts were used to measure VA and small letter CS (20/25 Snellen equivalent) in 13 emmetropic subjects."*
*"VA improved by an average of 10% (2-3 letters)"*Using this
*"Using this approach, binocular enhancement was identified in 8/13 subjects with CS, but in only 4/13 subjects with VA."*

*"Clinical assessment of visual acuity (VA) often is performed under both monocular and binocular viewing conditions, and most clinicians will agree that, in patients with normal vision in each eye, VA is better with two eyes as compared with one. However, the improvement is slight (7-l l %), typically in terms of letters rather than lines on a VA chart."*

*"The 10% improvement in VA is comparable to values reported in previous studies using gratings and letter stimuli, but represents an increase of only 2-3 letters on the acuity chart."*

Complex targets

Similar to normal monocular visual acuity.

0.6–1.0 arc minutes

Much simpler targets

Such as vertical rods.

| 2 arc seconds |

Blindness


| 6/60 | 20/200 |


Arcminute gap

Normal vision (20/20) : 1 arcminute gap

Arcminutes per line pair

2 arcminutes per line pair = 20/20 vision
Spherical aberration limits the resolution of a 7 mm pupil to about 3 arcminutes per line pair
At a pupil diameter of 3 mm, the spherical aberration is greatly reduced, resulting in an improved resolution of approximately 1.7 arcminutes per line pair*

Pupil :
  • 7 mm diameter (spherical aberration) : 3 arcminutes per line pair
  • 3 mm diameter (spherical aberration greatly reduced) : 1.7 arcminutes per line pair

2 arcminutes per line pair = 1 arcminute gap

Maximum angular resolution of the human eye : 1.2–1.8 arc minutes per line pair (=0.6–0.9 arc minutes)
Normal vision (20/20) : 2 arcminutes per line pair


Cycles per degree (CPD)

https://petavoxel.wordpress.com/2010/02/26/cycles-per-degree/

Maximum theoretical resolution : 50 CPD (= 1.2 arcminute per line pair)

https://en.wikipedia.org/wiki/Eye

Cycles per degree / FOV


Central island : 60 CPD
Foveola : 35-60 CPD
Fovea : 25-35 CPD
Parafovea
Perifovea
Periphery (includes blind spot)

Eccentricity


From The Nervous System In Action - Chapter 7 (1978 reference) :

external image blndspot.gif

Same reference on Wikipedia - Fovea centralis

From Ocular Examination (2015) :

external image B9780323036566500068_f01-02-97803230365661.jpg

From Visual Acuity by Michael Kalloniatis and Charles Luu :

external image KallSpat19.jpg

external image KallSpat20.jpg

[[iovs.arvojournals.org/pdfaccess.ashx?url=/data/journals/iovs/933063/528.pdf|Optical and neural resolution in peripheral vision]] - Lars Frisen and Anders Glansholm (1975)