Visual acuity ... need help!

[NobleKnight]

 

I find difficulty in understanding what the text want to say . i mean to discriminate two sources of light at a length of 10 meters we need them to be far from each other by 1.5 to 2 millimeters ... okay what if we want to see at further distance for example 1000 m or more ... i think the distance between the two sources shall differ .. I want to know what formula determines this

 

 and the second part of the text above is not really understandable for me ... i mean saying the fovea is less than 0.5 mm and this means that visual acuity is less than 2 degrees of the field ... i don't understand how ... so please if you can clarify it please do it or lead me to better source so i can read it

[DanielMB]

NobleKnight,

Hi, 

Rods are photoreceptor cells sensitive to low luminance levels (scotopic vision and mesopic -transition zone-)

Cones are photoreceptor cells sensitive to medium and high luminance levels (photopic vision and mesopic)

Rods and Cones consitute the first layer of photoreceptors in the retina that interact with light entered the eye

Cones are concentrated mainly in the fovea region (0°-10°) (look at the first attached image, where you can see the rods and cones distribution)

Rods are concentrated almost exclusively in the extra-fovea peripheral region (>10°)

There are three types of cones in the HVS, denoted S, M, L, each group is characterized by a proper responsivity curve to light wavelength, Short, Medium and Long wavelength respectively

SML are frequently related to specific color stimulus, but not in rigorously way

S is related to a Blue stimulus, M to a Green, and L to Red

The population distribution of Cones is approximately S:M:L = 12:6:1

Cones are responsible for our trichromatic vision in the photopic and mesopic regions

2º in the fovea region means to take as first aproximation the fovea as point from a distant object covering those 2º, look at the third image, this picture consideres the fovea as a point and the object following perspective laws, the law you are asking for is just:

S = 0.140 x   being x the distance from the object to the fovea and S the object "size" (height)

 

You have to take care with the scope of the perspective laws I've written, just because there are other phenomena appearing when distances are not short distances (i.e. 10 mts or less), visibility has impact on our HSV (human system vision) affecting our criteria, using a substractive filter model, you can take it into account through the Beer-Lambert law, where image intensity decreases in an exponential way with distance, but this is other question ...

Edited October 1, 2018 by DanielMB
just adding a comment

[swansont]

On 9/10/2018 at 11:35 AM, NobleKnight said:

I find difficulty in understanding what the text want to say . i mean to discriminate two sources of light at a length of 10 meters we need them to be far from each other by 1.5 to 2 millimeters ... okay what if we want to see at further distance for example 1000 m or more ... i think the distance between the two sources shall differ .. I want to know what formula determines this

Arc length (which will be approximately the separation distance) = r * theta  where theta is measured in radians, and r is the radius of the circle, i.e. the distance to the objects

So the distance between points varies linearly with separation distance for something of a constant apparent size.

So 2 mm at 10 meters means 2 cm at 100m, 20 cm at 1 km, and so on.

[DanielMB]

For 25 seconds of arc (app. 0.41º) the separability should be expressed as  

S(x) = 0.0292 x

[NobleKnight]

Thank You So Much .... I really appreciate your help Both of you ... Daniel and Swansont

[mistermack]

Just by way of clarification, when you look at something, such as this text, and focus on it, the part of the image that you are focusing on falls on the fovea. It's much richer in receptors. The rest of the image falls on lower concentrations of receptors, giving more peripheral vision.

So no matter where you are looking, up down or side to side, the light from the bit you are focusing on is still falling on the fovea.

If I look at this text, the limit is about one word in reasonably good focus. Without moving your eyes, if you look at one word, the words alongside it are a lot less clear. You read by moving your focus from word to word. In fact, if you look at the w in word, the rest of the word is not clear, unless you "zoom out" to take in the whole word. 

If all of what's in front of you is 180 degrees left to right, then the wo in word approximates to 1 or 2 degrees, and that's the sort of area that has maximum visual acuity.