Decerning light

[Graduate]

i read in one article that human eye can decern burning candle 14 miles away in right condition.
First is this possible? second what can be VA of this person?

[rbaker]

Probably true. With peripheral vision you can discern the light but you can not discern whether it is a candle or a tracer or a SAM. Only when it get closer and you fixate on the object with your central visual field does your visual acuity come into play.

Perhaps there is a military chancre mechanic (flight surgeon) out there who can shed some further light on this issue. Shed some light . . . heh! Cripes, I amuse myself.

[npdr]

i read in one article that human eye can decern burning candle 14 miles away in right condition.
First is this possible? second what can be VA of this person?

Dear Graduate - The limit to human visual acuity is the anatomical arrangement and organization of the retinal elements. This has been determined to be about 20/10. In the candle example you mentioned, this isn't a form of border determination of a contrast target. Rather the flicker or the movement of the flame can be detected much easier than line acuity.

[Dave Nelson]

I once had a guy ask me what is the farthest the human eye can see? I told him M-31 is a galaxy 20,000,000 light years away and is visible to the naked eye. So I guess the correct answer is 20,000,000 years into the past? (just for scale, M-31 is one of what astronomers call the "local group" of galaxies.)

[Graduate]

so scotopic vision has far better acuity than photopic vision,can we say that?

[Ian Jordan]

scotopic waterfalling effects are well documented with very low energy inputs.
This is important in night driving and one of the causes of headlight difficulties. Although difficult to address it is possible with sufficent knowledge and headlight glare can be stopped. Specialist instrumentation needs to be used and this is generally not available in practices.
Magnocelluar effects can also play a part in this problem.

[William Stacy O.D.]

so scotopic vision has far better acuity than photopic vision,can we say that?

No. Scotopic (night) vision only uses rods.

Photopic uses both rods and cones.

Your macula only has cones in it, which is why you need to look off to one side of a faint star to see it. The peripheral retina has few cones, which is why when you are looking <HERE> you can't read your quote above.

Rods are great for detecting faint light and movement, terrible for shape definition (acuity).

Cones are great for fine acuity and color vision, terrible for detecting faint light.

[Darryl Meister]

More importantly, the larger receptive fields associated with rods have better "temporal" and "spatial" summation than the fields associated with cones. They are able to capture more light photons over a longer period of time, which is why they are more sensitive to detecting light, but this also makes them less sensitive to resolution (i.e., poorer visual acuity).