The iris of the eye cannot control the amount of light?

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The iris of the eye cannot control the amount of light?

Postby infint on March 28th, 2017, 2:50 pm 

I believe that these statements are true -
"When the pupil dilates, more light enters the eye."
"When the pupil constricts, less light enters the eye."
"In bright light conditions, the pupil constricts a little."
"In dim light conditions, the pupil dilates a little."

and that this statement is demonstrably and logically false
"The iris/pupil controls/regulates the amount of light entering the eye."

When I search the internet, I find variations of that last statement everywhere and various doctors and opticians also seem to agree and yet, I can prove to my own satisfaction that it is false. It does depend on what you mean by the words "control" and "regulate".

Before I describe the experiments that "prove" my case, I would like to know if anyone else already agrees with me.
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Re: The iris of the eye cannot control the amount of light?

Postby someguy1 on March 28th, 2017, 3:12 pm 

The principle certainly holds for photography, where you control the amount of light hitting your digital sensor by "stopping down," reducing the size of the lens aperture. It's certainly true that the pupil of the eye works the same way. I know more about photography than physiology, so if you have info to the contrary I'd be interested. Perhaps you have specific meanings for "regulate" and "control," which you took the trouble to put in quotes. Of course there's nobody twirling a dial to adjust your eye's aperture, but the optical principle is surely the same. Smaller hole, less stuff gets in.
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Re: The iris of the eye cannot control the amount of light?

Postby infint on March 28th, 2017, 5:27 pm 

A good quality 35mm camera can probably adjust from f2 to f16 which would correspond to a change of light intensity of 64:1. If the pupil of the eye has a maximum diameter of 9mm, then the minimum would need to be about 1.1mm for the same result. The problem is that the camera has two more adjustments for light levels - the shutter speed and the film speed, so even for the camera the iris aperture is insufficient to manage all the expected variations in light levels.

According to this Wikipedia page
https://en.wikipedia.org/wiki/Adaptation_(eye) the human eye can handle a variation in light levels of 1,000,000,000:1. That would imply that the narrowest pupil would be about 0.0003mm. I have only looked closely at my own eyes, but my pupils do not have that range of adjustment. I find that if I stand in front of a mirror in a darkened room and use a small torch to adjust light intensity, my pupils vary in size by no more than about 2:1. Other Wikipedia pages say that the pupil dilates or constricts in response to pain, drugs, disease, interest and imagination.

I say that the main way that the eye adjusts for different light levels is in the individual light sensitive cells in the retina. If you use a digital camera to photograph something like a working wall light, I think you will find it impossible to obtain an image which looks the same as what you see with your eyes. The camera adjusts for light level but it can only adjust the entire image and not small parts of it.
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Re: The iris of the eye cannot control the amount of light?

Postby someguy1 on March 28th, 2017, 6:27 pm 

I agree with your points. If the pupil is the only mechanism for light control, it doesn't have enough variability in diameter to account for the vast dynamic range of human perception. Is that a fair paraphrase?

You mentioned retinal sensitivity, which sounds perfectly sensible. I have no knowledge of that physiology but it sounds fascinating. Does the brain tell the retinal cells to dial back their sensitivity, or perhaps send a less intense signal to the brain? I know nothing of these things.

The other really amazing factor, and one that I believe leads us right into philosophy, is that the mind changes the subjective perceptions of a given input signal. The way you see something for the first time, as opposed to seeing the same thing for the thousandth time. At some point you "stop seeing it" because it's so familiar. When you move to a new place you see it for about two or three months. After that you no longer have "new eyes."

I'm using this somewhat subjective and psychological example to illustrate that the mind alters the raw input in many different ways. What those mechanisms are I don't know. I said mind but I'm perfectly willing to stipulate "brain." Even so, there is a subjective psychological component to our vision that has much to do with perception. And light levels are part of that too I'm sure.

But still ... the pupil is "a" means of regulation, just not the only one, right? And of course we'd all agree that the camera is nothing compared to the mind/brain/eye system of a living thing, like a human or a hawk.
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Re: The iris of the eye cannot control the amount of light?

Postby infint on March 28th, 2017, 11:27 pm 

I must apologise for my mistake. My problem lies in my interpretation of the words "control" and "regulate". When I read statements like "The pupil controls the amount of light" coupled with other statements like "The pupil dilates in dim light", I misunderstood them as implying that the pupil is the eye's main mechanism for adapting to varying light levels. On more careful reading, they did not say that. Actually all they were saying was "A bigger hole lets in more light" and I have no quarrel with that.
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Re: The iris of the eye cannot control the amount of light?

Postby neuro on April 14th, 2017, 10:58 am 

The main mechanism in adjusting eye sensitivity to light is the burning of the photosensitive pigment (retinal) which occurs in bright light and reduces sensitivity, counteracted by the re-synthesis of the same pigment, which prevails on "burning" in dim light (or in the dark).
However, the latter mechanism is quite slow (you need a few minutes to adapt to dark), and the "burning" itself requires a few seconds (during which you are flashed, just after moving from dim to bright light).

These mechanisms, however, do not affect the amount of light that enters the eye. This is a chore for the iris...
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