When your iris changes in size from light why does does the area we can see not change?
In: Biology
The iris enlarges in dark conditions to increase light flow into the eye and o to the retina. We can’t see well in dim or dark light, but by adapting, we can adjust.
Also, you put “does” twice.
the light coming in from the very side of your vision when your pupil is small can still make it onto the back of your eye (the retina) so you don’t notice the change. Also, there are fewer light sensitive cells way out on the sides of the retina so you wouldn’t notice subtle changes like you would in the middle of your vision.
The size is mostly about focusing not picture size, so it’s like wearing glasses. You are always seeing generally the same image, but the quality and focus of that image is what is shifting
Let’s call them points. Every tiny point on the area of your iris all projects the same full image on your retina. The more points, the more of these full images overlap, making it brighter.
Your eye only sees a tiny dot clearly at any one time. It moves around quickly to get a bigger view and your brain stitches it all back together for you.
The iris is a lens not a simple hole. A window is like a hole, you see a smaller image in a smaller window. A lens is different because it bends light and direct the image on a surface on the other side. A good lens will bring the light of each point on one side to a different point on the other side. Between each on these pair of points, the light can take many path as long as it goes through the lens. The field of view of your eyes depends on the size of the retina behind the iris and doesn’t change. The size of the iris only changes how much light the lens gets to bend into the eye.
The diameter of your pupil cannot shrink (as far as I am aware) such that you can’t see. As it gets larger the amount of light that enters your eye increases. If you were to artificially dilate your pupil, for instance if you went to an eye doctor and they used drops to dilate your pupils, you would findthat normally comfortable light conditions, like daylight, would be painful due to the abnormal amount of light striking the back of your eye. You might also gain a small amount of blurry and not useful peripheral vision.
Interestingly, if you close one eye in any very bright situation, you can tolerate brighter light than if you kept both eyes open. Just thought I would add that, for no particularly good reason.
This is analogous to the aperture of a camera lens by the way.
It’s because the retina — where light is processed into signals for the brain — it’s kind of like a movie screen — is not immediately adjacent to the iris.
If the retina were adjacent to the iris, then yes, a smaller iris aperture would mean a smaller field of vision.
But light hits the retina (the curved movie screen) pretty much all over whether the iris aperture is tiny or large, because the retina is a centimeter or so away from the aperture.
For the same reason a pinhole camera works.
The light entering your eye all traverses a small hole – your pupil – that gets even smaller when your pupil constricts. It then diverges in a cone and hits the retina on the back of your eye, forming an image. (The image is inverted, by the way.) The size of the pupil controls the brightness of the image on your retina, not how big the image is on your retina.
What matters for the area we see is the angle of the light rather than the size of our pupil. I made a quick photoshop diagram [showing the path light would take if you were looking at a red dot right in the corner of your vision](https://i.imgur.com/nFAqd0I.png) for different pupil sizes.
It doesn’t matter whether your pupil is big or small, the light still has the same path to the same part of your retina.
There is a bit more to it as well, as the lens in your eye is also bundling up all the light that hits it so not even the edge of your vision would get cut off by your pupil because any light coming from that angle that hits any part of your lens gets bent through the lens and spat back out going in the correct directly.
It’s all just like how a [camera lens aperture can change from massive to tiny](https://houseofroseblog.com/wp-content/uploads/2013/04/aperture-post-2.jpg) and still capture a complete image.
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