When your iris changes in size from light why does does the area we can see not change?
In: Biology
Watch how a mirror camera / DSLR Apeture works. Samme princip.
Light enters one hole of a size, and the retina/mirror stays same size.
To highjack this, my iris always (for the most time) seems to be wide open no matter the lighting. Can anyone tell me why that is?
Short answer: because light travels through the center of the lens and focuses it onto a small point. You can cover up the edges of a magnifying glass but still see the same image magnified.
Closing your iris is analogous to this. The center of the lens still has the same field of view, thusly it focuses that field of view into the retina (and we perceive it as such).
The area you see depends on how far the iris is from the retina in the back of an eye. it’s the same in a camera except they are using special lens elements that distort distances to get the same effect. Two thing do change when the iris changes size: (1) the amount of light that comes in, and (2) how fuzzy the out of focus stuff is. The reason eyes have an iris is for the amount of light. That’s how you can see in dark places and bright places. You won’t notice out of focus fuzziness much because even wide open at night, your iris is still quite small.
I’ve been studying photography so I’m not an expert but I can share what I have found.
The iris affects two things, how much light is collected per unit time and how many angles of approach light has between the subject and the sensor (or film). When you open the iris wider you take in more light per second (or fraction of a second). You also allow more valid lines a ray of light has between what your camera is pointed at and the sensor. This is why the amount of distance which appears in focus shrinks, objects closer to or farther from the focal point have more lines they can take through that circle and still strike the sensor. A tight iris let’s in less light but only has a few lines it can take through the circle and strike the sensor, which increases the distance where things are in focus.
What affects how big the field of view is is something called the focal length. This answer is already pretty long so you can look into that if you like. HTH.
The iris is not placed where it can control what part of the image is visible. It simply acts as a gate, letting less or more light through as needed. Your brain also adjusts how sensitive your rod and cone cells are to light, so the scene might not look brighter or darker even though the amount of light coming in has changed.
When the iris is opened, each rod or cone cell is able to either take more light from the same spot (wherever the image is in focus), or from a larger area around the spot (out of focus). When the iris is closed, the rods and cells gather less light from a smaller area. This is the same reason why adjusting the aperture on a lens will not only change how much light is captured, but also the depth of field (the region where objects are in focus).
Although each individual cell can gather more light, it still reports only one brightness value (and also color if it is a cone). So all of the light that each cell receives is averaged out (especially where the image is out of focus).
I think it’s focusing so fast. Side note, I can control the iris size it’s a weird useless skill
Because the light hitting your eye is coming [from all directions](http://www.passmyexams.co.uk/GCSE/physics/images/eye_xsection_01.jpg). If you were looking at a source of light waves which are all traveling towards your eye at the same straight angle, changing your pupil size would change the size of what you see. That’s not the way normal light works though. It’s generally bouncing everywhere all the time.
Every part of a lens makes every part of the image. This is how [pinhole cameras work](http://questgarden.com/127/71/9/110611112530/images/pinholeprinciple.jpg). Larger lenses simply mean more light gathering. This is why if you mask out half the lens on you camera, the entire image is still seen—just darker. (If you test this, make sure you are masking out your lens on the lens surface, not on optical glass *in front of the primary element*.)
To my point, [watch this guy murder a lens](https://youtu.be/YcZkCnPs45s) with a crab claw, and the lens still works perfectly. Albeit probably just a few % points dimmer.
The size of the pupil does not change the size of the image because the eye captures an inverted image (as if it had a single tiny hole). What is high appears at the bottom of your eye, what is left on the right side, and so on. Contracting the pupil only makes the image darker.
It behaves like a camera, or a shoebox with a tiny hole. This is enough to make a complete image. It will be inverted, since it has to go through a single point. And it will be pretty dim.
To make it brighter you need a bigger hole which will make the image fuzzy. Putting a lens over the hole removes the fuzziness and focus the image.
The front part of the eye is a lens which focuses the image on the back of the eye (the retina).
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