Imagine if you will..a world where humans had large black pupils that covered most of their eye like the picture below
The question:
What would these humans see and could they see more/better/worse than us?
Would their vision be obstructed when looking around because it might block part of their pupil?
What are the social implications of these humans having black eyes?
A pupil that big allows very large amounts of light to enter the eye. In extremely low light conditions, this is a great adaptation since it permits the maximum amount of light to reach the retina. However, in any other lighting conditions, this is a huge disadvantage because it prevents the eye from managing how much light it takes in. They will need to continually squint in bright light to keep themselves from being blinded. Imagine permanent image burn on the back of your eyes; this is their view at practically all times in conditions humans would consider normal.
Any kind of sudden bright light will cause them considerable difficulty.
Obscured Pupil
For the portions of the pupil behind behind an eyelid, the person won't even notice they can't see there, similar to how normal people don't notice that they can see their own noses. It's just how they see things and won't notice a difference.
Social
Socially, if everyone has black eyes like this, it's normal. Just like people with blonde hair in a nation of blonde hair don't notice anything different about each other. Interactions between the black-pupiled people and other peoples will be strained at first but probably work out, eventually.
Alternative Structure
Instead of having a monstrously large pupil, the sclera and iris could also be black with a normal sized pupil. Building an eye this way maintains a normal human's adaptability to different lighting conditions but also gives the desired "black eye" the OP is looking for.
This can be achieved by introducing significant amounts of melanin in the iris and sclera. Brown eyed humans already have this, just bump up the amount of melanin.
A huge opening with suitable lens to match would be a much larger aperture, providing for lower-light vision and poorer depth of field. However, with no way to adjust the aperture and no other changes, it would not handle differing light intensity.
The implication is that (a) there is no bright light and you always want the highest exposure possible; or (b) there is some different mechanism to deal with different light intensity.
Some ideas for that include:
Adding the link for ND filters I noticed that it mentions
These are used on catadioptric telescopes mentioned above and in any system that is required to work at 100% of its aperture (usually because the system is required to work at its maximal angular resolution).
So, there are reasons why an instrument would need to work at a large aperture. This implies that there might be real advantages for why an eye is built this way. Something to keep in mind for alien eyes!
Finally, I’d like to point out that the compound eyes of flys etc. seem to do just fine without an aperture adjustment, which would need to be employed on each lens individually! They must use some other means to cope, right?
Others suggest that humans with such large pupils would go blind very quickly. While this would be the case for humans with the same eye construction as us otherwise, I propose an alternative:
The vitreous humor is the jellylike substance that fills most of the eye. For us, this substance remains relatively unchanged and largely inert through most of our lives. While I'm stepping a bit out of my depth in biology and chemistry, hopefully the following concept gives you an idea that you can research in greater depth to meet your needs.
If a chemical were added to the vitreous humor, perhaps a something like a polymer, it might be possible to adjust the opacity of the vitreous humor, and thus how much light actually reaches the retina. If the retina (or perhaps even an external organ, possibly including the skin near the eye) detects too much light, an electrochemical reaction occurs that charges the vitreous humor and causes the normally amorphous polymer to crystallize to some degree (ideally this would be an analog behavior, with the body being able to control the degree of crystallization based on light). When the polymer crystallizes, less light passes through it, and so less light reaches the retina.
There are a few consequences to this though:
They would likely see better than us in dark environments. With the large pupils, their eyes would be able to take in and process even the dimmest light. As outlined in consequence #1, they would probably have worse vision than us in bright light, unless the polymer's crystalline form simply absorbed the light vs. scattering it.
The caveat is that with such a large aperture (pupil) their depth of field would be smaller... their ciliary muscle (that adjusts the lens) would need to be able to react faster and be stronger; otherwise they would have trouble focusing between near/mid/far ranges.
Their vision wouldn't necessarily be obstructed; on the contrary, it would probably be slightly better, as they would likely have more sharp "central" vision, and less peripheral vision. Any physical obstruction by their eyelids/etc would be naturally filtered out as their brain processed it.
As others have said, the social implications will depend on whether these humans are the primary/majority of the species, or if they are a lesser percentage of the population. I'd imagine anything between 20-50% would result in getting picked on similar to redheads, while below 20% would have more serious social repercussions such as discrimination and marginalization.
If their eyes do indeed function poorer in bright light, it's possible that it could be seen as a disability or genetic defect. Depending on the general social structure of your world's population, this could result in better acceptance more along the lines of how we treat disabled people today, or even greater marginalization and possibly even attempts at sterilization and eradication.
What if instead of being black due to the pupil being open, say that the proteins in their lens instead of filtering out just UV Light - adapt a filter that filters out a lot of the visible light spectrum(possibly due to increased sun exposure or hostile desert like conditions), this may lead to the ability to see in the red-to-infrared section.
This increased opacity in the lenses may cause the giant dilated pupils as less light would get past the dark lens, and it would adjust to let in more of the other light spectrum. This may lead to an increased visual acuity in the bright sun - but they may be completely unable to see in the dark. Think of wearing extremely dark sunglasses. The colors they see may be completely monochrome, or if this was far down the line of adaptation, they may be able to see some of the infrared range.
Because of seeing in some infrared, there may be a stigmata in social circles, as they may have been outsiders that would live/wander the desert. Also in certain light conditions, they may see through regular clothing. They would probably be color blind to many colors we see, and sometimes see some dark colors as bright white, this could lead to circumstances as seeing many obscured stains - think dirt or pizza sauce on a black shirt, this could lead to a view that regular people are "dirty" as they would see many of the things we would not clean regular without seeing the dirt or stains. They may be able to see veins or features under the skin, this brings to mind lie detecting seeing blood flush in the face.
A group of people with these abilities, may develop flashlights that cast ir light, that only they can see, and regular people would not.
You would also need to adapt the retina in some way. Perhaps its possible to have a much higher dynamic range retina which can handle the increased amount of light let in by this giant pupil, while at the same time being more sensitive to details in shadows. See HDR photography for examples, like https://owlbrainwave.files.wordpress.com/2011/05/hdr-example.jpg
They would need a less sensitive retina, which would imply less accurate vision. Alternately, they could do with a less tranparent cornea, vitreous body, or acqueous humour, which would also dampen their vision (unless there is some mechanism that changes the opacity of any of those to fit the amount of light).
The focus of their vision would be much less precise in both cases, too, so they would need to either be myopic, or have really good iris muscles, to adapt focus for different distances. They would probably be nocturnal people, trying to live at night and avoid sunlight. Other things unchanged, they might, like albinos and blind people, prefer to work as entertainers or musicians, professions that demand night shifts or dark work environments, or depend more on hearing than seeing.
There is going to be social prejudice against them unless they are the majority of people; if they are nocturnal, such prejudice will probably be even more intense.
They could have several "eyelids" that filter out some of the light with one or more only allowing light of a certain polarization. That way at night they could fully open the eye to see all the remaining light while at increased light levels they could filter out some to reduce glare.
Depending on the kinds of filters available they could be better off under certain circumstances. For example light reflected by water often polarized and so the glare created by such reflections could be reduced with polarized filters. Honey bees use the ability to see polarized light for orientation.
1) DEPENDS.
Real world biology works different.
Pupil is actually the "hole" on the iris. It lets different ammount of outer light reflections reaching inner side of the eyes. The inside of a human eye is covered with a layer called 'retina', this contains the specific bodypart, which turns outer light reflections into electric signals possible to be processed by the brain.
First dependency: Is the retina same or better/less developed?
Second dependency: Is the brain image processing is same or better/less developed?
Third dependency: What is the light reflections perceptable around?
I believe this latter one is the point you are interested in, as visibly "big-black-eyed" creatures adapt better to low light (even if smallest measuarble ammount. With no emitted light, an eye structure is useless) environments. Should you expose them to overwhelming type and ammount of light (please everyone, don't do that), it will temp/perm damage their eyes or even brain.
Multiple other dependencies still follows, but the question is covered.
2) What do you mean at "looking around"?
The basic mechanics you can discover also with just a hand of yours using as obstacle, positioned in front of you.
In short, if you turn "your lens" behind a covering material (even if partly), it will of course block you from getting a complete image.
Forth dependency: Distribution of light processing types on retina? If ever wondered, why our retina exactly goes wide or pointy, it is because the different location of percepting 'sticks'. In sharp light, the middle ones work better, in dim light, perimeter ones work better. You can try to read a digital clock panel by night with no room lights on. You will be able to get clear numbers only if not focusing on the panel, but just close to it.
So in dim light, if the pupil is wide, and location of dim light processing units located on perimeter area, image is obstructed in the ration of blocked line cone of sight related to completely unblocked. The brain mght still try to complete the vision with memory stored segments, however it will be not a real-time vision, but this will not necessarily be known to the person. Might require extensive knowledge on the topic.
3) DEPENDS. On social culture.
It absolutely depends on what the people associate individually or collectively with this specifics. They can be superhumans, falsely mistified gifteds, disadvantage holders...it comes mostly from history and folklore.
All the other answers focus on light sensitivity, but I think there is another important issue:
They would see everything very blurry, because a large pupil requires a large lens behind it to focus the light, and such a lens might just not fit into the eye (for comparison: a normal human's lens is bigger than the pupil, even when the pupil is at its maximum size).
The larger lens also means that there will be less space behind the lens, which moves the focal point further back and thus requires an even stronger lens than humans have (so either it has to be even thicker relative to its diameter or made out of another material entirely).
And even if such a large lens fits into the eye, there would be hardly any space left for muscles that contract or expand it, preventing any adaption, i.e. they are permanently near sighted or far sighted.
I'm not going to answer questions 2 and 3, as others have done that well already.
This is an actual condition (assuming its a black iris). The iris should not have too much of an affect on vision, and it is very common in the animalia kingdom.
