How lenses work

So here's a normal eye

And here's an image this eye sees.

Now here's an eye that has half of the pupul obstructed.

Would that eye see the image like this?

No. The image would instead appear like this - nothing is obstructed, the image is just darker because less light enters the eye.

Now the reason why that happens is to do with how lenses work - basically every part of a lens is capable of forming the whole image. The size of the lens matters only to enable more or less total amount of light to be transported (brighter or darker image).

Here you can see that even with half of the lens obstructed, the whole image is still formed, just with half as many light rays.

However the shape of the opening has an effect on the image - but only on the unfocused parts of it. Here's a modified photography lens with a heart cutout in front.

The image taken with such a lens - as you can see the parts of the image that are in focus are unaffected, but the bright lights in the background have taken the shape of hearts. That's how bokeh works.

Here's a hexagonal aperture of a photography lens.

The bokeh in a photo taken by such a lens would also take a hexagonal shape like in this image.

But there's also another place where you can see that effect - lens flares. Here every secondary flare is also a hexagon. But there's another effect going on - there are 6 distinct spikes around the sun - those are formed by diffraction of light along the straight edges of the hexagonal aperture. If the aperture was an octagon, there would be 8 spikes etc.

You can also see that effect in the Hubble Space Telescope images - here every bright star has a four way cross around it.

This is because Hubble, like most Newtonian telescopes has a secondary mirror in front of the primary mirror - it's suspended in the middle of the telescope by four thin beams that are forming the cross pattern. The light entering the telescope diffracts along the edges of the beams and forms those cross flares around bright stars.