how does this work?
In: Physics
You can focus light in two ways. You can use a lens which will bend the light towards the point where you want it to be. You can also use a small hole which narrows the field of view, so that only the light from the object you want to see gets through. This works because everything reflects or emits light in all directions. Narrowing the hole makes it so that less light reflected or emitted from each point gets through.
Now put the hole at the end of a box that doesn’t let any more light in. Because there’s no other light source to overpower the image you can see it projected onto the end of the box. Think about how sunlight coming through a window or open door fills up a room despite the area of the room being much larger than the opening.
Modern cameras use both methods. Because the lens bends the light, it can magnify the image without needing all of that space for the light to spread naturally.
A blurry image happens when the light from the thing you want to see (or take a picture of) is coming in from a bunch of directions and angles. Lenses fix this by bending the spread out light and focusing it. Pinholes work by blocking out the light that isn’t coming in from a particular angle. It’s getting rid of the extra light that leads to a blurry image.
[Here’s a great Minute Physics video about it](https://youtu.be/OydqR_7_DjI)
Have you ever been in a room with a bunch of canned lights for illumination. You have a whole bunch of different shadows from each light source that darken where they over lap.
Now imagine a room with a ceiling filled with canned lights. You probably wouldn’t cast much of a shadow at all because every part of you and the ground would be lit by enough lights that your shadow would be very indistinct multiple lights.
If you make a camera obscura from a box without closing the end it doesn’t focus an image, because light from every visible object is projected across the entire space, just like you didn’t cast a shadow in the room filled with light sources.
If you extinguish all but one of the canned lights, you’ll have a very distinct shadow with crisp edges because now there’s only one light source so when you block it, there’s very little light reaching the area your shadow covers.
A pinhole makes all the light come from one tiny point, so all the light reaching any part of the screen is only coming from one part of the scene. Just like your shadow would be sharp in a large room illuminated by one small light source, the each part of the screen is illuminated by the light from only one object so it projects a sharp image.
A lens uses optical trickery to allow the size of the pinhole to grow, while the light from one part of the scene is focused to a single virtual point.
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