How a camera works
This is a short exercise I wrote explaining the fundamentals of light and cameras- I hope it helps you.
To understand how a camera works you need to be aware of the following,
Lets say you are in a plain room with a single light source and four walls. A few feet in front of you is a chair. In order for you to see the chair, light has to first exit the bulb, reach the chair, bounce off the chair and then reach your eyes.
Most surfaces (even ones that appear smooth) are bumpy and light will bounce off in many directions. We can imagine holding a pink sheet of paper close to a white wall- A pink ‘glow’ of the paper will be visible on the wall. Due to the irregularities of the surfaces involved (in particular the wall) , this reflection will be fuzzy and out of focus. A very smooth surface will lead to parallel light rays being reflected at the same angle, and result in a mirror.
Parallel photons can ‘build’ an image. If we use a mirror as an example, the light from an object reflected near the ‘top’ of the object will remain ‘above’ the light from the ‘bottom’ of the object.
Cameras (and your eyes) use a similar principle.
Going back to the room with a chair from earlier, lets add a second room right next to it, sharing a wall. This new room is has no light source and is completely dark. However, if a small hole is made between the room, a small amount of light will filter into the second room.
In this scenario, the only light that can make it to the back wall of the second room will have to be traveling in a semi-parallel path with its neighbor. But there is a twist- If we say the chair is perfectly centered with the hole, the the light that bounces off near the center of the chair will pass straight through (level with the floor) and reach the dark room without issue. Light bouncing off near top of the chair will have to be angled slight downwards to make it through the hole- this means the will end up below the center photons. Same with light bouncing off the bottom of the chair- photons traveling level to the floor will hit the wall- but those angled slightly upwards will make it through the hole, and end up above the center photons.
What does this mean? it means the image projected on the opposing wall of the dark room will be upside down. More accurately it is reversed across all axis (left and right as well).
But perhaps even more importantly, because we have filtered out so much of the non-semi-parallel photons from hitting the back wall, we are able to build a clear image, and have created a camera! Modern cameras have simply shrunken down the size of the ‘room,’ and use a digital sensor sensitive to light to capture the image.
A few common photography terms
Aperture- Is use to describe how large the hole for the dark room is. A larger opening allows fore more light- and a blurrier image.
Focal length- how far the opsing wall (or sensor) is from the aperture. If the sensor is closer to the hole, less of the scene in the room will be captured, but it will also take up a larger space than it did previously- this can function as a zoom.
Exposure time- sensors are very sensitive to light, usually the aperture is only open for fractions of a second for an image to be captured. The length of time the aperture is left opened affects how bright or dark an image will be. A longer exposure will usually lead to a brighter image, but can also end up blurry if the subject or the camera move.