Why do cameras go in and out of focus? Why can’t everything just be in focus?
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Stuff gives off light. Whether that’s an actual light source like an LED or just a regular object reflecting light, the light goes off in all directions.
When we take a photo, we are trying to get light from a particular “source” to hit a particular part of the camera sensor (film, chip, etc). But the light going in all directions means light would hit the sensor from everywhere, and we wouldn’t see anything. So we need to refine things.
One way to do this is to use a pinhole. You block the subject of the picture from the sensor by a board or sheet or something which stops the light getting through. Then, you poke a tiny hole in the middle of it. Because the light can only go in a straight line, stuff from the bottom left of the subject hits the top right of the sensor, and vice versa. The image is completely sharp and in focus. The problem with this is that very little of the original light gets in.
One way to fix this is to use a lens. This focuses bits of light that have come from the same place in different directions onto a single part of the sensor. Say you’re taking a picture of a tree. The light coming from the topmost branch goes in different directions and hits all different parts of the lens. But the lens bends it just enough to focus it so it all hits the bottom of the sensor. Similarly, light from the base of the trunk hits all parts of the lens but gets focused on the top of the sensor.
However, lenses focus light more or less depending on the distance the light is coming from. So even though the tree is in focus, light coming from behind the tree gets bent too much and tries to focus in front of the sensor, meaning it’s spread out by the time it reaches the sensor. Meanwhile, light from something in front of the tree doesn’t get bent enough and wants to focus *behind* the sensor, so it’s not yet in focus when it reaches the sensor.
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