A hole, a lens and a very short while
How Does a Camera Freeze a Moment?
A photograph is light that was allowed in for a fraction of a second and recorded before anything had time to move.
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Step 1 of 5
A bare hole already makes an image.
Light travels in straight lines, so a small hole in a dark box throws an upside-down picture on the far wall. That is a camera with no parts at all.
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Step 2 of 5
A lens lets in far more light.
A pinhole is sharp but desperately dim. A lens gathers light from a wide opening and bends it all back to one point, so the image is bright and still sharp.
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Step 3 of 5
The shutter decides how much movement is recorded.
The sensor records everything that happens while it is open. A thousandth of a second freezes a bird; a whole second turns it into a smear.
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Step 4 of 5
Brightness has to be traded three ways.
A short shutter needs a wider opening or a more sensitive sensor. Every photograph is a choice between blur, how much is in focus, and how grainy the result is.
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Step 5 of 5
Film and sensors do the same job differently.
Film holds grains that change chemically where light hits. A digital sensor holds millions of wells that count arriving light as numbers. Both record where light landed.
Every camera image arrives upside down and back to front. Film was simply printed the right way up, and a screen flips it in software.
The short version
A camera bends light onto a recording surface and opens for a fraction of a second, so the scene is captured before it can move.
Try it yourself
Make a pinhole in foil over a dark tube. The image on the far end is real, and upside down.