What actually happens when you press the button

The camera on the front page of this site takes about two seconds to do its job, and I have never met a client who could tell me what happens inside it. That is completely fine — you do not need to know how an engine works to drive somewhere. But the sequence is genuinely beautiful, and it explains a few things people find confusing about photographs, so here it is in order.

The light gets in

Light hits the front element of the lens and passes through five or six pieces of shaped glass, each one bending it slightly to undo the errors introduced by the one before. That stack is the single most expensive thing in the bag, and it is why a cheap camera with a good lens beats an expensive camera with a bad one every single time.

The iris decides how much

Behind the glass sit eight thin blades arranged in a ring. Rotate them together and the opening they form shrinks from a wide circle to a small polygon. That is your aperture — ƒ/1.4 wide open, ƒ/16 stopped down.

Wide open lets in a great deal of light and throws everything except your subject out of focus. Stopped down lets in very little and holds the whole scene sharp. This is the entire reason your portraits have a soft background and your landscapes do not.

The mirror gets out of the way

In a single-lens reflex camera, a mirror sits at forty-five degrees behind the lens, bouncing the image up into a prism so your eye can see exactly what the lens sees. The instant you press the button that mirror swings up and out of the light path — which is why the viewfinder blacks out for a fraction of a second, and why these cameras make that particular clack.

The curtains open and close

With the mirror clear, two cloth or metal curtains sweep across the film plane, one after the other. The gap between them is your shutter speed. At 1/1000th of a second they are not really opening and closing so much as running a narrow slit across the frame — the top of your picture and the bottom are, strictly speaking, taken at slightly different moments.

Something records it

Then it lands: on film, or on a sensor pretending to be film. Everything before this point exists purely to deliver a controlled quantity of light to a flat plane for a controlled length of time.

That is the whole machine. A box, a hole, a way to time the hole, and something at the back that remembers. Every photograph you have ever loved came out of some version of it — and none of that mechanism has any opinion at all about where you choose to stand or when you decide to press the button, which remains the only part that was ever really the photographer’s job.