What a camera actually does

Strip away the menus and a camera is a light-tight box with a hole in one end and a sensor at the other. Everything it does is a way of controlling how much light comes through that hole, for how long, and how strongly the result is amplified.

Light in, picture out

Light reflects off whatever you are photographing, passes through the lens, and lands on the sensor. The sensor counts how much arrives at each point and turns that into numbers.

Too little light and the numbers are small and unreliable, which looks like a dark, muddy photograph. Too much and they hit their ceiling, which looks like blank white where detail used to be. Getting a usable amount is called exposure, and there are exactly three ways to change it.

The three controls

• The aperture is a ring of blades inside the lens. Opening it wider lets more light through.
• The shutter speed is how long the sensor is uncovered. Longer means more light.
• The iso is how much the signal is amplified afterwards. Higher means a brighter result from the same light.

That is the whole system. Every mode dial, every automatic setting, every piece of advice you will read is about choosing between these three.

Where the lens comes in

The lens does two jobs. It decides how much of the scene fits in the frame — its focal length — and it decides which distance is rendered sharply, which is focus.

A short focal length takes in a wide view. A long one magnifies a narrow slice. Neither is more correct; they are different pictures of the same scene.

What the camera cannot do

It cannot see the range of brightness your eyes can. Stand indoors looking out of a window and you can read both the room and the view; a camera must choose. Learning where that limit falls is most of learning to expose.

It also cannot know what you meant. A camera can measure the light and guess, and it guesses well most of the time — but it does not know which part of the frame you care about. That is the gap the rest of these lessons close.

Key takeaways

• A camera controls how much light reaches a sensor, for how long, and how much it is amplified.
• Those three controls are aperture, shutter speed and ISO, and there are no others.
• Each also changes something else about the photograph, which is why the choice matters.
• The camera cannot see the range your eyes can, and cannot know what you meant.