Photography is all about capturing light so the very first thing you should get a grip on is exposure.
Exposure is the amount of light that your camera sensor receives. If the sensor doesn’t receive enough light, the photo might be too dark or underexposed. If it receives too much light, the photo could be too bright or overexposed.
There are three controls on the camera that determine how much light reaches the sensor: aperture, shutter speed, and ISO.
An aperture is an iris inside the lens that can be opened and closed to let light through. How much the aperture is opened is indicated in f-stops, which look like a stylized f and a slash followed by a number.
To get a brighter image, you use a bigger opening like f/1.4 to f/2.8. To get a darker image, you use a smaller opening. Super simple: bigger hole, more light.
The part where it gets a little bit tricky is understanding the numbers: an f-stop like f/1.4 to f/2.8 means that the opening is bigger, and an f-stop like f/11 to f/22 means that the opening is smaller.
This confuses a lot of newbies: how is a number like 1.4 bigger than a number like 22?
Because it’s a ratio: think of the f as a 1. So a 1/1.4 is bigger than a 1/22. The f/ is part of a mathematical expression that describes the speed of the lens.
Visualizing aperture effects on exposure
Imagine that the sewer pipes below are apertures and the water is light. The pipe on the left has a bigger opening, and more water is flowing through. The pipe on the right has a smaller opening, and less water is flowing through.
The pipe on the left represents a bigger or wider aperture, while the pipe on the right represents a smaller or narrower aperture.
In a lens, the bigger the aperture, the more light can come through and the brighter the image.
The shutter is a device (physical or electronic) that covers the sensor in your camera body. The “speed” in “shutter speed” refers to how long the shutter stays open, exposing the sensor to light, and is expressed as seconds or fractions of a second.
For a brighter image, use a longer shutter speed. For a darker image, use a shorter shutter speed.
Visualizing shutter speed effects on exposure
Imagine that the bucket below is the sensor and the water is light. The longer the faucet is open, the more water is collected.
In a camera, the longer the shutter is open, the more light the sensor collects and the brighter the image.
ISO is kind of like a last-resort control (and not technically an exposure control either). You would only increase this to let more light in if all else fails.
Unlike with aperture and shutter speed, there is no creative effect from increasing ISO unless you consider noise something desirable or artistic.