The exposure triangle, explained with one photo

Say you're taking a photo of a friend walking towards you at dusk. The photo looks bright enough, but their face is a bit blurry. You make the shutter faster and their face is sharper. Now the photo's too dark. So what do you change next?

That's basically what the exposure triangle is for. Aperture and shutter speed control how much light the camera captures. ISO affects how bright the camera makes the image from that light. The reason you'd pick one setting over another is what it does to the rest of the photo.

Aperture changes how much is in focus

Aperture is the opening inside the lens. A wider opening lets in more light. The numbers are a bit backwards at first. f/2.8 is a wider opening than f/4.

Say you've focused on your friend's eyes. At f/2.8, less of the scene in front of and behind their eyes will look sharp than at f/4, with the same lens and framing. That range of distances that looks sharp is called depth of field. A shallow one can help your friend stand out from whatever's behind them.

Shutter speed changes what movement looks like

At 1/250 of a second, the camera collects light for half as long as it does at 1/125. Your friend has less time to move during the shot, so there's less chance of motion blur. The camera also collects half as much light if you leave everything else alone.

You might want the opposite if you're shooting a waterfall. Leave the shutter open longer and the water has time to move across the frame, which is where that silky look comes from.

ISO helps when you want to keep the other two settings

Let's go back to the darker shot. You've got the shutter speed you want, and you're happy with how much is in focus. Going from ISO 400 to ISO 800 can bring the brightness back up. The sensor still captured the same amount of light at that shutter speed and aperture. In dim light, the result can look grainier, especially in the shadows.

If you'd rather keep ISO at 400, you could open the lens from f/4 to f/2.8, assuming it goes that wide. That lets in about twice as much light and makes the depth of field shallower. Both changes get you to roughly the same brightness. Which photo would you rather have?

Start with what you want the photo to look like

For someone moving, start with shutter speed. For a portrait where you want a soft background, start with aperture. If you want to see the difference for yourself, photograph someone walking past the same spot twice. Try 1/125 and 1/250 with the aperture fixed, and double the ISO for the faster shot. Then zoom in and look at their hands or feet. That's where the trade-off is easier to see than on the camera's little preview.