Camera basics - aperture

Aperture is the physical opening that allows light onto the film. On a pinhole camera, this would be the actual pinhole. On a modern camera, there is a ring that can narrow or widen to change the aperture. The aperture controls two things: (1) the amount of light let into the camera at a given point in time, and (2) the depth of field [DoF].

The aperture is expressed as a fraction of the full focal length of the lens. It is written f/22 or f/16... There numbers are in strange intervals because each "full stop" is twice the area of the previous stop. This would be an increase or decrease in diameter by √2. These are the f-stops.

It's important to remember that the f-stop is a ratio, not actual sizes. So, for a 40mm lens, f/8 would be 40/8 = 5mm opening for light. However, an 80mm lens on f/8 would be 80/8 = 10mm.

Very simply, the bigger the number, the smaller the opening. f/22 is a very small opening letting in very little light. f/1.8 is a very large opening letting in a lot of light.

Why do we even care? Well, if it's bright out like in sunlight or snow, we have to limit the light, but if it's very dark out, we'll need to let in as much light as possible.

The other reason we care is depth of field. This is the area of a picture that's actually in focus. It can be a few millimeters wide or nearly infinite. In this way we can control what we want to accentuate. The most simple example would be a picture where the subject's in focus but the background's all blurry.