The aperture is a mechanical device in a lens that controls the amount of light that passes through the lens and reaches the sensor or film. It consists of a series of blades that open and close to change the size of the opening through which the light passes. The size of the opening is indicated by f-stops or aperture numbers (e.g., f/2.8).

A larger aperture (smaller f-number) allows more light to pass through and creates a shallower depth of field, while a smaller aperture (larger f-number) allows less light to pass through and creates a greater depth of field. Depth of field refers to the range of the photo that is in focus, from the foreground to the background. A shallow depth of field results in a blurred background and highlights the subject, while a large depth of field keeps more of the scene in focus.

The aperture also affects the exposure of a photo. A larger aperture allows for a shorter shutter speed, which is helpful for freezing fast-moving objects, and it also creates a shallower depth of field. Conversely, a smaller aperture requires a longer shutter speed in low light and provides a greater depth of field.

Photographers can adjust the aperture manually or automatically, depending on the type of shot and the desired results. Some cameras also have an aperture priority mode, which allows the user to set the aperture manually while the camera automatically calculates the shutter speed to ensure correct exposure. The aperture is a crucial aspect of exposure.