An aperture is a hole or an opening through which light travels.
If the admitted rays also pass through a lens, highly collimated rays (narrow aperture) will result in sharpness at the image plane, while uncollimated rays (wide aperture) will result in sharpness for rays with the right focal length only.
This means that a wide aperture results in an image that is sharp around what the lens is focusing on and blurred otherwise.
The aperture also determines how many of the incoming rays are actually admitted and thus how much light reaches the image plane (the narrower the aperture, the darker the image).
You can control aperture with your lens in the front.
In combination with variation of shutter speed, the aperture size will regulate the film’s degree of exposure to light. Typically, a fast shutter speed will require a larger aperture to ensure sufficient light exposure, and a slow shutter speed will require a smaller aperture to avoid excessive exposure.
Diagram of decreasing aperture sizes (increasing f-numbers) for “full stop” increments (factor of two aperture area per stop)
close depth of field
Middle depth of field
Far depth of field
