In a traditional camera, the aperture controls how much light enters the camera per unit time. However, because the aperture has a finite diameter, it also creates depth of field effects. That is, if one uses a larger aperture, less of the scene will appear in focus. Likewise, if one uses a smaller aperture, then more of the scene will appear in focus. In the limit where the aperture is infinitely small, we have a pinhole camera.
Photographers often like to utilize depth of field as a creative effect.
However, once the depth of field effect has been captured on film, it cannot be changed. By allowing a variable aperture, we are letting the user add and modify the depth of field effects in after the pictures have been taken. Since we are using a discrete set of rays in our database, the 'closest' ray is usually not identical to the desired ray. By increasing n , we are in effect increasing the aperture of the a discrete camera lens. In the above pictures, we weighted rays from every camera using a gaussian.