ratio of lens focal length to aperture diameter). Reducing the aperture diameter (increasing the f-number) increases the because only the light travelling at shallower angles passes through the aperture so only cones of rays with shallower angles reach the image plane. In other words, the circles of confusion are reduced or increasing the .

For a given size of the subject's image in the focal plane, the same f-number on any focal length lens will give the same depth of field. This is evident