aperture.....

ap·er·ture (pr-chr)
n.
1. An opening, such as a hole, gap, or slit.
2.
a. A usually adjustable opening in an optical instrument, such as a camera or telescope, that limits the amount of light passing through a lens or onto a mirror.
b. The diameter of such an opening, often expressed as an f-number.
c. The diameter of the objective of a telescope.