Yes, it's a term from photography. You could set the F stop on your lense to control the amount of light coming through and adjust the depth of field. A small F stop, or larger number, would mean you needed a longer exposure for the film but it would give you a long depth of field, so focus was not as critical. The opposite, a large hole, lets in more light so exposures could be quicker but field depth is shorter so you had to be more careful with focus. The same principle applies to a rifle scope. A small hole will allow a wide focus range but makes the image dimmer so you need bright light to use it. It also allows you to adjust the parallax for closer so your head position is not so critical as it would be normally.