pinhole

Example: "Sam, you have no idea how big this thing was…It was like putting like a watermelon into a pinhole."

But the least crevice is sufficient for the purpose; a pinhole will do the business.

Peculiar diffused effects can be obtained by using a fine slit in place of the pinhole.

I have seen a prize-winning picture in an exhibition that was made with a cigar box, with a pinhole in one end for a lens.