Rendering sharp shadow on cycle rendering

Question

I would generate a clear shadow projected on a floor. I am wondering what is the essential node used to achieve this.

I am using these nodes enter image description here

and I get this image, I remark that the shadow is blurred (a lot of penumbra). I fix the focus at the floor and I try to change the Aperture (F/stop) to fit the depth of field. enter image description here

on sun, point and spot lamps, you can reduce the size to get sharp shadows — Bithur, Feb 23 '15 at 17:14
@startingBlender Scale it down alot. The larger a light source the softer the shaodw. If your light source is tiny, you shadows will be sharp — GiantCowFilms, Feb 23 '15 at 17:18
@GiantCowFilms, you mean I scale the point light? I scale it 0.01 but it does not change the result! — BetterEnglish, Feb 23 '15 at 17:19
@startingBlender with secondary light(s). can be set to no "cast shadow" if needed — Bithur, Feb 23 '15 at 17:20
possible duplicate of Unwanted sharp shadows when using a point lamp — Jaroslav Jerryno Novotny, Feb 23 '15 at 17:27
@Jerryno That question is very similar, and the answer to that question may solve this one, but the question itself is asking a different thing (how to get sharp shadows vs. how to get blurrier shadows with a point lamp). See this meta post, the answers cover the same thing but the questions are different. — PGmath, Feb 23 '15 at 20:57