Thank you for your answers but, the problem isn't the focal point, I have got it. And when I render, I have a little bit of blur; it work.

The problem is that I don't know how to define the quantity of blur and the distance which is really net on the photo. I mean that I want to have a little net distance and after a quite big blur effect. I joined to my message a preview of my work.

Thank you for your help about this.

I'm trying to add a depth of field on a render but, I don't arrive to have a good and big blur effect on the picture. How can I make this... the aperture does not change anything in the blur... I don't understand. HELP PLEASE !