Why are my pictures blury?

Okay so I just got a rokinon 14mm f/2.8 lens for my Nikon d3300 and all the pictures I ve taken with it have come out blury. It s definitely the lens because my basic 18-55mm lens takes crystal clear pics. I ve been testing it by taking pictures of trees about 250ft away. Is there settings I need to adjust my camera to for this lens? I ve just been using auto focus but I can t get it to focus right. Is my subject too close and that s why they re blury? Because this lens is usually used to take pictures of the sky and stars. Or is it a problem with the lens on an inside piece of glass that I wouldn t be able to clean without taking it apart? Please help.

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