Doors and Windows - Door frames

I'm building a shed. I need to know how to build the door frame. Can I attach the door directly to the studs (4x4's, they were free), or do I need to build a separate frame to hang the door on. This will be a very heavy door and it's also over sized (approx 40"). If I use the studs, it will not affect my center pattern as I already have to move the studs on either side of the door towards the corners of the shed just to allow the door to fit. If I have to build a separate frame inside the studs, I'll merely have to move them more.
If I do have to build a frame, would you be so kind as to explain how to do it. This seems very simple to me, and I think I may be over-engineering here.

Thanks.

Jowboy

It's a shed. There is no reason that you have to use a door jamb in it. However, a 40" wide door will need heavier hinges than the 3-1/2" interior type. I would use 3 galvanized 4" hinges.

Another suggestion (this is what I did) -- make the opening 48" wide and use (2) 24" doors for it. Each door is lighter, and at least 1/2 the time that I go into the shed, one 24" door is plenty wide enough to use. Latch was simple enough -- a deadbolt one one door (keyed just like the house), and a cane bolt for the other door. (I used (3) 4" galvanized "tee" hinges for each door, and through bolted them thru the studs.)