Now to your original question, as frank said you cannot rely on the screen on the back of your camera when it is zommed out, but as frank said, zoom ALL THE WAY IN in on a star or whatever your focussing and you will see what your focus is like. As mentioned DSLR focus is great for acheiving really good focus with a DSLR. It takes a bit of practice but as they say practice make perfect.
  