I use a Canon 5D Mark III for Landscape & Night Photography. In order to get pin point sharp stars at night, I've been trying the live view method to zoom in on a bright star and manually focus until it becomes tack sharp. The problem I'm having is that when I zoom in 5x or 10x, I cannot see anything but noise. I've attached a photo to show you what I see. I apologize for the quality as it was snapped in the dark with my iPhone. I've tried with exposure simulation both on and off with no luck. Does anyone else have this same issue and if so, how can I get around it?
Just focus to infinity. You might be focusing on a very dark object.
Use software to focus the camera for stars. BackyardEOS is relatively inexpensive. Also gives you a number of other features useful for astrophotographers. You need a computer and your camera usb cable. Once you get focus just switch the lens to manual mode.
I've also found that it's good to let the camera and lens temperature stabilize for about an hour. Focus can change during the cool down or warm up period.
You can also ask one of our better astro-photographers here in canonrumors, JRISTA.