A lot of my dark sky shots might be 20 or 30 second exposures. How does Live View work in that case? Couldn't I just as easily take sample shots? I don't have any experience trying to focus from an LCD. Maybe with the new 3" LCDs it will be easier.
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It wouldn't make any difference at all. You'd just be turning on LiveView to focus, then turning it off before you shoot. When you go to shoot, the camera is back to the way it was. Again, you could activate LiveView, focus manually (using an enlarged view), then deactivate it in the space of two seconds.
I suppose you could use it to frame as well, but you only activate it when you need it any case. If it's turned off, it doesn't do anything to exposures.
Sample shots would only tell you if your focus was off. They would not help you focus correctly in the first place.
Really, LiveView is no big deal. At first, a lot of experienced shooters complained that adding it to DSLRs was "dumbing them down" or something, as if they expected to find people holding DSLRs at arm's length with the screen on, just as they do with point and shoots. As it turns out, a lot of serious shooters find it useful as an electronic focus magnifier. Just the thing for macro, or any situation where focus might be difficult.
And there's no shame in admitting difficulty manually focussing a DSLR. In the manual focus film days, we often had split prisms and other focusing aids to help. As autofocus and digital came along, the focus screens in SLRs were altered to fit the new technology. For example, slow zooms turned split prisms dark. Since so many consumers were buying slow zooms for their autofocus cameras, the manufacturers just got rid of split prisms. Great for autofocus, not so great for manual focus. Recognizing this, newer DSLRs with interchangeable focus screens usually have one optionally available that has been specifically designed to make manual focus easier.