Well admittedly I am very new at this DSLR thing, and the truth is the Canon Xsi is my first venture into SLR's since my nikon F5.
Changes in employment and otherwise caused me keep my 40 year hobby to a point and shoot for the last 9 years. Latest version being Canon's SD880IS.
No, really, I am going somewhere with this...sooooo, I am very used to using the screen for everything since that is all I have had on the last 2 cameras.
Consequently I was excited to have the 'viewfinder' back on my new camera with the screen as an option using live view. I was discouraged when looking at the first few pictures I took using the viewfinder after I discovered that in order to get the framing I intended I would have to crop the viewfinder almost into peoples foreheads, since it (the viewfinder) only shows about 95% (less in my opinion) of the actual frame.
So when shooting portraits/groups or anything else where you would like an accurate framing of your subject it is easier to use the live view, especially with the 'grid' option selected.
I'm not convinced I have the 'focus' subject nailed yet using live view, ( following the instructions in the manual ), but all my photos have come out sharp so far...
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Just my $0.02...
Jim