HDV is imported with not a problem in final cut and i-movie. When you go to save for a dvd, it will down convert for you without a problem. The only issue is that with HDV the files are huge. Even the a short 30 second clip will chew up a gig or more of space - massive it is. So if you want to shoot for dvd, just record in DV mode for now. I too use a powerbook g4 (and a g5 imac). I love the H3C camera.
As for you decision on which format to use from the get go, if you final cut pro, no worries it will import both without any special attention needed. If you use i-movie, then you will need to decide because it will ask you when you open a new project, which format you want to use. If you say 1080i, and the file is SD, you will only see a blue screen. You will also have to decide with i-movie between wide screen and 4:3 prior to importing as the same issues of a blue screen will arise if the video does not match your settings.
hope this helps,
I'm thinking of buying the SONY HDR-HC3 camcorder. I will need footage for regular DVD players and Projectors as well as wanting the highest quality footage for future use with High definition equipment. Is it possible to shoot everything in HD and use it for both HD and DV. I have a Mac Powerbook with system Tiger. Can the HD footage be reduced to MPEG2 so that I then have a copy of my video in both resolutions?
Or do I have to decide before filming which formate to use?

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