Nikon D5000 and Mac. Why can't I watch AVI-format video at 1280x720p on my Mac?

I have a Mac OS X 10.6.8 and I can't watch AVI-format video at 1280x720p. This is strange, because I haven't the same problem with the first two formats, 640x424 and 320x216. What's the problem? Moreover, my iMovie doesn't support AVI-files of D5000. I tried to convert them with MPEG Stremclip, but I get a reduced quality. Have you any suggestions? I hope so. Please, help me. I want to try to create a video-test, but everything is getting so complicated (or maybe I'm the incapable. It's sure -.-).

You only want to watch?

You have at least two options:

1) Download and install VLC player (free). QuickTime can deal with MOV and MP4 files (among a few others), but not AVI… Unless…

2) Download and install Flip4Mac (the free QuickTime plug-in to play Windows media files).

If you are "getting reduced quality" when converting the video in MPEG StreamClip, it is likely because you are using too much compression for the converted file.

You did not tell us which Mac. For all we know, the CPU is too slow - or there's not enough RAM - or there's something else going on that we do not know about in the machine - that is causing the playback issues you are experiencing at 1280x720.

That (standard, regular, non-modified) iMovie does not support AVI files is no surprise. The AVI file type is Microsoft proprietary like WMV files.