expectations and what sort of video you think will be in your future.
As for AVCHD working with Macs and FCE4, the "experience" will depend on the Macintosh you are using. I've used a HF S100 with a 2 year old iMac and using "Log and Capture" it works great.
After using (and continuing to use) miniDV tape I did not find the FCE4 process dealing with decompressing MTS files from the camera at all "tedious". For what it is worth, all editors will need decompressed files in order to deal with them. In this case, your alternative is to copy the MTS files to the Mac and use something else to transcode them (like MPEG StreamClip) - resulting in large file size. But I found using the Log and Capture flow was easy. The HF S21 has a lens filter diameter of 58mm. This means you can add lenses or a DoF adapter then add lenses to it.
I have no experience using any dSLRs which capture video as a "convenience feature". Vimeo shows so nice video from many of them.
Once the video is in the computer, the source of the video has little impact on uploadable formats. FCE has a ton of file types it can render for exporting. I've not hit any "roadblocks".
I'm looking to purchase my first video camera, and looking for a prosumer model that offers HD, 24p, preferably a Cine mode, and a decent amount of manual features (man. focus & shutter speed are a must!)
I was fairly set on the Canon Vixia hf s21 (the 21 because it features an electronic viewfinder) because it seems to offer all these things and excellent looking video for its size and price. I have, however, only ever used mini DV, so the AVCHD/SD card feature on this camera worries me a little. Does the format/media work well with Macs and specifically Final Cut Express 4? I've heard you have to use an Apple codec to transcode the files into a format (AIC) that FCE supports- a tedious process that ends up creating files much larger than the originals. Is this camera more hassle than it's worth? I'm looking to make indie films and want to be able to edit them and upload them to the internet without hitting constant compatibility roadblocks.
In addition, it was recently suggested that I look at dSLR's, and specifically the Canon EOS 7d, as it's an 18 megapixel camera along with HD video. I was told it's superior in quality for only slightly more money, offers more lens options, and is far more compatible with Macs for editing purposes. Is the 7d a smarter choice than the hf s21 if my main objective is quality cinematic-looking HD video from a camera that offers a multitude of manual features?

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