Some codecs, such as h.264, AVCHD, and MPEG2 are "Long Group of Pictures" codecs. This means that the codec encodes one full frame and then for the next 24 or so frames, just records partial information. For editing in iMovie, it is recommended to convert these movies to an i-frame codec (in other words, a codec where every frame is a full frame. (This is not referring to the I-Frame codec that is marketed in some cameras.)
Motion JPEG-A is already a full i-Frame codec, so you should get excellent results in iMovie using it as is.
For more on this topic, see my post here.
Thanks for your explanation. Previously, as I described above, I thought I _had_ to convert to HD footage to AIC.
I imported some footage with the "Optimize video" box unchecked, and it worked. With the box unchecked, I saw that the choice (Full vs Large) was grayed out, but it was the same as what was in the Preferences pane. So I wasn't sure what to expect! Opening the iMovie Event file in QT showed it was in JPEG-A format. And at that point I recalled that JPEG-A contains independent frames just like AIC does, and thought, "So why shouldn't it work?" The book called iMovie '09 by Pogue and Miller says that there is no loss in quality when converting HD to AIC, but I have found differences. And now you say that JPEG-A should work functionally just as well in iMovie. Cool! So converting to AIC is only necessary when the original format is not a "full I-Frame codec". OK.
What's interesting here is that the AIC files are half the size of the original JPEG-A files! I did a couple of quick comparisons. From one movie I selected a scene that includes a sunlit front lawn, a driveway, a tree, a flying disc in mid-air, and some bushes in the back in the shade. Switching from JPEG-A to AIC shows that the AIC version looks a just a tad sharper or has slightly more contrast or both. I actually prefer the AIC version, but the difference is small enough that I'm not sure if one could recognize which is which without a direct comparison done by overlapping the two and switching between them using Cmd-` (in other words, showing a person only one of them). So for this, AIC wins, and more so since the file is half the size of the JPEG-A! But how can that be? How can a file half the size look just as good, if not slightly better than the original size file?
I also made exports of the aforementioned scene using H.264 at High quality and single pass. The results are indistinguishable.
I tried a sunset scene from another movie. Here the AIC is definitely lighter/brighter. I can see more detail in the dark areas. Also, the colors look slightly less saturated. For this I prefer AIC, but it's a mixed bag. Actually, making an H.264 export of this movie (the one I already did, which used and AIC event file) also did that (at least for the sunset scene)! But posting it on smugmug.com undid that.
At least several months ago I did a similar comparison with yet other footage. I found that AIC lightened things up a bit. In cases like that I would consider using an import without "optimizing". But it might be just as easy to adjust the contrast and brightness and such in the app.
So, evidently, the difference depends on what's in the picture! It looks like sticking with AIC causes little or no harm.
Thanks for your input.
If you think about it, photos are a single frame codec.
TIFF, JPEG, RAW, and PNG all contain the info for a single frame.
You might use TIFF or RAW if you wanted to preserve as much as possible of the original so you could take it through many transformations and still retain fidelity to the original. You might choose JPEG if all you need is a compressed version to show on a computer screen or to print.
In this analogy, ProRes 422 ( a codec used in Final Cut Pro X) would be the codec that you would use if you wanted to be able to do a lot of manipulation without losing quality. Motion JPEG and AIC are useful for light manipulation (like iMovie, with only one render pass). h.264 is useful for the final product, where you just need to watch it, but are not editing it further.
I have to report a new, recent discovery. The JPEG-A files I got from the transfer company all have the Brightness control slightly to the left! This explains why I often saw that AIC lightened things up a bit. (I never opened the A/V Controls panel before, and so noticed this only now.)
Above I wrote
I did a couple of quick comparisons. From one movie I selected a scene that includes a sunlit front lawn, a driveway, a tree, a flying disc in mid-air, and some bushes in the back in the shade. Switching from JPEG-A to AIC shows that the AIC version looks a just a tad sharper or has slightly more contrast or both.
For this comparison I must have moved the Brightness pointer to the neutral position, as I don't report the AIC being any lighter. I just did the test again to confirm it. The two are pretty much indistinguishable. For the others where I reported the AIC version being lighter, it must have been for the above reason: the Brightness control was slightly left of center.
OK, just wanted to correct myself here. Looks like AIC is pretty d_a_m_n good!