Summary: The easy workaround to get iMovie work beautifull with 60fps AVCHD video footage is to convert them to Apple InterMediate Codec(AIC) MOV which ensures the best result when using in iMovie 08/09/11.
The 1080 60fps (i.e., 59.94 progressive frames per second), also known as 1080 60p footage, is a new format above and beyond the standard AVCHD spec, which is now widely used by many videocams. Sony uses AVCHD in its videocams like Sony NEX-5N, NEX7, A99, A65, and Panasonic has 1080 60p on its videocams since the TM700 and includes the recent videocmas like Panasonic GH6, etc. The advent of 60fps video saves for pixel peeping or private playback of a sports clip, but the trouble is that the existing editing products can't handle the stuff, except with a very strong computer and in limited amounts, and the output may be confined to lower bitrates and incur quality loss. Therefore you will always find it a painful experience to import 60fps video to iMovie, even with the lastest iMovie 11 (V9.0.9). And there're some frequently questions quoted as below:
"I am a novice iMovie user. I have footage in a 60fps camera that I imported into iMovie. When I import it, either with or without optimization, the video is very jumpy and the audio doesn't time with the footage at all. My project is 30fps, since I thought that was closest to what I want. Also, the end of the clips is cut off. I think it is importing it as 30fps, which makes it play the video slow and cuts the rest of the clip when the clip duration ends. When I view the movies in finder, I don't see this problem, suggesting that this is an iMovie issue. 30fps footage imports into iMovie correctly. Is there any way to import the video correctly so that I don't have this problem, even if it means downgrading the quality?"
"I am trying to import some video from an SD card. When the import screen comes up, it shows 60fps with the circle and slash through it for all the thumbnails. Is it still possible to get this imported into iMovie, even though it doesn't support 60fps?"
The reason that we can't successuflly transfer the recorded AVCHD footages at 60fps to iMovie is limited by iMovie itself. Currently 60fps footage is a very non-standard format and it's unsupported by iMovie. With the lastest iMovie 11, I was hoping that Apple would add support for additional codecs like the 60fps or 50fps (In Europe) videos, like the relatively new (yet, increasingly popular) AVCHD 2.0; however, it did not. As a result, working with video from cameras shooting 60fps footage in iMovie brings a bit of a problem when importing the files from your camera.
To be able to edit your HD video in iMovie, we suggest you set your camcorder to record in AVCHD 60i or AVCHD 24i or MP4. However, working with AVCHD 2.0 files at 60fps requires another step to get the footage into iMovie 08/09/11. The easy workaround is to copy the video files to the hard drive, and then rewrap the 1080 60fps (.mts and .m2ts) videos into Apple InterMediate Codec(AIC) MOV movies ensuring maximum editing quality and efficiency when working with iMovie.
I don'd know whether there is a free AVCHD Converter to use. But I know Brorsoft has an great utility Mac AVCHD Converter. It is free, clean, safe to install and use. If you are editing 60fps video with iMovie smoothly, chances are you already installed this great free utility. If not, download the free trial from here.
[size="4"]The Steps: Converting 60fps AVCHD 2.0 footage for iMovie[/size]
Step 1: Download, install and launch the program and then click the "Add" button to load the .mts videos. You can also use the drag-and-drop method to load the 60fps files to the application UI.
The program is an easy-to-use yet professional app which can be used on Mac OS X 10.9 Mavericks, 10.8 Mountain Lion, 10.7 Lion, Snow Leopard 10.6, Leopard 10.5. With it, you can effortlessly transcode AVCHD MTS to AIC MOV for iMovie 08/09/11, Apple ProRes MOV for Final Cut Pro X/7/6, DNxHD for Avid MC, etc. and it works well and converts flawlessly.
Step 2: Click the Format box and select iMovie --> Apple InterMediate Codec(AIC) (*.mov) as output format.
Tip: Please set 1920*1080 as video size, 30fps for users in NTSC area and 25fps in PAL area as video bitrate to get the best video quality. If you want to get a smaller ones, please decrease the video size from 1920*1080 to 1440*1080 or 1280*720.
Step 3: Click the Arrow button under the preview window, and the Mac AVCHD Converter will start transcode 60fps AVCHD MTS/M2TS to AIC MOV for iMovie 08/09/11 on Mac.
After the conversion, click "Open" to locate the result videos and then launch iMovie,choose File > Import > Movies..., to import the converted 60fps AVCHD footage into iMovie; prepare for editing. You will find that it's just a piece of cake to edit 60fps video files in iMovie.
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Source:Does iMovie'11 support 60fps video?