Export At 25fps, not 30fps?

So my question is, how can I export a video from QuickTime and specifically choose a framerate of 25 fps?

You can't using QT X if you imported/captured the HDV content as HDV1080i50 rather than HDV1080p25. QT X employs a context adaptive approach to the H.264 1080p exports which limits frame rates in excess of 30 fps to 30 fps and matches frame rates less than or equal to 30 fps. Had you captured the source content as HDV1080p25, then the H.264 1080p export would also be 25 fps. However, as things stand, you will have to re-compress the HDV1080i50 file using software having a manual target frame rate option. As previously mentioned, this may include QT 7 Pro, MPEG Streamclip, as well as, the video editor used to capture the original source content.

The file that I have originated from an AVCHD Progressive camcorder. It's 1080/50p (progressive, not interlaced).

Sorry, misunderstood your work flow. Had thought you were importing HD content via a video editor and were exporting to H.264 with QT X. If possible, best approach would be to record as 1080p25 if camcorder settings allow this to avoid this problem. QT based apps which allow you to change the frame rate normally apply a simple "drop frame" algorithm. MPEG Streamclip allows you to "blend" frames but this has a tendency to "soften" or blur the combined frames. Haven't tried the latest version of Compressor from the App store, but it may (or may not) have the best algorithm for changing the frame rate. In any event, I normally look for a work flow that avoids changing frame rates if at all possible.

Apple's iMovie '11 and QuickTime Player X should be the apps required to solve my consumer-level problem, but they're not modern enough.

Are you saying iMovie '11 no longer imports AVCHD directly from a supported camcorder by converting it to AIC/Linear PCM at the size and standard specified for the new project? (Not a major iMovie user so I was unaware of such major changes if true.)

Thanks for posting the information. As I see it you have two work flows here. You can record the content as 1080p50 or 1080i50—not much of a surprise. If you recored at 1080p50 and want to reduce the frame rate to 25 fps, you will have to recompress the file to eliminate half the frames but the result remains progressive without comb artifacts. If you record at 1080i50, then the file will have to be deinterlaced or decombed (I believe QT X may automatically do this) but may have to deal with combing artifacts or decombing motion blur in merged progressive frame areas with high magnitude motion vectors.

Both types of recordings can be delt with using various software applications. If you are copying raw footage directly to your platform, the two that come to mind most quickly would be Voltaic and HandBrake.

Since I do not own an AVCHD camcorder, I don't have any personal experience with Voltiac but AVCHD users have frequently recommended it for the conversion of content copied directly from a camcorder to computer without benefit of conversion by a video editor. As I understand it, this app is basically a QT 7 Pro converter with built-in support for raw AVCHD/AC3 video files. Like QT 7 Pro, it will allow you to either deinterlace 1080i50 content or reduce 1080p50 content to a user selected frame rate at 1080, 720 or SD resolutions using custom or Apple mobile device target settings.

My personal choice would probably be HandBrake here but that is because I am both more familiar with it and it offers more user options which you have yet to indicate may be of importance to you. For instance, I normally convert using an anamorphic strategy so as to transcode the souce file using its original encode matrix and may or may not correct the output display as needed or change non-anamorphic content into anamorphic content to both conserve storage space and increase bandwidth efficiency for multiple TV device sharing my home networks. In addition a can create multiple custom export presets based on the resolution, complexity, or quality of the content being processed.

And, while frame rate is of less importance to HDTV users than analog TV viewers, this app also contains some specific frame rate options which will likely be of interest to you. For instance, your 1080p50 content can be exported using a 25 fps "Peak Framerate (VFR)" popup settings that will limit the final frame rate to a maximum of 25 fps and vary the time between successive frames to try and retain the perceptual quality of the 50 fps source file. Or, if you prefer, you can switch to a 25 fps "Constant" frame rate if you prefer. In addition, if you end up recording any 1080i50 files, you can select between the deinterlace or decomb options. (I tend to prefer the default setting for the the latter myself.) In addition, you have not indicated the target use for your movies and HandBrake seems to provide a very good balance between single-pass conversion time and final quality while probably cutting your file size in half for the same resolution depending on you actual settings.

Wanted to test Compressor so I stated a 2 hour plus file this morning. Unfortunately, it has been running for 9 hours and now says it won't finish for another 24. So much for speed in converting BD content using the 2-pass 10 Mbps TV2 target with both DD5.1 AC3 and AAC stereo audio. And that didn't include putting the data in an MOV file container so it would open in Compressor 4. (Same source file only takes 2-3 hours for the HandBrake single-pass conversion.) Guess I wont be using this work flow again but i figure I'll let it continue so I can evaluate the final results. (Probably just my luck the program will crash sometime tonight while I'm sleeping.)

In any event, you may want to run a couple of short test files through one of the conversion work processes and see if the results meet your requirements or create more problems than they cure.

In the meantime, I have lots of AVCHD Progress (50 fps) footage that I need to convert.

Still do not have an AVCHD camcorder but have been playing with AVCHD M2TS files sourced from Blu-ray. VLC indicates the original files are 1080p48 content and I have been testing use of a number of work flows and software options to include re-wrapping M2TS files to MKV and MOV file containers, VoltaicHD, RoadMovie, ClipWrap, and HandBrake.

I looked up VoltaicHD. It's available on the Mac App Store for £13.99. Unfortunately though, it's only got one review, and it's very negative. So I'm not sure about that solution:

http://itunes.apple.com/gb/app/voltaichd/id423957622?mt=12

On the one hand, I did not have the crashing problem mentioned by the first reviewer under either Lion or Mountain Lion. (I suspect the main problem was a combination of inappropriate settings and/or a failure to reset options properly after warning messages.) On the other hand, I too was not overly impressed with the options available. (60i to 24p reverse detelecine function seemed to work on test files downloaded from the internet but that would not really help you.) Apps used to rewrap 1080p48 files to MKV and MOV automatically changed 47.952 frame rates to 23.976 but then HandBrake also performs a similar function without having to purchase these applications. Was initially impressed by ClipWrap's ability to copy AVCHD video to MOV file container and convert audio to 6 channels of Little Endian Integer compressed audio which played well in QT 7 v7.6.6 and converted normally in HandBrake to H.264/AAC/AC3 but quickly became very disappointed when I learned that the MOV file had a problem playing its video in most other QT-based apps like QT X v10.2, iTunes, the TV, iMovie, MPEG Streamclip, etc. In short, I still consider the free HandBrake app to be the best option most of my work flows.

I have Handbrake already, and I'll give that a go, but I do find it very complicated compared to QuickTime Player 10.1.

Suggest you make two short test files. Start by making sure that you have the latest presets installed. Then recommend you try the TV 3 preset to convert one of your AVCHD files. (I.e., just load your file, select the TV 3 preset, and press the "Start" button.) If this option works the way I expect it to, it should automatically convert your Source file to a 1920x1080 file have an approximate frame rate of 25 fps using a variable frame rate (VFR) strategy that first halves your original frame rate and then removes any duplicate frames and adjusts the frame rate as needed to maintain playback that is as smooth as possible. The data rate is also lowered to create a measurably smaller target file.

For the second test, use the same source file and preset but change the output file name, change the "Framerate (FPS):" setting to 25 fps, and change the "Peak Framerate (VFR)" setting to "Constant Framerate" before pressing the "Start" button. When the conversion is complete, compare playback of the two files. In many cases the first file will have a frame rate of less than 25 fps but play smoother than the second file which has an exact frame rate of 25 fps. In any case, you can then decide if there is any visual difference between the two and select which settings best suits the needs of your current project.

ADDED:

In case it wasn't already obvious, I was referring to use of the free HandBrake app above to create the test files. I have a couple of test files containing Ken Burns type effects which better demonstrate the difference in the two frame rate strategies (VFR vs. Exact Constant) but since my examples were based on a 23.976 x2 (47.952) fps source file, I thought it best for you to make/evaluate your own content. However, I can temporarily post my sample files if you wish for comparison. Let me know one way or the other...