This is a complex topic, but here’s likely what’s happening: There are two methods for recording video camera material intended for slow motion:
- Higher frame rate recording: Each frame is recorded at a high frame rate (e.g., 120 fps), and the file metadata reflects that same frame rate. When imported into an NLE and slowed down (e.g., retimed to play back on a 30 fps timeline), the result is 25% speed or 4:1 slow motion. For example, a 1-second recording at 120 fps, if retimed to 25% speed in Final Cut Pro, would have a 4-second duration at 30 fps. On a 30 fps timeline, all frames of the original 120 fps clip would play back, providing smooth 4:1 slow motion.
- "In-Camera Retimed" Slow Motion: Often referred to as “HFR mode” (Blackmagic), “S&Q mode” (Sony), or “High Frame Rate Movie” (Canon), this method samples the sensor at a high frame rate (e.g., 120 fps) but retimes the clip in-camera to a lower playback frame rate (e.g., 30 fps). The resulting file’s metadata reflects the lower frame rate. Using the same 1-second recording example, the out-of-camera clip has a 4-second duration at 30 fps, with "baked-in" 25% slow motion.
Why Manufacturers Use This Method:
- Reduced Data Bandwidth: Retiming in-camera reduces the encoder's data bandwidth and the SD card’s performance requirements. For example, a 1-second recording at 120 fps does not need to write 120 fps per second but might spread the data over 2 seconds.
- User Convenience: It provides users with an out-of-camera slow-motion clip without requiring retiming in a “complicated” NLE.
Downsides:
- Non-Standard Terminology: Different manufacturers use inconsistent terminology, such as “Off-Speed” or “HFR” (Blackmagic), and include settings like “project frame rate” or “sensor frame rate.” Camera manuals rarely explain this well, and NLE documentation often doesn’t address it because it’s a varying camera-specific feature.
- Accidental Activation: On Blackmagic cameras, for instance, the hardware "HFR" button is poorly placed, leading to accidental activation. This frequently results in footage that’s difficult to manage in post.
- Metadata Issues and NLE Confusion: Non-standard clip metadata can confuse NLEs, producing unpredictable results. Traditional 120 fps clips can be retimed predictably in any NLE. However, with “baked-in” slow motion (e.g., 120 fps recorded as 30 fps), playback at 30 fps generally works fine, but attempting to retime such clips may cause erratic behavior due to the NLE’s rate-conforming algorithm.
- Camera-Specific Limitations: Certain cameras impose restrictions in HFR mode, such as disabling audio or limiting codec options. For example, the Canon R3 only supports "All-Intra" in HFR mode.
Regarding Dropped Frames:
It’s important to clarify the term “dropped frame.” Dropped frames during playback differ from a persistently missing frame in a clip or frames dropped during encoding.
In Final Cut Pro (Settings > Playback), the default setting is “If a frame drops, stop playback and warn.” This is an aesthetic courtesy, not an indicator of a critical issue. Many users, including myself, disable this setting.
If the "dropped frame" warning is being triggered because the setting is enabled, simply disabling it resolves the issue.
The fact that a “dropped frame” warning now appears in FCP (when it didn’t previously) or that one NLE flags the issue while another does not might be caused by non-standard method combined with a metadata “kludge." We can investigate further if this issue proves significant enough to warrant time-consuming analysis by experienced volunteers.
