Lazaclip

Sony FX3 left an .RSV file instead of an MP4

Updated 3 November 2026

The FX3 is often the camera that runs the longest takes of the day: a ceremony, a speech, an interview, a stage performance. That is also when a battery runs flat or someone knocks the card door open. Afterwards the card holds a file like C0057.RSV where the C0057.MP4 should be, and Premiere, Resolve and Final Cut won't touch it.

This guide is specific to the FX3: where the file is, what the dual card slots mean for you, what to try first, and how to get the clip back on a Windows PC.

What happened to the file

While the FX3 records, it writes video and sound into a working .RSV file. Pressing stop adds the index (the table that tells a player where each frame and each block of audio is) and renames the file to .MP4. A power cut skips both steps. The recording is in the file; the index isn't. The full explanation is in What is an RSV file?

The FX3 records in three Sony formats that end up as .MP4: XAVC S and XAVC S-I (both H.264) and XAVC HS (H.265). An interrupted recording in any of them leaves the same kind of .RSV.

Before anything else: protect the card

  • Don't format the card, and don't accept a format prompt from the camera.
  • Don't record anything else on it. If you are still on the shoot, put a fresh card in and keep that one aside.
  • Don't delete the .RSV, however broken it looks.
  • Copy the card first. In a card reader, copy the whole PRIVATE folder to your PC or an external drive. Clips are in PRIVATE/M4ROOT/CLIP; copying the whole M4ROOT keeps the camera's other files next to them.

CFexpress Type A cards need a reader that supports them; an ordinary SD reader won't take them. If you only have an SD reader at hand, wait rather than improvising with the card.

Check the second slot

The FX3 has two card slots. If you had it set to record to both cards at the same time and only one card was pulled out, the other card may hold a finished .MP4 of the same take. Check it before you repair anything. If the battery died, both cards will normally hold an .RSV, and both are worth copying: you then have two copies of the same data.

If you were recording to one card and using the second slot for overflow, the take is on the card that was active at the moment of the cut.

The free first try: the camera itself

Some Sony cameras try to recover an interrupted clip on their own when you put the card back into the same camera with a charged battery. Whether that happens depends on the model, the firmware and how the recording was cut off, so treat it as a free attempt, not a sure thing. Do it only after you have copied the card:

  1. Charge the battery (or use a fresh one).
  2. Put the card back into the same FX3 it was recorded in, in the same slot.
  3. Switch on and wait. If the camera shows a message about recovering or repairing data, let it finish and don't switch off in the middle.
  4. Check PRIVATE/M4ROOT/CLIP again in the card reader. If a new .MP4 with the same number appeared, you are done.

Recover the FX3 clip with Lazaclip

If the .RSV is still there, Lazaclip is our Windows 10/11 program for this (we make it, so we are not neutral). It reads the recorded data, rebuilds the index and saves a normal .MP4, on your PC and without uploading anything:

  1. Open the card in the reader, or the folder you copied. Every .RSV on it is listed with a check of whether it can be rebuilt.
  2. Click Preview 10 s: the first ten seconds of the take, with a trial watermark, open in your video player. The scan and the preview are free.
  3. With a license, save the full video to a drive on your PC. If both FX3 cards have an .RSV, or several takes were cut off, Repair all handles them in one go.

The program never changes the .RSV and won't save results back onto the card. You need free space for about the size of the .RSV: a long 4K take can be tens of gigabytes.

What we have and haven't tested

We tested Lazaclip on recordings from the A7S III in XAVC S (8-bit 4:2:0 and 10-bit 4:2:2), XAVC S-I and XAVC HS: rebuilt without any reference clip, with frames and audio identical to the original up to the last unfinished group of frames before the cut.

The FX3 shares its sensor and much of its video system with the A7S III and uses the same XAVC S / XAVC HS recording format. We haven't tested FX3 files yet; the free preview shows whether the program can read your file before you pay. If it can't, write to us at support@aphexcoding.tech; a description of the recording mode helps.

After the repair

  • The last second or so before the power cut is lost: it was never completely written.
  • S-Log3 footage looks flat in the preview, exactly as it would straight out of the camera; the saved file is the original data, ready for your usual grade.
  • Import the repaired .MP4 like any other clip. The camera's sidecar .XML file for that take may be missing, so if your editor relies on it (for example for timecode), check that clip after import, and sync sound from an external recorder by waveform if needed.

Avoiding it on the next job

  • Start each long take with a charged battery, and keep a full spare in your pocket, not in the bag.
  • For ceremonies and long interviews, consider external power; check the FX3 manual for the power options it supports while recording.
  • Stop recording before you swap a card, and wait until the camera has finished writing before you open the card door or switch off.
  • Recording to both slots at once costs a second card but gives you a second copy of every take.

Other cameras: FX30, A7S III, and the general RSV file explainer.