ffmpeg -i input.mov \
  -c:v libx264 -crf 18 -preset medium \
  -force_key_frames "expr:gte(t,n_forced*2)" \
  -c:a aac -b:a 192k \
  output.mp4

That example forces a keyframe every 2 seconds (template). For your use, you give an explicit list:

ffmpeg -i input.mov \
  -c:v libx264 -crf 18 -preset medium \
  -force_key_frames 12.345,27.900,55.033 \
  -c:a aac -b:a 192k \
  output.mp4

Those numbers are seconds.

2) Force keyframes by frame number (exact frames)

If you truly mean “frame 1234, 5678, …” you can do it, but FFmpeg’s force mechanism is timestamp-based, so you convert frames → seconds using the frame rate.

For CFR material with fps = F:

• time = frame / F

Then:

-force_key_frames  (frame1/F),(frame2/F),(frame3/F)

If your source is VFR (common in phone MOVs), “exact frame number” is slippery; you’ll want to normalize first (see “Third option” below).

3) Make them seek-safe cuts (IDR, not just any I-frame)

A “keyframe” request can result in an I-frame that still references earlier frames depending on encoder settings. For clean random access and splitting, you usually want closed GOP / IDR behavior.

For x264, add:

-x264-params "open-gop=0"

And it’s often wise to avoid scenecut inserting extras if you’re trying to make GOP structure predictable:

-x264-params "scenecut=0:open-gop=0"

(You can still force your scene-boundary keyframes; you’re just disabling the encoder’s own scene-triggered extras.)

So a more “editing-friendly” command is:

ffmpeg -i input.mov \
  -c:v libx264 -crf 18 -preset medium \
  -force_key_frames 12.345,27.900,55.033 \
  -x264-params "scenecut=0:open-gop=0" \
  -c:a aac -b:a 192k \
  output.mp4

4) Also align the GOP length so you don’t fight the encoder

If your forced points are far apart, the encoder will still place keyframes according to max GOP. You can set it explicitly:

• -g = maximum GOP length (in frames)
• -keyint_min = minimum distance between I-frames

Example at 30 fps, if you never want an automatic keyframe more often than every 10 seconds unless forced:

-g 300 -keyint_min 300

Then your forced list adds additional I-frames at scene cuts.

“Third option”: If your goal is future splitting with zero re-encode

If you want later splitting/modification to be painless, you have two fundamentally different strategies:

A) Interframe, but keyframes at cut points (what we just did)

Good компромise: small files, but later cuts are clean if you cut on keyframes.

B) Use an edit-friendly codec instead of H.264/H.265

If you’re going to do lots of slicing/rework, consider intraframe codecs (every frame is a keyframe): ProRes / DNxHR. Huge files, but trivial editing.

Example ProRes:

ffmpeg -i input.mov -c:v prores_ks -profile:v 3 -c:a pcm_s16le output.mov

Practical cautions (the stuff that bites)

• MP4 requires re-encode for keyframe changes. You can’t “mark existing frames as I-frames” without encoding (unless you already encoded with those keyframes).
• Accurate frame indexing needs CFR. For VFR sources, force CFR first (or work in timestamps only). If you must nail “frame N”, do:
  • transcode to CFR intermediate (-vf fps=...) or
  • use timestamps derived from the actual packet times (more annoying).

Japanese (inline, simple)

強制（きょうせい） キーフレーム は できます。— You can force keyframes.
場面（ばめん） の 切（き）れ 目（め） に キーフレーム を 入（い）れる と 編集（へんしゅう）しやすい。— Putting keyframes at scene boundaries makes editing easier.

Omitted but relevant

How to generate the scene-cut list automatically

FFmpeg can detect scene changes (select='gt(scene,0.3)') and you can extract timestamps, then feed them back into -force_key_frames. It’s doable, just a bit shell-script-y.

If you tell me whether your “exact frames” are literal frame numbers, or timestamps, and whether the MOV is phone/VFR, I can give you a command that’s actually correct for your case.