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Choosing the appropriate frame rate for your underwater video

Discussion in 'Underwater Videography' started by Interceptor121, Nov 22, 2019.

  1. Interceptor121

    Interceptor121 PADI Pro

    Because the bit-rate is the same for 30 and 60 so if you are increasing frames you are decreasing image quality in your case
  2. spoolin01

    spoolin01 Solo Diver

    # of Dives: 500 - 999
    Location: SF Bay Area, CA
    Looked better to me! But I get your take.
  3. Interceptor121

    Interceptor121 PADI Pro

  4. matdiver

    matdiver Nassau Grouper

    # of Dives: 500 - 999
    Location: Belgium
    Hey Interceptor,
    interesting thread. I generally use my GH5 to shoot 50 fps, 180° shutter (i.e. 100) underwater, but often vary it, e.g. for filming something under an overhang if I don't have lights, or I just don't want to shine something very bright at something like a porcupine puffer. Then, I drop the frame rate and shutter to 50, or even shutter 25, if the subject is still/very slow moving. The main reason I shoot 50 fps, is so that I then have the option of slowing down something quick and unexpected, interesting fish behaviour e.g., when it comes to editing. It is also useful for taking stills from your video, but I agree that it's boring when it's used for slo mo too much. Stills taken from 4K video have a resolution of about 8 MP, so they're more than enough for social media or A4 prints. At 50 fps, I think there's a better chance of getting the action sufficiently frozen to make a decent still.

    Anyway, what I wanted to ask you about was this. On your website you compare the different bitrates of 50 and 25 fps (150 vs 100 Mbps respectively), but I'm not sure how meaningful a comparison this is. Surely, with 50 fps, you have 2 "I" frames per second, whereas with 25 fps you only have one? One second of real-time has to be split into 50 or 25 frames, and I don't see that it would be a direct comparison. The 25 fps setting would have more motion to interpolate (I mean "to account for") since there are fewer frames in that second, but the subject will probably still be moving. Isn't it possible that that would affect the efficiency of the codec?

    On slo mo VFR with my GH5, the bitrate always seems rather "low" anyway, around 40 Mbps for 1920p at 20% speed, which is capturing at 125 fps for playback at 25 fps. However, the only hit to image quality (apart from the obvious problem of comparing 4k and FHD) is that with such high framerates you really need to respect the 180° rule if you you want to have clear images to play back slowly, which means a shutter of 250, and so possible exposure issues leading to a grainy image.

    Btw, have you ever looked at that 6K photo function on the GH5 from a video perspective? It basically gives you 30 "I" frames a second, 4992 x 3744 resolution, and works out at about 200 Mbps.

    neehooya likes this.
  5. Interceptor121

    Interceptor121 PADI Pro

    Hi Matt
    I have actually looked into the GOP structure and while for single frame rate there is one I frame in a GOP of 12 for double (50/60) the GOP is 24 so in both cases there are around two I frames per second and therefore the 50/60p options are not better at predicting changes in the frame. As the 50/60 p have a lot of predicted frames the compression is very efficient and therefore only a 50% increased bitrate gives the same visual quality. The codec that better detects changes in motion is the 400 all intra as it doesn’t use predictions at all. Regarding shutter speed 1/100 is not enough to freeze motion you need 1/200 if you don’t use a strobe and I would never make prints off a frame grab anyway
    The analysis i did on the GH5 is here
    Panasonic GH5 Demystifying Movie recording settings

    I haven’t looked at that function as 200 mbps is low I use 400 Mbps ALL intra that gives me 24/25/30 fps all I
    neehooya and matdiver like this.

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