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image extraction from 4k clip

Discussion in 'Filmmaking' started by adwb, Dec 3, 2018.

  1. adwb

    adwb TalkEmount Regular

    Sep 30, 2015
    Bristol UK
    this I hope a simple question for any one wiser than me

    if I shoot a clip in 4K with a Sony A 7r2 in full frame mode 48 M and then in crop mode 18 M will there be a difference in the limit that I can zoom in on a portion of that clip when editing?

    Similar question also if using a 1 inch 20megabit sensor ? Compared to the above.

    this assumes all shot at the highest bit rate available
  2. WNG

    WNG TalkEmount Hall of Famer

    Aug 12, 2014
    Arrid Zone-A, USA
    Well, since 4K is defined as X by Y pixels, the pixel density and total number of pixels will be the same regardless of modes or sensors. Bit rate won't affect the available pixels either.
    If my statement(s) are wrong, someone kindly correct me.
  3. bdbits

    bdbits TalkEmount All-Pro

    Sep 10, 2015
    I am mostly a video newbie and not 100% sure of the math here but... 4k is 3840 × 2160, or 8M pixels per frame. If we assume 12 bits for each pixel, that's over 12M bytes per frame. If shooting at say 24 fps (frames/second), that would be nearly 300M/second. I assume the camera is doing some on-the-fly lossy compression or something as it is obviously not saving that much data. I think that is where the bit-rate becomes a factor - the higher the bit rate, the less lossy the compression and thus greater image quality.

    If I am correct, to get back to adwb/Alistair's question, I think the higher the bit-rate, the better IQ you will get. The larger sensor size will give a better starting point (FF vs APS-C vs 1-inch) though, so I would think FF would trump APS-C would trump 1-inch, yet I often run across articles suggesting that video is best shot in crop mode. So I am clearly out of my knowledge base and must be off somewhere.

    It's an interesting question. Surely someone here is video savvy and knows how this all works. :hmmm:
  4. WNG

    WNG TalkEmount Hall of Famer

    Aug 12, 2014
    Arrid Zone-A, USA
    I was simply addressing the specific any limit to zooming/cropping during post editing. Which there shouldn't be given the fixed and uniform pixel count.
    How good the starting footage is a different story, that may dictate the detail quality. I think 6K downsampled 4K yields the best results. The recommendation to use crop mode (super 35 ?) I believe comes from high res FF sensors that incorporated line-skipping to achieve 4K.

    I'm also speculating from time reading and theory, not actual production experience.
  5. Kinlau

    Kinlau New to TalkEmount

    May 3, 2012
    For extracting 4K clips, I’ve only done it with my Panasonic G9 and FZ2500, and the quality can be very good as long as you keep something’s in mind.

    If the 4K is down sampled instead of line skipping, the quality will be very good. Line skipping often causes jagged edges, so the cropped mode might be better, but it also depends on the subject.

    The Sony’s also have much less jello in cropped mode, so if the subject is moving, the cropped mode will again be better.

    I also find 4K 60p footage much better for frame grabs than 30p or 24p, but I don’t think you have 4k60p available.

    1 inch sensors can actually be better for this as the readout is usually faster, less jello, better optimized for video.
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