Filming cine film from screens - optimizing DR - hmmm, I guess a little bit 'zoney'

Discussion in 'Exposure Discussion' started by nick mulder, Feb 27, 2010.

  1. nick mulder

    nick mulder Subscriber

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    Hi,

    I want to shoot video footage off an LCD (or an HD CRT monitor if I can get one) with a 16mm camera. I'm trying to learn how to optimize the output so that it would appear as if the footage were shot on film in the first instance so I'm keen to learn how to optimize the DR/gamma/luminance distribution from screen to film.

    (Although the discussion may involve a digital component the end goal or most of the work is done with film itself so I hope I can keep this 'APUG')

    Anyways, imagine I had a video of a contrasty outdoor sunny scene in which its max and min luminance values hit the actual %100 and %0 values of the screen (I'll make them do that) - I also have dialed in an %18 grey and have taken the following quick readings with a spot meter from the screen:

    EV % 0 = 4
    EV %18 = 7
    EV %100 = 10

    So that is a DR of 6 and I have my baseline exposure set with the %18 grey.

    I think I'm going to be needing to push the film to get the required range of exposure values distributed on the film as it would have if I were to shoot the footage in camera in the first place - right ?

    Is it as simple as that ?

    What don't I know about ? Am I about to get the range correct but send all the nice visual data to peak or shadows ? Will I lose the shadows in the push and have to pump them up a little in the file ?

    I do have some old old film here that was used in the 80's to shoot strictly off CRT monitors, I have noted it is very contrasty compared to standard films, perhaps I should start with that stock as a baseline (?)

    It's not lost on me that I have control over both the file going to the screen (I can play with it 'in post'), the screen itself (brightness, contrast settings) aaaand photochemically with the film itself - I'd like to nail this with the end goal being that I can have all my composite effects and CGI footage have a 'shot with cine and developed in a bucket in the backyard' aesthetic (well, there's more to it than that, but I don't want to get off topic)

    Also - as an aside why isn't it %50 grey ??? How did %18 grey become the mid point here ? The readings show it follows er... 'properly' too :confused:

    Kind Regards
     
  2. ic-racer

    ic-racer Member

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    Negative or reversal? If you are shooting negative, it will be easier as you can fine tune when you print. Or are you going back to digital/video?

    If doing reversal, I'd just get a ballpark exposure an send off 25 feet for processing and adjust from there with trial and error. Otherwise you will need the sensitometric data from the film you are using to figure it out beforehand. If shooting reversal you would need to know the range of the film and then base the exposure on the high value on the screen, etc.

    Are you doing this single frame at a time?
     
  3. nick mulder

    nick mulder Subscriber

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    I can home develop in both neg or reversal - not sure which I'll use in the end - I think neg would make more sense but would require more tuning than reversal once you consider reversals lower range might suit the screen more than the neg which will soak up the light with ease...

    Single frame yes so in essence it's a still shoot - no worries re. refresh rates, flicker etc...

    The sensitometric data will be hard to come by - I'm just going with the idea that at least the DR will be more than 6 stops, I mean you might even get 6 stops on the linear part of the curve huh (?)
     
  4. nick mulder

    nick mulder Subscriber

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    I should have mentioned it will be black and white also - likely some flavor of Plus-X or the old CRT stock I have here
     
  5. Stephen Benskin

    Stephen Benskin Member

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    Have you considered the sinc problem? Film is 24 fps and video is 30. When they show video in motion pictures, they have to use a special process in order to eliminate flame lines, otherwise it will look like the screens at NASA during the Apollo missions.

    18% gray is the perceived visual middle. When you take a gray scale, 18% is the approximate value we see as the middle. It is now considered 19 point something. From a photographic standpoint, 12% is considered middle when the highlight is at 100% Reflectance and the average luminance range is 7 1/3 stops and when it is measured at the film plane. When you incorporate average flare, the highlights at 100% Reflectance fall approximately 0.94 log-H units above the exposure value at 12% Reflectance and the shadows fall approximately 0.94 units below, thus making 12% the middle. The middle for the average scene Reflectance outside of the camera is around 8% to 9%.
     
  6. nick mulder

    nick mulder Subscriber

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    Film can be shot in TV speeds also, any frame rate really ... video is any frame rate also - 30 fps (29.97fps) is just NTSC, a frame rate popular in a certain area of the world.

    I really don't want to bog down the thread with this stuff though - as I said above "in essence it's a still shoot - no worries re. refresh rates, flicker etc..." - just really interested in nailing the DR/luminance aspect.

    "18% gray is the perceived visual middle"

    Which again, begs the question: why isn't it %50 grey ? Is there a log/lin going on ?

    Anyway ... I'm just keen on the DR/luminance issue so I shouldn't go off topic myself :tongue::rolleyes:
     
  7. tiberiustibz

    tiberiustibz Member

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    Because if one averages a typical scene, throwing it completely out of focus, on average that will be 18% grey. Therefore, averaging meters are designed to assume that every scene will produce an average of 18% grey.

    You should be able to set the refresh rate to 24 or a multiple thereof.

    This just strikes me as a difficult proposition. To start just shoot it straight and see if it's passable. With B+W you can probably push to your hearts content. Maybe try an N+1 first?
     
  8. ic-racer

    ic-racer Member

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    In terms of the 18% not seeming to be in the middle, remember the HD curve is drawn with log axes. There is a good description of log vs linear representations of light intensity in that Kodak PDF.

    Since you are going to be processing the film yourself, I would tackle this similar to printing a still negative on paper. Adjust exposure and contrast by trial and error.
     
  9. nick mulder

    nick mulder Subscriber

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    "on average that will be %18 grey" - hmmm, so typical scenes are darker on average ?

    I'm just going to shoot it one frame at a time - automated - but there is no distinction between cine and stills using this method... I've done it before, It works :wink: no need to discuss it - ugh! (sorry if I sound ungrateful for the advice, but I did suspect we'd end up talking about this issue as for whatever reason it's the first hiccup in the process)

    This really is just a zone system question - cine doesn't need to come into it.

    Hmmm, I just don't think straight will be acceptable - if the footage is of a scene that should have a DR of sunlight to shadow this will be in excess of the 6 stops that the monitor is giving me...

    I will underexpose and push to compensate, which will solve the DR issue but I want to know how to regain my shadow detail - the luminance distribution might be pushed left/right/up/down the curve in an unnatural fashion - so I'm wondering how to fix for that - how to counter the effects of the push, but only the distribution of the information on the curve not the actual shape of it (which is set for the DR expansion)

    make sense ? Likely I'm wrong somewhere ...

    I think I'll just have to test test test - was hoping to get in the ball park from word go however :smile:
     
  10. nick mulder

    nick mulder Subscriber

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    Right - as I expected log/lin - ok, solved! :wink:

    Hmmm, the film will either be scanned/telecined or simply shot off a wall with a 4K video camera (I know, I know ... hoops to jump through galore and seemingly contradictory processes but there are reasons why I'm doing it this way, I'm purposefully not bringing them up for fear of prompting off topic discussion :D)

    I'd like to optimize the neg for the scan/telecine as it will be going through a 'one-light' process and would be best to match the rest of the er, real footage which will be shot in camera (16mm)
     
  11. polyglot

    polyglot Member

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    Obviously, your shutter speed has to be slow enough to encompass a full scan of the screen, which is about 90% of the refresh period; otherwise, you will see only a narrow stripe of image on the screen.

    More sync issues: monitors normally run with a refresh rate that is much higher than the frame rate of the video they're displaying. 60, 72, 75, 80, 85Hz (PC monitors), 100 and 120Hz (frame-doubling LCD TVs) are all fairly-widely used refresh values. This may well cause you problems if the monitor's refresh rate is not an integer multiple of the frame-rate of the video that is being shown on the monitor and it will definitely cause problems if the monitor's refresh rate is not synchronized with your shutter.

    For example, 25Hz (50 fields/sec interlaced, PAL) video on an 80Hz PC monitor will mean that most frames are displayed for 3 refreshes but some frames will be displayed for 4 refreshes, which causes a slight staggering effect that's not visible to a human. If you shoot it at 80fps and then throw away a bunch of frames to get back to 25fps, it will work, but I'm guessing you can't do that... If you shoot it at 25Hz, the monitor's refresh will not be synchronized with your shutter, which means you will get black bars walking up/down the screen.

    If using a CRT, you must ensure your shutter is synchronized to the vertical refresh of the monitor. It's not enough to have them at the same rate, they must occur at the same time or you get black bars through your image. Wrong rate and the bar moves; right rate but unsynchronized and the bar just sits there in your image.

    With LCDs, it's a little easier because the image doesn't scan, but it is refreshed using a scanning process. If you shoot an LCD with an unsynchronized shutter, you will get horizontal tearing artefacts where you capture part of one frame and part of another. That looks bad because the frames are obviously different, and you can see the discontinuity at a tear-line across the screen. This line will move around in the same manner as the black bar on a CRT depending on your synchronization or lack thereof.

    If you're shooting a device with a VGA or DVI interface, you can pull the vertical sync signal from the cable using a bit of electronics and if your shutter is flexible enough, synchronize it to the vertical sync pulses. That will require a bit of electronics to process the sync signal and some sort of interface to the shutter.

    Can your 16mm camera shoot single frames? If so, it might be much easier to set it to a longer shutter speed that encompasses a few refresh cycles of an LCD and then capture the video one frame at a time to your film. That way, you avoid problems synchronizing your shutter with the monitor refresh and you also avoid telecine and pull-down problems with the video frame-rate as long as you have a video source of the same frame rate that you want your film to be at.

    One more thing: "full HD" resolution is only 1920x1080 or about 2MP, which is a small fraction of what you can get on 16mm. If your 16mm camera is focused on the screen, you'll be able to see pixels. Might want to defocus it just enough to blur the pixel edges out, or at least make sure the screen-door effect (black lines between the pixels) is not recorded on film.

    I would definitely recommend neg film instead of reversal because I think you'll need to use some adjustments in printing to control your black and white points. 80's CRTs had very very poor contrast; modern monitors are better but in some cases, their "black" isn't very black.
     
  12. polyglot

    polyglot Member

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    Oh yeah, I think there are hybrid print-to-film services. You give them a video file and a spool of film and get exposed film back. Might not be cheap, might not fit with your aesthetic, but there are people who will digitally print images directly to film for you.
     
  13. nick mulder

    nick mulder Subscriber

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    Thanks - but I'm afraid this is just going to get off topic...

    Have you read what I've written ???

    In reply:

    Super16 with a modern 50ASA stock levels out at about 2K res - any more and you're just making the grain look more 'correct'... I know this from both experience and other professionals advice online (if that holds any weight, who knows)

    I've tried (read what I written already) - but I dont know how to say this more clearly:

    I DONT NEED ADVICE RE. TIMING/SYNC ISSUES

    I've done this before, I've succeeded in it - I know about it...

    What I did was high con (titles) and could be individually maximized at the scanning step - this time I'm pretending out of interest that I can only get a 'one light' scan/telecine and that the LUMINANCE needs proper care in a more continuos scene...

    IMAGINE I WANT TO TAKE ONE PHOTO OFF A SCREEN - AS THIS IS IN ESSENCE ALL I'M DOING (JUST A LOT OF THEM)...

    Put simply:

    What are the effects of the required push to bring a scene (** forget it's a screen **) with a DR of 6 to optimize the latitude of film *on* the distribution of shadows and highlights along the curve ?



    excuse the caps, bold and italics - I'm frustrated, long day ...




    :munch:
     
  14. rmolson

    rmolson Member

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    filming cine

    This has absolutely nothing to do with your question, but it reminded me of the story of the very first public demonstration of TV. The press showed up with their photographers and 4x5 speed Graphics And one by one they lined up in front of the set and shot a picture using flash bulbs! The engineers demonstrating were flabbergasted and never said a word Never heard what happened when they all got back to their papers and developed their film!
     
  15. nick mulder

    nick mulder Subscriber

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    heh heh - nice story

    I've found I've got a bit further with the same question at hybridphoto with one user and cinematography.com with a cine tab tech - interesting stuff !

    I'm sidetracked with other work at the mo' tho and cant get down to it quite yet ...