ND for video: Variable vs Fixed

Discussion in 'Video' started by Fiodor, Sep 18, 2020.

  1. Basically, variable NDs are more fast and practical, with a bit lower quality, and more uneven tint.

    And the problem with the fixed ones is that, for example, you have one filter which reduces 6 stops, but you need to reduce 7 stops. So you have to put a 1 stop ND filter on top (which could affect definition or color), or sacrifice one variable that you wouldn’t like to change (aperture, shutter speed or ISO).

    1- I would like to read experiences from people who use fixed NDs.

    How do you adjust the entrance of light if one filter alone doesn’t give the proper exposure?

    If there aren’t fast movements, would you increase the shutter speed? Because you won’t notice the difference and you don’t want to increase the depth of field closing the aperture.

    2- I learnt that variables, because of the polarization effect, could flat skin tones, as if they had too much make-up, because of the reduction of reflections. Have you noticed this effect? This video shows a technique to optimize the VND to have better colors and to reduce skin flatness:



    What do you think? Have you practiced this technique?

    3- Let’s say you need continuity between shots in a scene. But at the same time you could be using the sunlight, which might change a bit and so as a consequence you may rotate the VND ring to adjust the exposure. I guess this adjustment could modulate color a bit, which is not desirable in contiguous shots in a scene, like in a shot reverse shot sequence.

    So, in this scenario, would it be advisable to leave the VND untouched? (if that is possible, because I guess you could move the ring accidentally).
     
  2. The raison d'être for ND filters in video is to manage shutter speed and depth of field. If the shutter speed is faster than 1/125, objects in motion tend to show a stroboscopic effect. Likewise a shallow depth of field helps isolate subjects from the background, and allow a change of focus for dramatic effect. I would estimate that 90% of my video is shot with the lens wide open with a shutter speed of 1/30 to 1/60 second.

    Many professional and semi-professional video cameras have a built-in variable ND filter, typically 2,4 and 6 stops. Since professional video lenses use 95 to 105 mm filters, it becomes very expensive (and bulky) to carry sets of screw in filters. Although hybrid cameras, like a Sony A7iii, take cinematic quality video, they are not equipped with the same features. For this reason I opted to buy a variable ND filter, sized 95 mm, for for this purpose. An alternative is to use square filters in a matte box, which is one-size-fits all and can stay in place when lenses are changed. A 4" matte box is a big piece of hardware, and must be mounted on rails, which in turn, requires a cage of some sort.

    I do not change the ND setting within scenes, rather for a particular setting. A top of the line filter, like B+W, does not have a significant color shift or polarizing effect, but isn't exactly cheap in that size. I have single value ND filters in smaller sizes for video and other purposes, and a matte box with flags for the exceptions.
     
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  3. @Ed_Ingold

    Thanks for the reply.

    Yeah, you are right. The rule for normal shutter speed is to set it to twice the frames per second. The lower the SS, the more the amount of motion blur. And the higher it is, the more the stroboscopic effect, as you noted.

    So, what VND do you use for your Sony? A B+W?
     
  4. Yes, B+W, and I'm almost embarrassed how much it cost. However I shoot video as a profession, and I'd rather be embarrassed at the price than the results. The B+W doesn't have a pin to turn the filter, but it's still oversized. I have to put the hood on first, or use a matte box. That's how a 2 pound Sony and 3 pound lens becomes a 12 pound shoulder rig.

    The standard 180 deg shutter angle harks to the Mitchell camera era. There's nothing keeping you from using a 360 deg angle (or larger) with digital cameras. I rarely deviate from 1/60 second at 1080p60. The shutter can actually span frames for extreme low light situations, and still play back at the correct rate (albeit blurred and jerky).
     
    Last edited: Sep 21, 2020
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  5. The VND filter in my Sony FS5M2 is behind the lens, and has several options: off, step-variable, continuously variable and auto. Sony takes ND very seriously, and added the same feature to the FS7M2 and FX9. The off setting substitutes a clear filter for the (LCD?) ND filter, since glass of any thickness affects the focus point in that location. ND filters for still use are located in my camera bag, where they have remained, untouched, since 2017. They were last used to photograph waterfalls in Iceland, but I got over it quickly.

    Video in bright sunlight benefits greatly from log gamma profiles, or a simplified version called "HDR". REC.709 color has a gamut smaller than sRGB, and handles high-contrast situations poorly. REC.2020 (4K) is better, but still no cigar. Log 2 or Log 3 gamma always requires post processing, whereas HDR can be used directly with many TV sets and editing software.

    I'm not being snarky about ND use, rather relating my real-life experience. Yours may vary. They have their place, especially for video and to give a sense of motion in water. Almost as effective for moving water is shooting several frames and blending them in software. You don't even need a tripod.
     
    Last edited: Sep 22, 2020
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  6. How do built-in ND filters work like I have in my Sony P&S RX100iv?
     
  7. I checked specifications for the Sony RX100iv, and find no mention of a built-in ND filter.

    For purposes of discussion, modern cameras with built-in VND filters probably use liquid crystal technology to twist polarization between two polarizers. Twenty years ago, I bought a large Panasonic news-type (ENG) camera, which had a mechanical filter with several windows for different densities. I'm pretty sure Sony shoulder-mount cameras I used afterwards were mechanical as well.
     
  8. Ed, The RX100iv has an ND filter. It was dropped in the latter models' revisions. You may be looking at manual for the latter models. Here's my manual for the M4 release.
     
  9. Here's from the manual
    ND Filter.jpg
     
  10. I don’t know how the NDs of your cameras work like. You could record some video with and without ND and compare them.

    The only thing I know about Sony and NDs is that a couple of times I shot with cameras like the Z1, a semi-pro HDV camera, and I used the ND. I never did a quality test, I just used the ND to shoot with a wider aperture, and it worked fine.

    But I guess a Z1 has nothing to do with your camera, which I think is a pocket one.

    Just try it.

    I wish my Fuji X-T3 had built-in NDs.
     
  11. When I set 1/60 or double the 30 frames per second I'm recording at, the ND will kick in so I'm not over exposing on bright sunny days. It works fine. My question is, how is the ND implemented? Is there a software adjustment? Or is there some hardware ND filter in the camera that is used?
     
  12. If the ND is continuously variable, it is almost certainly based on liquid crystal technology. If it is stepwise variable only, it can be either mechanical or liquid crystal based. In the absence of technical data, that's the best I can do.
     

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