To ND or Not to ND...

Andy_L wrote on 11/17/2011, 9:18 PM

That is the question. I'm trying to get comfortable with a Sony NX70U, and I'm kind of mystified by its base sensitivity/ISO rating. Properly exposed in bright sunlight in full manual, the camera shoots 1/400 sec fully stopped down (to f9.6)!

Obviously, this aperture with a 1/3" chip gives a lot of lens diffraction, even if the shutter speed were acceptable.

In full auto mode, some kind of ND filter or gain attenuation kicks in, allowing you to expose in sunlight at f4 and 1/288 of a second. A little better, I guess.

It just seems kind of bizarre to me that a modern video camera can't expose using a standard shutter speed (ie, 1/48 or 1/60) in bright light without adding ND filters in front of the lens.

Is this common in professional cams? Maybe people are getting so used to the 'video' look that these higher shutter speeds are considered normal, even for motion. Or am I really supposed to use a 6-stop ND filter in daylight?

Thoughts?

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farss wrote on 11/20/2011, 6:20 AM

"I'll have to do some experimenting with pans and shutter speeds and Vegas' motion blur and see what the result looks like."

It is almost impossible to recreate the motion blur from a shutter in post.
Both AE and Vegas can create the effect for objects that you animate in the application as can most CGI applications. You do this in Vegas using Supersampling, it does nothing for footage already shot, period. The Motion Blur that exists in Vegas blurs multiple frames, which can be very useful for certain things and effects. It could be useful if you'd shot say 60p and wanted 60i from it.
Of no use if you've shot 30p at 1/500 and you want it to look like you'd used 1/60.

I have found one plugin for AE that has a go at real motion blur but you have to supply the motion vector manually. That's probably quite tedious and a real problem with lots of things moving in different directions.

One approach that I've never found the time to try would be to use something like MVTools to convert say 30p to 120p, then use Vegas or AE's MB and then resample back down to 30p, that might work, maybe. A lot of processing involved even if it does work.

Bob.

Barry W. Hull wrote on 11/20/2011, 7:36 AM

Great discussion.

Back in 1992 at the Indy 500 I had a Sony camcorder in Hi8. If I recall it had some sort of "high speed" button on it, so of course I used it to film the race (had great seats last turn before the home stretch). The video was entirely unwatchable, so jerky it gave a headache. I was very dissapointed and never touched that button again.

Now I understand what those ND filters on my current camera are for, and also realize my still photography background, wanting a crystal clear picture, can be counter-intuitive to nice smooth video where a bit of blur is the way to go. I will use the ND filters on those bright sunny days.

This makes clear my recent post about what is the best recording mode, that choppy look in 30p and I could not figure it out. I think I just raised the bar on my video work.

musicvid10 wrote on 11/20/2011, 9:27 AM

I ran a bit of Bob's footage through motion blur in Vegas and i have to agree. My idea would only work to smooth video that is a "little" too sharp, and maybe not very well at all at IVTC or PAL frame rates.

It also occurred to me that the effects of fast shutter may be more pronounced to the human eye at 25p than at 30p -- moreso than the math would suggest (15%). Maybe perceived flicker reaches some kind of threshold at some point.

johnmeyer wrote on 11/20/2011, 12:42 PM

As always, there are plugins for AVISynth that can do "real" motion blur, using motion estimation. This would probably be at least as good (or bad) as AE, and certainly better than any simple-minded motion blur that just averages frames (ugh!). The search word for this is MFlowBlur.

farss wrote on 11/20/2011, 2:01 PM

"It also occurred to me that the effects of fast shutter may be more pronounced to the human eye at 25p than at 30p -- moreso than the math would suggest (15%). Maybe perceived flicker reaches some kind of threshold at some point. "

Absolutely.
Obviously the place where the most work has been done on this is for cinema. A 35mm film projector uses a 2 blade shutter, the eye is flashed twice with the same image. 8mm projectors use a 3 blade shutter to avoid noticeable flicker and stobing because of the lower frame rate of 8mm. Screen brightness and venue illumination are both carefully controlled and there's a considerable number of paramters to consider when shooting film at 24fps and nothing changes when shooting digital. Well actually something does change because digital film cameras also have Detail settings and yes, it seems edge enhancement is also a factor in human perception of this problem.

If you look at the video of mine on YT at 360 in YT's native image size it's kind of hard to see the problem. At 720p full screen it is more noticeable.

So if you take all that on board then mathematically you can see that the difference between 24/25 fps and 30fps is more than just the linear difference in the frame rate would suggest. 30p is oftenly promoted as a good compromise, it still retains some of the aesthetic of 24p without all the problems of acquisition and presentation.

Bob.

Laurence wrote on 11/21/2011, 5:31 PM

One of the issues with using DSLRs for video is the lack of built in ND filters. Purists get around this by using a matte box an square ND filters, but I find that an adjustable ND filter works quite well. Basically an adjustable and filter is made up of two polarized lenses attached together. As you turn the front one, it gets progressively darker. It works quite well in a run and gun situation where you don't want to be constantly changing ND filters.

musicvid10 wrote on 11/21/2011, 7:55 PM

That's a still photography technique that has been around as long as I can remember. Because the polarization effect varies with the angle and intensity of reflected light, the amount of reduction is "about" 2 stops with both rings at horizontal, but the fully "stopped down" amount (polarizers at 90 deg relative) actually varies quite a bit from scene to scene. At that position the sky can turn unnaturally dark.

But I agree, it would be a great added tool to have without the clutter of a matte box.

Serena wrote on 11/22/2011, 6:25 AM

In run-n-gun one doesn't generally wish to shoot at constant aperture and won't often be wanting to carefully control DoF, so I'm always surprised that people are so uptight about needing variable NDs because they don't want to be swapping NDs in a matte-box. Large sensors (such as super-35 and certainly not 5D) don't suffer diffraction degradation of definition until apertures get smaller than f/11. So in most instances a single ND (screw in or matte) will cover the circumstances of the shoot. True you do need a set of NDs, but doesn't everyone? And, as musicvid has pointed out, polas bring their own problems (e.g. pan a short focal length lens across the sky -- not nice!).