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-   -   panning & distortion (https://www.dvinfo.net/forum/open-dv-discussion/479652-panning-distortion.html)

Lars McCall May 30th, 2010 08:16 PM

panning & distortion
 
Please help me understand this "effect."

Video looks clear during a still shot, but when panning there is a "jitter" in the image- a loss of clarity. It's not a major distortion, but it is a subtle loss of focus and image integrity as the camera pans.

When I look at good films, I see pan shots that are very clear. For example, Wim Wender films have this very smooth image consistent panning (as well as many other films.) Is this due to the use of analog film? Do digital consumer camcorders lack this ability, yet digital professional video cameras are able to "smooth pan?"

I just bought a Canon HF S100 and see this subtle distortion when I pan- even if it is a slow pan. I shoot 60i with this camera. The idea of "High Definition" for this camera seems a bit misleading. Stills are fine, yet pans have this inherent jitter.

What is the concept called that I'm talking about? Is 'smooth panning" achievable with a lower end camera like the HF S100, or is it a matter of technique that I'm unaware of? I'm shooting at 60i with this video camera.

I am really hoping to achieve smooth panning. In tests I panned as slowly as I could (hand held yet steady) but there is still this lack of "focus" as the camera pans. What is this?
Thanks for any help.

Chris Soucy May 30th, 2010 09:29 PM

Hi Lars..............
 
It's pretty simple really.

What you're seeing is bandwidth, or, more correctly, the lack thereof and/ or motion smear due to too slow a shutter speed.

Taking the last first:

Just as with a still camera, if there is appreciable camera shake/ movement during the time period the shutter is open, you will get motion smear as the pixels move relative to the subject.

Increase the shutter speed and the smearing decreases and vice versa for any particular pan rate.

Back to the first:

Every Long GOP compression scheme has a maximum bandwidth. For HDV it's about 25mbps.

With a static shot, the frames in the GOP group don't change much, if at all, so there's bags of bandwidth and a nice clear picture.

Start a pan and pixels start changeing, the faster the pan, the more they change and the more data has to be crammed into the frames. At some point the rate of change exceeds the ability of the codec to cram data into these frames and it just starts throwing data away, hence the infamous HDV "smear", tho' any long GOP codec can be provoked no matter what bandwidth it has (tho' obviously, if you double the availabe bandwidth, you don't throw so much away nearly so fast).

The "trick" to working around this problem is pretty simple in theory but not alway quite so easy in practice.

DO NOT just do "general" pans without a target subject in the frame, it always looks awefull because there's nothing to take your mind off the smearing.

Locate a "target", a car, say, and keep it in the same relative position in the frame as you pan to track it.

This has two effects, first the target pixels don't change all that much frame to frame, thus require less bandwidth. Second, because there is a visible "target" for your eye to follow, it draws attention away from the background which can smear as much as it likes without being noticed (unless you're a cranky perfectionist like me, and I alway notice it).

As video is a narrative medium it's usually telling a story, make your target part of the story and those pans can look pretty damn good.

Make sense?


CS

Annie Haycock May 31st, 2010 05:47 AM

Clear explanation of this effect much appreciated.

Thanks

Lars McCall May 31st, 2010 09:55 AM

I understand now, thank you.

David Morgan June 5th, 2010 08:54 PM

Chris,
so in comparison:

1080i/ 60 which I believe is interlaced at 1/60 per field = 30 fps

1080p30 which I think is 30fps progressive but the shutter speed is? My guess would be 1/30 which would produce smearing.

So I wonder what the difference in look is between 1080i 30 (as above) and 1080p30/ 1/60 shutter speed? Probably need more light for the latter but what else?


dave

Chris Soucy June 6th, 2010 01:07 AM

Hi David............
 
Er, not entirely sure I'm with the program on the question.

If you're shooting progressive you only have whole 30 FRAMES to play with. Increase the shutter speed past 1/60 and the gaps between what was on one frame and the next doubles, making the jerking more obvious.

With interlace you have 60 FIELDS to play with, which means that you can push the shutter speed to 1/120 or even 1/240 without it being too obvious.

True, those 60 fields are only half the picture, but they convey a great deal more info than you'd think.

This wasn't actually covered in my original post so I'm still a bit vague as to what you're asking.

Hopefully I've nailed it.


CS

David Morgan June 6th, 2010 11:32 AM

Sorry, don't intend to be vague. I shoot a lot of theatre shows in high contrast, variable light and plenty of dancing.

I've always shot interlaced on my Canon A1. Primary outlet is SD DVD, not the web.
Lately, I've been shooting HDV 1080-60i. However, If I upload this footage to websites I'm going to want progressive.
I think I get your point about increasing the shutter speed (causing strobing) if your shooting progressive. Sometimes it's a bit tough to wrap your head around the shutter speed business. I always shoot in manual mode with the shutter at 1/60. I use the Iris to control exposure. Having said that, I'm always looking for other ways to shoot these shows.
Really wondering if there is any way to shoot progressive with fast movement and not introduce blur. i thought maybe just going to 1/60 might be a workable formula for 30p but I think I get your point. The picture could end up strobing, or if not, I wonder if the look would be different between 60i (30 frames interlaced) or 30p shooting at 1/60 shutter speed. I guess that's what I'm curious about. (Can risk shooting a show to find out however)

Hope I explained this a little better.
dave

John Meeks June 6th, 2010 01:46 PM

Have you tried 720p? You might be noticing some kind of deinterlacing unless you're watching it on a 1080i native display (like a CRT).

I just did some experiments, and while 720 is ever slightly less sharp from the smaller resolution, the motion seems so much smoother from having 60 full frames per second.

Chris Soucy June 6th, 2010 02:07 PM

Hmm, well.............
 
OK, try this (early in the morning and still working on Java intake, so bear with me...).

Imagine you were shooting these shows with a bog standard film SLR and you could, somehow, shoot 30 fps.

Now, if you know anything about SLR film cameras you'll know that with the lighting conditions you've described and the movement taking place, 1/60 isn't going to cut it if you want pin sharp images.

Maybe 1/250 or maybe, even faster. Assuming you can push the ISO that fast AND you have the telephoto lens from Heaven (F1.0, say) you now have 30 frames shot at super high shutter speed and crystal clear.

The problem is, stick them all together and you find there are huge chunks missing because 30 X 1/250 = 1/8 or thereabouts, leaving 7/8's of the fun MIA.

Now, go back and shoot with a much lower (half) resolution film but shoot at 60 fps.

60 X 1/250 = 1/4. Hey, you've got twice the movement detail, if not the "full Monty" detail in any of them, tho', given the way the human eye/brain system works, that'll do.

Ideally, of course, you'd be able to shoot full resolution @1/250 @ 240 fps, but then you'd need to buy/ rent a Phantom @ $150,000 bucks a throw and find a display system that could show it at the same rate, unless you find snail watching an exciting pastime.

[I'm not actually sure even a Phantom can shoot full 1080 @ 240 fps, but that's beside the point].

Given that you're stuck with good 'ol HDV and 30 and 60 are your only options, i it will have to be if you want to crank that shutter speed up. Quite how you square the circle needing progressive for the web is a different story altogether.


CS

David Morgan June 6th, 2010 02:20 PM

thx Chris. Experimentation is clearly necessary here.


John,
My camera (Canon XH-A1) will not do 60p, even at 720. However, I have a Focus Enhancements FSH-200 DTE recorder that will do 720p @60. thanks for the suggestion. I'll find a way to try it.

dave

David Morgan June 6th, 2010 02:35 PM

Oops, think I'm screwed here to. the DTE will record the signal sent to it. So I'm out of luck for 60p

Bryan Cantwell June 7th, 2010 08:25 AM

David,

If all you need progressive for is posting on the web, is there any reason that de-interlacing in post won't work?


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