GH2 Why 24-frame video motion will never look like film, and 1/48th doesn't matter.

DPStewart

DPStewart

Veteran
Lot's of good questions being asked around here lately!

I admit my thread title is a little overly-provacative...but I couldn't see a way to edit the title. It sould say:

Part 1. Why 24-frame 1/48 speed video motion doesn't look "exactly" like film:

It's the spinning shutter of a film camera and the rolling shutter of CMOS chips.
So...the shutter on a film camera spins and 180-degrees of it is open and exposes the film. We all get that. But look at that process in super-slow motion for a moment. The film camera's shutter does not expose the entire frame equally over time. The shutter's opening is spinning and the film is being snapped into place OVER TIME. Of course it's a VERY short period of time, but in the world of film exposure every little bit makes a difference. So what's happening ON THE FILM? Well, think about it - anything in the frame that is MOVING is going to receive less exposure time per grain of film than anything that is more stationary. So moving elements by definition receive less exposure time. Less exposure time equals slightly softer focus and less saturation.
I was lucky enough to have an actual FILM editor in Los Angeles explain this and show me stills taken from a film during a moment of action. Aside from the obvious motion blurs of some areas, it was quite possible to notice a "less exposed" quality to parts of the frame that were in moderate motion. The effect was even easier to see when he darkened the lightbox we were looking at it on. This "less exposed" effect allows the eye to naturally focus less on these areas therefor giving a smoother "sense" of motion. There's even a name for the uneven shutter exposure but I can't remember what they called it - I've heard it referenced here on the forum.

Part 2. Why 1/48th shutter speed doesn't matter:
Rolling shutter and over sharpened lenses have already been well established as things that screw with motion smoothness. Once we get "frame at once" CMOS chips - all cameras with rolling shutters will probably become nearly worthless overnight [monetarily speaking]. But this still does not address the "less exposed" phenomenon of film shutters - although that will probably be simulated in the future by software.
All the decent DSLR video cameras seem to have their own characteristics when it comes to perceived motion smoothness at different frame rates to start with and that makes splitting hairs over 1/48 - 1/50 a bit dubious, especially considering all the factors already mentioned. 1/40 on the GH2 is indeed very smooth, but if someone waves their hands around in your frame - you will most likely get motion blurs. 1/50 will tighten that up but at the cost of potential artifacting from Panny lens over-sharpening and possible rolling shutter effects.
So shutter speed on a DSLR is such a different ball game than on a film camera that the best advice is to simply do some shooting at various speeds and get to know what your camera and lenses will deliver for you. Shutter speed differences can, if used thoughtfully, provide a bit of extra mood or emotional impact to a scene. Get to know them, for they are your friends.

~Cheers
 
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In the meantime people are actually shooting amazing projects with Video cameras...... I can't wait till the GH3 comes out so we can finally have something good to talk about......
Back to work
 
DPStewart said:
Lot's of good questions being asked around here lately!

~Cheers
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I'm almost positive that one of the things Arri and high end digital camera manufacturers supplying Film film camera replacements, are not 'worried' about, it would be the knife edge effect of a spinning Film film camera shutter.

In fact I don't think digital film projector manufacturers are doing anything about the equivalent physical mechanism in transitioning from Film film projectors.

What I think the Arris are working on is less noise at higher spatial resolutions and higher ISO values for quality image formation, at a 'reasonable' cost.

They maybe working on 'blanking' technology to eliminate a major source of shutter 'distortion', just as 35mm still Film film camera makers made better shutter screens to eliminate distortion due to the traveling 'slit' shutter, and of course, there always was the 'leaf' shutter in the lens....

But since these were mechanical devices and had various issues when used at 'highspeed', Edgerton used strobe lights to get faster than a speeding bullet exposures... which had short duration, and high lumen counts... But even then lights have finite rise times... for which I think the "Banana and the Bullet" is an appropriate image...

edgerton_banana.jpg
 
j1clark@ucsd.edu said:
I'm almost positive that one of the things Arri and high end digital camera manufacturers supplying Film film camera replacements, are not 'worried' about, it would be the knife edge effect of a spinning Film film camera shutter.
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Well they are definitely doing something, because the Alexa moves like film and not like the GH2. It's not the operator and it's not the extra dynamic range. There is something in the motion itself that they are doing differently and I would like to know what. Possibly figure out if there is a way we can emulate it with software.
 
roxics said:
Well they are definitely doing something, because the Alexa moves like film and not like the GH2. It's not the operator and it's not the extra dynamic range. There is something in the motion itself that they are doing differently and I would like to know what. Possibly figure out if there is a way we can emulate it with software.
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Perish the thought that an $89K Alexa kit should work 'better' than a $1.5K GH-2 kit...

The Alexa has a mechanical shutter option. Even so, the electronic 'shutter' read out is very fast, and 'reports' indicate unless the camera or subject movement is 'designed' to show the difference, there's not much difference between the two.

I think much of the 'rolling shutter' issue that people seem to have with DSLRs stems from the fact that they really don't know how to handle a 'movie camera'. Despite its name... a movie camera has limitations on what can move and how, and how that is captured, relative to the film speed and shutter speed.

There are tables which indicate how fast a pan should be or how fast a moving object should cross the frame to avoid these artifacts. While the specifics may be different for the DSLRs, it is not like capture with a Film film camera is 'no problem'.

Personally I think ever car should have provisions for a buggy whip to be held close at hand for the driver.... because you never know when you may need to be pulled out of the mud...
(As it was the Renault Dauphine did have as late as the 60's the provision for being started with a handcrank... which I used when the battery died...)
 
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j1clark@ucsd.edu said:
There are tables which indicate how fast a pan should be or how fast a moving object should cross the frame to avoid these artifacts. While the specifics may be different for the DSLRs, it is not like capture with a Film film camera is 'no problem'.
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AMEN! I think that should be printed on a piece of yellow HAZARD/WARNING Tape and strapped to every new DSLR sold! LOL!
 
roxics said:
Well they are definitely doing something, because the Alexa moves like film and not like the GH2. It's not the operator and it's not the extra dynamic range. There is something in the motion itself that they are doing differently and I would like to know what. Possibly figure out if there is a way we can emulate it with software.
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Could they be using a global shutter image sensor and a mechanical shutter?

The image sensor could be triggered to expose by the position of the mechanical shutter wheel as it spins. So as the mechanical shutter turns into position just before the edge of the window allows light to hit the image sensor, the sensor starts exposing, then once the window has passed, the sensor offloads the data.

I came up with this idea a year or so ago, but I never had any way of testing it, to see if it would give a "film like" response.
 
There is a gadget (can't remember the name) that you can put in a mattebox like a filter. It simulates an mechanical shutter and works very well - pricey though.

Best solution I saw so far.

Frank
 
roxics said:
http://tessive.com this?
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I believe this is what Frank was referring to... When I visited the Tessive booth at NAB, the fellow explained that the 'tessive' device works as a better 'tuned' low pass optical filter, which results in more 'blur', but less 'aliasing' of higher frequency spatial/time data. Also, it probably would 'remove' the artifacts related to the 'mechanical shutter' that people are wanting...

I think the 'adhoc' method of shooting at 1/40 sec. that I think Kholi advocated may be the 'cheap' solution...

Here's a description of the Tessive filter compared to a 180 deg electronic and 180 deg. mechanical filter available on the Sony F65. From the graph it looks like the 'tessive' filter is 'open' for more of the 1/24 frame time, but the transmissivity of the filter is 'shaped' over time to give a low pass filter effect.

http://tessive.com/mechanical-shutter-comparison

Here's a picture of the 'wave forms' of the different shutter operation...

Shutter-waveforms.png


From the graph, one would set one's 'shutter speed' to be identically the frame rate, that is 360 deg. and let the filter 'shutter' the optical path.




I asked the fellow if the Tessive device could be used as a variable filter and not just 'full frame' as it is basically an LCD pixel based device, one could make a feedback loop of the image, and cut sections of the scene down independent of other areas of the filter... sort of a dynamic split filter... the guy looked at me funny... and smiled...
 
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1/40 on gh2, 1/45 on canons. That's close enough for me on either camera.

In response to Stewart, however, Alexa gets pretty close when shot at it's best if not nearly exact, and if you haven't seen Blackmagic footage yet, especially johns new stuff from puberty blues, you ought to go and check that out.

As someone that's hyper sensitive to this, whatever Blackmagic did they got that part as right as I think you could get it.
 
Kholi said:
As someone that's hyper sensitive to this, whatever Blackmagic did they got that part as right as I think you could get it.
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Agreed. Blackmagic does it right like Alexa. But I'm still curious what "it" actually is if it's not an effect of a mechanical shutter. And why cameras like the GH2 don't have it and if there is a way to emulate it in post.
 
I still think 1/40th on the GH2 isn't quite right. It's closer, and I use it over 1/50th, but still something missing.
 
roxics said:
I still think 1/40th on the GH2 isn't quite right. It's closer, and I use it over 1/50th, but still something missing.
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Well, I too am somewhat of a stickler when it comes to motion issues on video cameras. Most will never see it, and think it's a funny topic to even raise up... but it's a major eyesore for me. Out of all the DSLR's including Nikon / Canon / Sony and the smaller brands... the GH2 hands down gets closest to that "filmic" 1/48 shutter motion you see in film cameras. Now, both Alexa and BMC and leagues ahead in this area and provide a vastly superior look if we're talking about emulating the aesthetic of a mechanical shutter on a film camera.

This is one of the major reasons why after 2 years, I still use the GH2 and have not yet upgraded to a newer or "proper" video camera (FS100 / 700 / AF100 etc)., despite having some funds saved up for just that. The only single camera I've seen in which I could be set up and ready to go with every accessory I'll need for 10k or under, that gives me, what I would consider "filmic" motion, is the BMC. And honestly, if that camera hadn't popped up unexpectedly from nowhere like it did, I don't think I'd even consider upgrading right now.
 
Kholi said:
1/40 on gh2, 1/45 on canons. That's close enough for me on either camera.

In response to Stewart, however, Alexa gets pretty close when shot at it's best if not nearly exact, and if you haven't seen Blackmagic footage yet, especially johns new stuff from puberty blues, you ought to go and check that out.

As someone that's hyper sensitive to this, whatever Blackmagic did they got that part as right as I think you could get it.
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Ha! Yeah Kholi - the BMC stuff I've seen looks AMAZING. I just hope that thing really becomes an available reality on the market!

To be fair - I really wasn't thinking that the $60,000 - $80,000 Alexa was the kind of device we are talking about. I meant to keep the topic to DSLR's because that's what we're using and discussing here. [most of us anyway]
It looks to me like the BMC isn't really even a DSLR at all, but more of a modular cinema camera like the REDs.
 
Sage said:
Really, camera has not been a problem for some time now..
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That is the funny part about 99% of the posts here - just about any modern camera will do the most jobs quite well if you just learn how to use it properly ;)

But then I guess pixel-peeping is what camera forums are for.
 
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