Is the 180-degree rule a happy accident?

[Chris Hurd]

Originally Posted by Steve Mullen

I don't understand why something I wrote about HD-DLRs (NEX-VG10) was moved to the NX5U forum.

Well, I understand why... it's due to the onset of senility and general scatterbrainedness. Now put back to VG10 where it rightfully belongs!

[Dave Blackhurst]

Chris -
Steve does make a good point - "DV" is now present in many smaller/consumer devices - I remember when you were trying to start up "HDMOM". Still could use a forum for Sony "handycams", which seem to end up just having scattered "orphan" threads here and there. Maybe there's some commonality there thematically, as I imagine many Sony users buy them for the auto functions, but sometimes want to explore the potential from any "manual" functions.


You may want to consider that there is a HUGE opportunity for some of the new shooters to find info here as they try to figure out the "basics" of how cameras "work".

I know that a lot of consumer oriented devices are designed for full auto or as close to it as possible, but there are many "bridge" devices starting to have some manual control - many with some pretty impressive imaging capabilities, some for pretty close to dirt cheap.

Understanding "what's under the hood" is handy so you have some idea what's going on when you're shooting, even if "auto" is fine most times.

[Marcus Durham]

Originally Posted by Brian Drysdale

The UK 50 Hz decision was made way before television was a consideration.

Utility frequency - Wikipedia, the free encyclopedia

Interestingly, although off-topic, part of the power shortage problem in Japan at the moment is caused by half the country being on 50Hz and the other half on 60Hz with only a limited amount of capacity to exchange between the two systems in operation.

[Steve Mullen]

Originally Posted by Daniel Epstein

I believe Nikola Tesla determined 60 HZ as the inventor of AC Power which Westinghouse bought and funded. Various reasons are given for the choice of 50Hz in Europe including economics as well as protectionism.

Inventor Nikola Tesla Biography

Protectionism sounds right to me. Just like japan used the American NTSC system but changed just the RF parts so the channels weren't in the same frequencies as used in the USA. They gambled our companies would be to dumb to build versions of TVs to ship to japan.

So why 220 volts? This is more dangerous?

[Rainer Listing]

The 180 degree shutter rule simply expresses that 180 degrees is the largest opening you can have on a mechanical rotating shutter before you start to double expose part of the frame. For 24p this gives you 1/48th of a second. Most higher end film cameras allowed you to adjust the opening to a smaller size.Bolex used less than 180 degrees and let you adjust the angle without taking off the lens. It was common to shoot half the opening in bright light, around 120th of a second. No-one complained it looked stroby. Electronically we can shoot from the equivalent of 360 degrees to nothing. The real mistake is being bound by an archaic rule or arbitrary standards of what looks acceptable rather than using all the resources available to obtain your desired artistic vision.

[Richard Alvarez]

Quite a few of my super 8 cameras have 210 or 220 degree shutters. These are "XL" designated - or 'low light' models.

[Brian Drysdale]

Originally Posted by Rainer Listing

Most higher end film cameras allowed you to adjust the opening to a smaller size.Bolex used less than 180 degrees and let you adjust the angle without taking off the lens. It was common to shoot half the opening in bright light, around 120th of a second. No-one complained it looked stroby.

Yes, with the Bolex you usually only noticed the strobe effect when the shutter was closed 2 stops,

The 16mm Mitchell Professional has a max shutter angle of 235 degrees.

There is a range of acceptable shutter angles - depending on the action. The use of a mirror reflex systems (with mirror shutters) on motion picture cameras tends to put them into a band around 170-200 degrees, with the possibility of adjustment to a narrower angle on some cameras. 180 degrees is a handy figure that fits in that band and strikes a good balance for light level requirements and motion blur.

[Steve Mullen]

[QUOTE=Rainer Listing;1629585It was common to shoot half the opening in bright light, around 120th of a second. No-one complained it looked stroby. Electronically we can shoot from the equivalent of 360 degrees to nothing. The real mistake is being bound by an archaic rule or arbitrary standards of what looks acceptable rather than using all the resources available to obtain your desired artistic vision.[/QUOTE]

You've mixed three concepts:

1) When shooting 24fps video, folks may say they are following a "rule" but there's no reason to worry that folks are "... being bound by an archaic rule ...:"

2) Nor need you worry they being bound by "... arbitrary standards of what looks acceptable ...."

They are trying desperately to find away to eliminate the excessive motion judder they see in their video. They are adjusting shutter speed to get enough motion blur to help reduce the excessive judder, but not so much the motion blur turns the image into a smeared mess or so little that the image strobes. It turns out that the rule WORKS. But not all that well with low-cost video cameras!

3) "No-one complained it looked stroby."

A) Anytime a single frame is presented twice -- an "eye tracking artifact" is created. A double bladed shutter; a 50Hz presentation of 25p; a 60Hz presentation of 30p -- all force the our eyes to see doubled images. Motion blur from a slow shutter speed helps cover-up this artifact. Super sharp frames from a fast shutter speed makes the artifact more visible.

B) Although the eye-tracking artifact is inherent, it is NOT the reason why one doesn't see strobing from film and very expensive HD cameras, but do see it from low-cost video cameras.

Low cost video cameras do not have an extended frequency response mostly because their lenses have too low an MTF. To get acceptable FINE detail, the signal is boosted. The good -- the high frequency FINE DETAIL is strengthened. The bad -- the middle frequencies which are already strong, are boosted even more.

The middle frequencies represent EDGES. Simply put, low cost video cameras have edges that are way too sharp and thus very visible. (Making things worse, is that low-cost video cameras ADD ringing to edges which further makes edges stand-out.) The stronger the edges, the stronger the eye-tracking artifact. The faster the shutter speed, the more clear the edges and the more clear the artifact.

Walter Murch, who edited one 3D film back in the 1980's -- "Captain Eo" -- noticed that horizontal movement will strobe much sooner in 3D than it does in 2D. This was true then, and it is still true now. "The more conscious we are of edges, the earlier strobing kicks in."

The soft images from 16mm film were ideal because edges were not "hard." So you could go to 120-degrees -- to cut light in very bright light -- and not see strobing.

[Daniel Epstein]

Originally Posted by Steve Mullen

Protectionism sounds right to me. Just like japan used the American NTSC system but changed just the RF parts so the channels weren't in the same frequencies as used in the USA. They gambled our companies would be to dumb to build versions of TVs to ship to japan.

So why 220 volts? This is more dangerous?

220 Volta actually uses thinner wires since the amperage is smaller so it could be considered a trade off. Not much more dangerous and less of a conversion is required from the higher line voltages. In Europe they don't need Ballasts for smaller HMI's so 220 does have some advantages for production.

As far as the lengths other countries went to to protect their home markets it seems to have paid off for the most part.

[Rainer Listing]

Originally Posted by Steve Mullen

It turns out that the rule WORKS. But not all that well with low-cost video cameras!

Steve, I was actually agreeing with the principle of your initial post, where you said you didn't think the "rule" should be applied to 60fps. And as you also said, it depends on the motion present. As an other end example, the default on my (now rarely used) XL2 at 25p (I'm in PAL land) is 1/25th second, equivalent to 360 degrees - at times especially nice for weddings, although most users (self included) mostly automatically set it to 1/50th for most situations. Agree 180 degrees is a happy accident and may be a useful rule of thumb for many situations, but you shouldn't follow it blindly (even if you are after a "film look").

[Steve Mullen]

I agree -- and find the rule with video only partially helps.

I forgot to mention that the more common name for the eye-tracking artifact is "motion judder."

When cinematographers carefully shoot -- they keep motion judder to a minimum. What you see is the nice filmic "low sampling rate judder" of 24fps.

Motion judder and low sampling rate judder should not be confused with "2-3 pulldown judder."

Because video has harder edges, excessive motion judder looks so bad that we often call it "shudder" or "strobing." This we shouldn't see, but too often even by adding lots of blur we do.

It also happens even with very expensive HD cameras. I remember a scene in The Tudors where the camera (a Genisis I think) moved down a church isle and as each window (very bright) and in-between walls (dark) passed through the frame, the window edges shuddered badly. The upright backs of each bench also shuddered.

I would have hoped that p60 would be better that p30, but 60i still looks smoother. To the eye there seems to be a difference between 60 whole frames -- each at 1/60th -- and 30 frames composed of two field where each was exposed at 1/60th.

In the former, the eye integrates BETWEEN frames while in the latter the eye integrates both BETWEEN frames and WITHIN frames. You could describe the latter as using both temporal integration and spatial integration -- where the integration time for both is 1/60th second.

I'm beginning to appreciate how well interlace video actually works with our eyes. It really was perfect for CRTs!

Which means that although we now think of progressive as the best -- its very first use in film involved a happy accidents. Strobing becomes more pronounced as screen brightness increases (Larry Thorpe, Sony and Canon, made me aware of this at a JVC lunch at NAB). Because the initial projector light sources where so dim, and the film was so soft, motion judder was no problem. Over time as film stock became better and projectors brighter -- controlled lighting (not sunlight) shooting rules, and 3-bladed shutters all worked to keep judder mostly invisible.

Larry described how much engineering it took to make the first Cine Alta work. VERY expensive high MTF lenses, VERY expensive processing to keep the edge frequencies from being boosted -- were all needed. And, the addition of an anti-judder setting that actually decreases the strength of the edge frequencies were all needed. I think he spent a decade working on Cine Alta.

The solution might seem to shoot interlace -- except that flat panel monitors need to deinterlace the signal. And, it is VERY hard to do this well!

[Brian Drysdale]

Roller captions, which worked fine on interlaced CRT, can now have problems on modern flat screen televisions because they're running too fast. This something you have to consider even when shooting roller film titles, we had to re-shoot the end titles of a 35mm short film because of it.