Is the 180-degree rule a happy accident?
 

We all know the 180-degree rule.

But why is it valid? Why should the frame-rate require a certain shutter-speed?

1) Let's step back to shooting stills of moving objects. As a still photographer I know that I set the shutter-speed based on the motion speeds of moving objects -- including the movement of the camera frame caused by unsteady hands, especially at long focal lengths -- and how much motion blur I want to SEE.

The usual rule is that below 1/60th second blur will be too high and picture detail will be lost. As motion speed increases and/or I want a clearer image, the faster the shutter-speed I use.

Shutter-speed does not depend on how rapidly I take pictures. :)

2) there is an inherent benefit to a slower shutter-speed. It allows more light to reach film. So were I designing a movie camera in the 19th Century, I would aim for as long a shutter-speed as I could get away with.

3) I would also try a low frame-rate to keep film cost low, as long as folks saw smooth motion. That might be 15fps to 18fps.

4) BECAUSE THE CAMERA IS MECHANICAL the shutter disk had to rotate once per frame. Once meant that opening angle was an free variable. However, the obvious design was half open and half closed. At 15fps, the shutter would be open for 1/30th second. This worked!

But, it's likely that as film gained more resolution, folks noticed the blur was a bit too much so the shutter angle was reduced to 150-degrees (Bolex) that increased the shutter speed by 1/3rd to 1/40th second.

5) as the frame rate increased to 18fps, the shutter speed naturally became faster.

6) when the frame rate was increased to 24fps, the half open and half closed design increased the shutter speed to 1/48th second. With a 150-degree angle, the speed became 1/64th second.

7) I very much doubt any science was involved. Movies developed on the basis of what worked. And, over time the movie shutter speed moved closer to the photography shutter speed of 1/60th.

8) Hence, the 180-degree is not a science fact, it is a byproduct of camera development.

9) If I'm correct, then blindly applying the rule as the frame rate goes above 24fps--30fps may not be valid. It may be that for MOST AMOUNTS OF MOTION a speed of 1/60th second is optimal. Which means that when one shoots 50fps or 60fps, the shutter speed need not follow the 180-degree.

10) Because I can shoot 60p with the FZ100, I am finding that 1/60th looks better -- less strobbing -- than 1/125th.

11) Using a slower shutter speed provides a full stop of greater sensitivity! Of course in bright light, it requires a 1-stop stronger ND filter.

12) When one shoots 60p and uses 2:3 to obtain 24fps -- many claim the motion blur looks wrong. But, if one used a 1/50th or 1/60th second speed rather than the 1/125th required by the RULE I'll bet motion will look correct.

Comments?

Re: Is the 180-degree rule a happy accident?
 

Interesting observations.
I always thought the reason for 1/60th for NTSC video was that the format always used to be 30 fps interlaced, which meant 60 "frames" (fields), and therefore a minimum of 1/60th would give you a full shutter exposure for each field.
Using that logic, I would think that 1/60th for 60p should look similar to what we get with 1/60th for 60i in terms of motion rendering, etc.

Re: Is the 180-degree rule a happy accident?
 

Originally films were shot at 16 frames per second, the frame rate increased because of sound and the need for a faster linear speed for optical sound. Silent speed projectors have 3 shutter blades to prevent flicker.

Regular 8mm is traditionally shot at 16 fps for silent films and Super 8 at 18 fps, however, motion does look better at 24 fps. I used to use the higher frame rate when shooting 8mm because the motion looked less blurred. However, motion blur is a requirement for smooth movement, unless you increase the frame rate, which does have economic ramifications, even in the digital world

Originally the camera and projector were the same piece of kit and not separate items.

LUMIERE BROTHERS FILMS - HISTORY

Re: Is the 180-degree rule a happy accident?
 

Moved from Sony NX5 to TIP since the topic is more universal than the camcorder itself.

Re: Is the 180-degree rule a happy accident?
 

Not like me to be a pedant :-) but I know the 180 degree rule OK and it's to do with where you put the camera relative to the talent.

This is the 180-degree SHUTTER rule we're talking about here isn't it? Or am I just easily confused?

Re: Is the 180-degree rule a happy accident?
 

My bad. I saw "180 degree rule" and immediately thought it was about crossing axis.

Thread moved back to NX5, sorry for the interruption.

Re: Is the 180-degree rule a happy accident?
 

I thought the same thing. Maybe we can edit the thread title to add the word "shutter"?

Re: Is the 180-degree rule a happy accident?
 

Here's a good and interesting article:

XDCAM-USER.COM Shutter, shutter speed and shutter angle.

Very good information and some things to think about when setting your shutter speed or angle.

-Garrett

Re: Is shutter-speed and frame-rate reciprocity a happy accident?
 

That really makes sense!

Interestingly, it has the same logic as a photographer uses. How can I get the greatest light sensitivity? In photography this is limited by hand steadiness -- which for most people is 1/60th second. I suspect that most cameras where one can't control shutter speed, the shutter speed is set to 1/60th.

In video, you are right, the longest possible duration is a field time. And, originally video cameras were very insensitive so one wanted to use the entire duration.

Which means "shutter-speed and frame-rate reciprocity" for still photography and video came out of the need for maximum sensitivity, human hand steadiness, and technology (a frame rate that matched power line frequency that is just high enough so we don't see lighting flicker -- a brain function).

Amazingly, the amount of motion blur created by 1/60th second LOOKS RIGHT FOR STILLS and for video causes a series still frames to BLEND INTO MOTION. (And, motion blur to look right.)

As I pointed out in my first post, film camera development led to a shutter speed between 1/48th and 1/60th second. Here again shutter speed enables a series still frames to BLEND INTO MOTION. (And, motion blur to look right.)


If this describes how we got to 1/60th second (in the USA), then "shutter-speed and frame-rate reciprocity" is the result of 1/60th second intersecting three other processes:

1) the fact that images need to be presented at a certain rate for us to see MOTION. The minimum rate is about 15fps. But, motion looks more accurate as the temporal sampling rate increases -- another brain function.

2) the need for at least 24fps to get the linear speed needed to record audio with an acceptable frequency range. This is pure technology -- although what defines "acceptable" is a function of our brain. (I'm sure developers also saw better looking motion as a positive side effect.)

3) the need for a projector presentation rate of at least 48Hz to avoid flicker with relatively dim light sources. (this frequency, like the creation of motion, is built into our brains.)


I think it's fair to assume that had film stock been cheaper or someone had said they wanted a "hifi" soundtrack, or had Edison invented AC and thus wanted to use synchronous electric motors in cameras and projectors, we would have a 30fps cinema.

In this case, not only would "shutter-speed and frame-rate reciprocity" exist, the same shutter speed would be used for all media.


PS1: we know the 50Hz power frequency was not well chosen for three reasons: lighting flickers, TV flickers, and 1/50th second exposures require VERY steady hands.

I wonder why Westinghouse chose 60Hz? Perhaps he saw Edison's DC powered lights were steady so he wanted the same for AC driven lights. So, he went to 60Hz. Which also made synchronous clocks EZ because "60" is a magic number for time. And, 666 is ...


PS2: Anyway, Robert is right, when we use 1/60th with 60p we are getting the same amount of motion blur for each frame as we get for each field when shooting 60i.

We are also getting the same amount of motion blur as my Bolex that had a 150-degree shutter and thus a 1/60th exposure at 24fps. Which means 60p should be able to be converted, by adding 2:3 pulldown, to 24p.

AND IT ALSO MEANS MY TITLE IS WRONG. The rule really isn't 180-degrees because not all film cameras have, or are used, at 180-degrees.

Re: Is the 180-degree rule a happy accident?
 

Chris, I understand why you would it move to TIP, but I don't understand why something I wrote about HD-DLRs (NEX-VG10) was moved to the NX5U forum.

My interest is in the NEW products that are in the gap between camcorders and RED. The VG10 is in this gap -- as are the new large-chip cameras from Sony and Panasonic. (Because I don't have the money, I write about products in the under $2000 space.)

Which makes me wonder if there could a be forum for those who are buying these new cameras that shoot video. (Photographers are often confused by formats and codecs and NLEs.) And, for use by those who used to use camcorders, but now use cameras. (Videographers are often confused by DOF, the need and use of Vari-ND filters, crop factors, and CMOS-read-put technology.)

For example, my shutter-speed story got mentioned in a photo forum because photographers are now able to shoot video, but can get zero help from photographers.

This really is the Digital Video growth area. And, it's not always product specific.

Re: Is the 180-degree rule a happy accident?
 

The widest shutter angles found on film cameras vary from 135 degrees to 200 degrees, with most being between 170- 173 degrees and 180 degrees. A pure 180 degrees figure isn't hard and fast, although the look of the mechanical shutter is slightly different to the electronic shutter, but 180 degrees tends to be the one that looks best in most occasions and is the simplest to remember.

However, there are times when you do need a non standard shutter angle.

Re: Is the 180-degree rule a happy accident?
 

"but 180 degrees tends to be the one that looks best in most occasions."

That's an important point. Not only is speed dependent on the motion blur "needed" to create smooth motion based upon the speed of objects -- it is dependent on whether you want a typical look a an atypical look (more smear or less smear). This is equally true of still photography, although in my mind faster than 1/60th is used because of camera shake. To stop fast action one would not go to 1/250th but to 1/2500.

With video cameras, even though a continuous adjustment would simple to implement, we usual get increments. My Bolex could be adjusted while shooting from 150 to 0. All mechanical!

Re: Is the 180-degree rule a happy accident?
 

The standard shutter angles used are extremely close as makes no difference visually, 172.8 degrees is the other key shutter angle in Europe, this allows flicker free shooting at 24 fps using 50 Hz lights.

Re: Is shutter-speed and frame-rate reciprocity a happy accident?
 

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

Re: Is the 180-degree rule a happy accident?
 

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

Utility frequency - Wikipedia, the free encyclopedia

Re: Is the 180-degree rule a happy accident?
 

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

Re: Is the 180-degree rule a happy accident?
 

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.

Re: Is the 180-degree rule a happy accident?
 

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.

Re: Is shutter-speed and frame-rate reciprocity a happy accident?
 

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?

Re: Is the 180-degree rule a happy accident?
 

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.

Re: Is the 180-degree rule a happy accident?
 

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

Re: Is the 180-degree rule a happy accident?
 

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.

Re: Is the 180-degree rule a happy accident?
 

[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.

Re: Is shutter-speed and frame-rate reciprocity a happy accident?
 

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.

Re: Is the 180-degree rule a happy accident?
 

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").

Re: Is the 180-degree rule a happy accident?
 

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!

Re: Is the 180-degree rule a happy accident?
 

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.