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Old March 17th, 2011, 08:06 PM   #1
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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.

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Old March 18th, 2011, 02:08 AM   #2
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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.
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Old March 18th, 2011, 11:18 AM   #3
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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.

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Old March 18th, 2011, 11:42 AM   #4
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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.
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Old March 18th, 2011, 11:52 AM   #5
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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?
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Old March 18th, 2011, 12:00 PM   #6
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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.
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Old March 18th, 2011, 12:23 PM   #7
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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"?
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Old March 18th, 2011, 12:55 PM   #8
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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.

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Old March 19th, 2011, 02:45 AM   #9
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Re: Is shutter-speed and frame-rate reciprocity a happy accident?

Quote:
Originally Posted by Robert Young View Post
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.
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.
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Old March 19th, 2011, 02:48 AM   #10
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Re: Is the 180-degree rule a happy accident?

Quote:
Originally Posted by Chris Hurd View Post
My bad. I saw "180 degree rule" and immediately thought it was about crossing axis.

Thread moved back to NX5, sorry for the interruption.
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.
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Old March 19th, 2011, 02:52 AM   #11
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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.
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Old March 19th, 2011, 07:17 AM   #12
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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!
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Old March 19th, 2011, 07:33 AM   #13
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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.
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Old March 19th, 2011, 09:53 AM   #14
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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.

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Old March 19th, 2011, 10:38 AM   #15
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Re: Is the 180-degree rule a happy accident?

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

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