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View Full Version : 180 degree rule, ND filters and 24p



dsp_418
02-05-2011, 01:09 AM
I've stumbled upon an article about the 180 degree rule and why it's important to respect this rule as long as we don't have particular needs. So far shooting outdoors in a bright daylight I pumped the shutter speed way over 1/200 (much, much more actually) because I didn't know about the 180 degree rule (and I didn't know what a ND filter was about). Now I'm getting the reason why ND filters exist, but I still don't know exactly how they work. I assume that depending of the time of the day I'm shooting the shutter speed needs to be tweaked accordingly. Therefore if I use ND filter to keep the 180 degree rule safe, I've to use a particular stop to don't go over 1/48 if I'm shooting a clip. So the question is: how can I know which ND filter I need since over the day the light will significantly change?

Last thing. The same article pointed the 24p look that most of the people like (the reason for the 180 degree rule). Now I don't remember anymore what happened the few times I've tried the 24p on my camera! but the thing is I'm trying the 24p right now and it stutters a lot! It seems that's the normal look of a 24p shooting: is that correct? am I doing something wrong?
How do you guys shoot usually? 30fps and then you down convert to 24p in post? Or is it better to shoot in 24p?

Thanks for any help :)

dsp_418
02-10-2011, 05:58 PM
Anyone?

cyclone
02-12-2011, 07:05 PM
The 180 degree rule refers to camera placement, unless there is a rule I am not aware of that deals with shutter speeds. Am I just not understanding the question?

IVHM
02-12-2011, 07:15 PM
The 180 degree rule refers to camera placement, unless there is a rule I am not aware of that deals with shutter speeds. Am I just not understanding the question?

I was also confused at first, what he's referring to is setting the shutter to twice the frame rate to mimic a 180 degree shutter on a film camera.

cyclone
02-12-2011, 08:07 PM
Ahhh.
Thank you.

Chris S. Alexander
02-13-2011, 04:23 AM
The ND filter choice comes with a few other stipulations to it: How fast is your lens (meaning how low can your f-stop go)? How high is the sun? What shutter speed are you running at? ND filters work in terms of stops. A .3 ND filter is 1 stop, a .6 is 2 stops, and a .9 is 3 stops. The way ND filters work is by applying a sort of "sun glasses" effect for your lens. The higher the number on the filter, the more stops, meaning less light going into your lens and hitting your sensor. You just have to eye ball it. Start by putting on the .3 filter and work your way up. Eventually you'll just know.

When you're talking about a 180 degree rule, you are in fact talking about a 1/48 shutter speed (Canon DSLR can't do 1/48, so we compensate by shooting 1/50). When applied to 24 frames per second, your camera takes a picture every other 1/48th of a second. This is the closest that we can get the shutter to allow for enough motion blur to make things seem "realistic." When you crank your shutter up (say to 1/96, 1/200, etc) you get a juttery effect where there is less motion blur and the movements are more noticeable (this works great for action shots). However if you feel confident you can use the 360 degree shutter, which is 1/24 (or 1/30 on a Canon DSLR). This allows for more motion blur, and less jutter between frames. My only suggestion when you use 360 degree shutter is that it be on a shot without a lot of movement (example, a close up of someone talking), because putting a 360 shutter on a shot of someone running will be a blurry mess.

I'm not sure exactly what problem you are having with shooting 24p, I would have to see some footage to help determine that. However I can give you advice on converting 30fps to 24fps in post...if possible DON'T DO IT! It won't look great and your time (and money) will be wasted. Shooting 30fps or 24fps is something that you need to decide before you shoot. Instructional videos, reality shows, and documentaries are great at 30fps...feature films, commercials, narratives, and short films are better done at 24fps.

It's 4AM where I am and I'm feeling a little tired. I'm hoping that all of that makes sense. If not, feel free to ask me to explain and I will do so when I am coherent. Good luck with your shooting!

GrahamH
02-13-2011, 05:52 AM
Yes, at different times of day you might find you'd like different levels of ND filtering - if you look online or on fleabay you with see "fader ND" filters that let you vary the level. Unfortunately they tend to soften your image and add color casts, unless you pay substantial $$$.

If you look at how 24p is used in movies, judderbis minimised by supporting the camera well (tripods, glidecams, cranes and dollys) and being very circumspect with pans and other camera movements.

Alex H.
02-13-2011, 06:13 AM
The 180 rule is all well and good, but there's a complication when shooting with DSLR cameras. The rule works just fine outdoors under natural light, but once you get under artificial light it is much more important to match your shutter speed to the electrical system. In the NTSC territory (US, Canada, etc.), it's a 60Hz system so you need a 1/60 shutter (or a multiple thereof). In PAL territory (Europe, etc.) it's a 50Hz system, so the shutter should be 1/50 (or a multiple thereof). Otherwise, the footage may have flickering/strobing artifacts from the lights.

Bruce Foreman
03-27-2011, 10:28 PM
C2V is correct.

24fps is also an "archaic" holdover from film days and represented the best compromise between economy of film consumption and still allowing persistence of vision to occur when the film was projected. That frame rate by itself does not guarantee a "filmic" look.

Actually modern projection equipment in theaters actually projects each frame twice to smooth out any "flicker" effect (some can see this at 24fps others can't) to prevent some folks from getting headaches so what we experience in the theater is actually 48fps.

The "film" look so sought after is really a combination of several things, composition, depth of focus, lighting, and a dynamic range that emulates what films were capable of. The frame rate by itself doesn't do it. Often 24fps can result in a kind of "stuttery" look that doesn't happen so much at 30fps, and although Blu-ray is recorded commercially at 24fps TV's do a pretty good job of adjusting when playing something we have recorded and rendered at 30fps (I view my own created HD content rendered to an HD file format and played through a hardware media player connected to my TV with HDMI. It doesn't seem to "care" what frame rate I use).

With my Canon DSLRs (7D, T2i, and now 60D) I start out with shutter at 1/60th, I'm in NTSC "country" (still close enough to 1/48th to allow some motion blur), aperture of choice for desired zone of acceptable sharpness (DOF), and ISO temporarily on AUTO until I see where the camera want's to set it. I press lightly on the shutter button to make settings read out on the bottom of the LCD (with display set to show those) and then manually dial in that ISO value, exposure is now "locked" and will not change as I pan (that's the way I want it).

If I'm going to try to judge exposure effect on the LCD I use a viewfinder loupe whether indoors or out, and if I like the exposure effect I see I leave it there, otherwise I can dial the ISO up or down a bit until I see what I want (I also pay attention to the meter scale and indicator at the bottom of the LCD).

I also use ND filters for added exposure control especially if I'm going to try for selective focus with a wide aperture but you have to be aware that "stacking" these regardless of brands may result in some color shift. Mine are Tiffen brand and when I stack .6 and .9 for about a 5 stop reduction I do get a very slight magenta shift but that is easily correctable in my NLE if it is at all noticeable.

bwwd
03-28-2011, 04:28 PM
oh bummer,projection is in 48 but capturing is still 24p,24/25p is a must IMO for starting to make a movie that looks like a movie,30p looks liek cheapo cam or some 90's tv comedy series for kids,everything above that looks even more cheaper and too smooth to be serious for filmmaking,this also depends on shutter speed,but if we want the look of the movies which were shot in 24p then how about using... 24p ?

Barry_Green
03-28-2011, 05:01 PM
Projection is 24fps, period. Just because the frame is flashed twice, doesn't mean it's showing more frames per second. The film runs through the projector at 24 frames per second. Flashing the frame two (or three times) does absolutely nothing to change the feeling of motion; instead it is to even out the flicker that comes from bright frames being projected and then the shutter being held dark for a long time. By using a two- or three-bladed shutter they can even out the flicker.

j
03-28-2011, 06:13 PM
> I've stumbled upon an article about the 180 degree rule and why it's important to respect this rule as long as we don't have particular needs.

Link?

GarageBoy
01-13-2014, 12:00 PM
So how do I use an exposure meter and obey the 180 deg rule?

GrahamH
01-13-2014, 12:37 PM
To obey the 180 degree rule, set your shutter speed to one half of the frame rate (e.g., 1/48th or 1/50th if you are shooting 24fps).

Then use your light meter to determine what aperture is required for correct exposure at that shutter speed.

If you don't like the aperture value that is required, adjust your ISO, lighting and/or amount of ND filtration ... while leaving the shutter speed unchanged.

JMtheDP
01-14-2014, 09:38 AM
I'll try and help here by giving example of a scenario and solution.

Generally we like to decide stop in advance and stick to that stop for a consistent "look". The same reason (one of many) we use the same shutter speed.

Outdoors, I like to shoot around T4. My lenses are pin sharp there. There's just enough DoF to both look nice and give my focus puller a chance.

I'll have my lightmeter set to 25fps (I'm in Europe) and, say, 500 ISO. My lightmeter will say something like T16 (possibly a lot more). Now it is telling me I need T16 to get the right exposure. But I want to shoot at T4. So work out the stops difference.

T4 T5.6 T8 T11 T16

I can see that my lightmeter is telling me there is four stops of exposure I need to get rid of. So I will use an ND 1.2. Then my exposure will be correct. Hope that helps.

GarageBoy
01-14-2014, 10:14 AM
Thanks, so ND is the extra variable

JMtheDP
01-14-2014, 01:44 PM
Yes.

And I'm not sure if anyone answered the OP's question as to what ND actually is... it is a filter (usually glass, often built-in to some cameras) which darkens the image without affecting the colour balance in any way. It stands for Neutral density.

David G. Smith
01-14-2014, 06:13 PM
So, what is the "180 degree rule"? I shot a lot of film but the widest shutter angle I ever worked with was 170 degrees. Now, with digital acquisition, I pretty much stick to 1/48th a second shutter speed and use lighting, aperture settings, gain (Or ISO settings) and ND filtration to control exposure. I have done some casual experiments with shutter speed changes to affect exposure but found it to not to be to my liking. Am I missing something?

Michael Schrengohst
01-15-2014, 08:42 AM
So, what is the "180 degree rule"? I shot a lot of film but the widest shutter angle I ever worked with was 170 degrees. Now, with digital acquisition, I pretty much stick to 1/48th a second shutter speed and use lighting, aperture settings, gain (Or ISO settings) and ND filtration to control exposure. I have done some casual experiments with shutter speed changes to affect exposure but found it to not to be to my liking. Am I missing something?

I think people are getting confused over the "180 degree rule"
There is this "180 degree rule"

https://vimeo.com/2690589

and this "180 degree rule"

http://tylerginter.tumblr.com/post/11480534977/180-degree-shutter-learn-it-live-it-love-it

Noiz2
08-06-2015, 08:51 AM
Projection is 24fps, period. Just because the frame is flashed twice, doesn't mean it's showing more frames per second. The film runs through the projector at 24 frames per second. Flashing the frame two (or three times) does absolutely nothing to change the feeling of motion; instead it is to even out the flicker that comes from bright frames being projected and then the shutter being held dark for a long time. By using a two- or three-bladed shutter they can even out the flicker.

That is not entirely true. The brain perceives the second frame as being split between the first film frame and the second film frame. So while in the real world yes there is no change in the second flash, humans "see" it as an in between frame and it does add to the smoothness of the motion.

Video though is a totally different beast. It has no "black" between frames, which also helps hide motion issues in human brains, and it paints the image down the screen instead of painting it across the screen. Both are so fast that it may not make a big difference but film is going "with" the most common motion (side to side) and video is going at odds to the motion. So while film would tent to smear the image the same way as motion blur, video is creating a "rolling shutter" type distortion.

JMtheDP
08-07-2015, 09:14 AM
24 fps with a 1/48th shutter = 180 degree shutter. It relates to a film shutter, which being a mirror is best measured in degrees. Much better way of doing it than fractions. Lots about this online.

Razz16mm
08-08-2015, 05:19 AM
There is a black interval between video frames too. True 24 fps flickers horribly on electronic displays. The normal refresh rate for TV displays these days is at least 120Hz: 5x per 24 fps progressive frame, 4x for 30fps, not including any "smooth motion" interpolation BS. This eliminates the need for 3:2 pulldown conversion at 60Hz NTSC rates.

Liam Hall
08-08-2015, 06:41 AM
24 fps with a 1/48th shutter = 180 degree shutter. It relates to a film shutter, which being a mirror is best measured in degrees.
...which being a rotating disc, is best measured in degrees.

JMtheDP
08-08-2015, 07:27 AM
...which being a rotating disc, is best measured in degrees.

You are correct, that'll teach me for posting after a long drive :)

I would say they are spinning disc mirrors, but not all are - some are just there to blank the sensor.

I believe the Viper had a spinning mirror in it, despite having no optical look-through