Frame Rates

[Tim Lawrence]

Hi All,

Can you guys share your opinions on frame rates?

I understand that lowering the frame rate increases light intake, but can makes things blurry, and increasing frame rate helps capture motion better but requires more light and can look stuttery. It seems that using the iris is the primary way to adjust exposure, not frame rate. My question (which seems to go unanswered in even some of the best books I have) is what is the standard?

From what I have gathered so far, it seems to be:

1/48 for 24p
1/60 for 30p
I am unclear as to whether it is 1/120 for 60i/60p or 1/60 for 60i/60p

Can somebody offer a succint explanation of this subject?

If I am filming in HD in 24p for a cinematic short movie where I have total control of the lighting, should I go for 24p at 1/48?

If I am filming a training video in 30p, should I go for 1/30?

If so, when do you step outside of these parameters?

Thank you for any help,

TL

[Sam Hancock]

Hey Tim-

What you are referring to is Shutter Speed. The 24p/30p/60p part of the equation is the frame rate, and refers to how many frames are shot per second. The appropriate shutter speed for a "normal" look at any frame rate is double the frame rate, as you say:

1/48 for 24p
1/60 for 30p
1/120 for 60p

If you lower the shutter speed in relation to the frame rate, ie. 1/24 shooting 24p, then yes, you will gain more light, but the images will look a little smeared or blurry- this can be great if your using it for a particular effect in like a dream sequence, or a psychic vision, or something...

If you raise the shutter speed in relation to frame rate, ie. 1/100 shooting 30p, then yep- the images will look a bit more stuttery, but they will also have a crisp sharp quality that can be used to great effect- especially in action sequences where there is a lot of stuff being kicked into the air (dirt, dust, snow, water...) Ridley Scott seems to often use this type of photography in his action sequences.

Hope this helps!

[Garrett Low]

Hi Tim,

As general rule of thumb, you would usually set your shutter speed 180 degrees from your frame rate. That's where setting the shutter at 1/60th for 30fps (which includes 30p or 60i) and 1/48th for 24fps comes from. This all goes back to film cameras where the shutter rotates.

So basically if you double the frame rate you would have the shutter speed that would be considered the general rule. But it is only because that is what we're use to seeing from film cameras. Just as with watching a movie we're use to seeing 24fps and on television we're use to seeing 30fps.

As Sam noted the frame rate, shutter speed and f-stop you choose depends greatly on the look you want. All lenses have a sweet spot so knowing your equipment is essential. For instance on a Sony EX3 stock lens going above f8 will give a soft picture.

Most camera produce a fine picture if you increase the shutter speed to a certain point. But go above it an you get that strobing effect that Sam mentioned. 1/100th will most likely be fine and will help stop the motion. It may not seem as natural again because we are use to seeing some motion blur.

A couple of things, you should never change the shutter speed within a clip. I also don't like to change the frame rate within a project. Some people do it and I have had to, especially in documentary, but I find it distracting. For a training video I would actually consider shooting 60i if there will be a lot of high speed action. That is unless you have a camera that can shoot 60p, but I'm guessing if you don't.

When do you step outside of the basic rules of thumb? As they say, when the shoot calls for it. In most cases the basic parameters will give you a very pleasing and familiar shot. If you do go outside the general rules, have a good reason for it. And, make sure you do some test shots first. You don't want to find out that the shots you captured aren't what you wanted.

Good luck,
Garrett

[Tim Lawrence]

Thanks for the info guys.

I actually did know it was called shutter speed and not frame rate, was just in a hurry when I typed that this morning.

I am still unsure with 60i/60p though.

Sam said 1/120 for 60p and Garret, you said 1/60 for 60i, so is that how it is:

60i=1/60
60p=1/120

(even though they are both 60fps?)

Thanks,

TL

[Garrett Low]

Tim,

Unless you have a viewing system that you can view at 60 frames per second you'll be watching 60p material in slow motion. TV's traditionally could only show 30fps. 60i is really two half frames put together every 1/30th of a second. If that makes sense. The i means interlaced so when played back it plays back at 30 frames per second.

The p is for progressive which means that each frame contains all of the lines of video to reproduce that frame. So in 30p every 1/30th of a second you capture a full frame of video. In 60i you would have every other line of video captured every 1/60th of a second. Then those the two adjacent frames are combined every 1/30th of a second to reproduce the complete frame.

Hence to stay 180 degrees out for 60i you would shoot at 1/60th and for for 60p you'd shoot at 1/20th.

Does that make sense?

Garrett

[Jay Webster]

Don't know if this is of any help here but...
It seems that people get confused about shutter speed (how it affects motion etc.) more in film/video than they do with still cameras. So if you have experience with still photography, here is a simple equation that I have carried around in the ole noodle for a while:

Exposure Time = fps X 360 divided by shutter angle in degrees.
So at 24fps it looks like this 24 X 360 equals 8,640.
8,640 divided by 180 degrees (the standard film shutter degree) = 48 or 1/48th of a second.
works at any frame per second rate.

Cheers!

[Garrett Low]

Jay, all your equation is doing is multiplying the frame rate by 2. 360/180 = 2.

Garrett

[Jay Webster]

Yup,
Garrett you are absolutely correct. I guess I just never looked at it that way.
Coming from a still background, I just found it useful to "simplify" in my own mind the shutter speed thing.
I have used this with car footage (over cranking) etc..
It just seemed to solidify in my own feeble mind just where I was within a given scenario.
I guess the interesting thing about all of this is that we all have our individual ways of approaching the shot.
Cheers and thanks for your useful clarification!

[Tim Lawrence]

I'm confused as to why 60 p at 1/120 will be in slow mo??

That seems to be a standard for most of the HD cams, 720 60p mode.

Are you saying that if I film something in 720 60p, edit it on FCP, and then play it as a QT or DVD it will not look normal?

Thanks,

TL

[Garrett Low]

Tim,

Shooting at 720/60p is called over cranking. It essentially is running the recorded frame rate faster than the playback frame rate. Unless you have a system that can playback footage at 60fps you will be watching it at 50% slow motion on a 30fps playback system (remember 60i is really 30fps slit into two halves so you only see one complete frame every 30 seconds), or at 40% of normal speed if shown at 24p. So yes, you will be watching slow motion.

Under cranking is the opposite, recording at a slower frame rate than the playback frame rate which yields a speeding up when viewed.

Garrett

[Shaun Roemich]

Originally Posted by Garrett Low

Shooting at 720/60p is called over cranking. It essentially is running the recorded frame rate faster than the playback frame rate. Unless you have a system that can playback footage at 60fps you will be watching it at 50% slow motion on a 30fps playback system (remember 60i is really 30fps slit into two halves so you only see one complete frame every 30 seconds), or at 40% of normal speed if shown at 24p. So yes, you will be watching slow motion.

60 frames per second destined for 24P/30P playback is overcranking. As 720P60 is a VALID distribution format, shooting 720P60 for 720P60 distribution is just plain old "normal" video.

[Tim Lawrence]

So, just to finalize.

60p looks normal on playback and should be recorded at 1/120 shutter for normal applications?

Thanks,

TL

[Shaun Roemich]

Tim: I shoot 60P at 1/60th under NORMAL conditions. It plays back at "normal" speed. I hesitate to say it "looks normal" as 60P has it's own "look", much the same as 24/30P and 60i have their own "look".

Even when I drop 60P material into a 24P timeline in FCP, it plays back in "real time" with motion characteristics of 24P. IF I want to create a 2.5x slow down effect using every frame I need to use Cinema Tools to change the playback frame rate flag to 24 and then I get slo-mo (or "overcranked")

Hope this helps.

[Tim Lawrence]

Hi Shaun,

Thanks for the answer.

I'm curious why you shoot 60p at 1/60. That would not be 180 degrees, correct?

I thought that the aim was for 180 degrees?

Thanks,

TL

[Shaun Roemich]

My experience is that most people tend to START noticing the strobing effect of faster shutter speeds once once gets into the 3 digits (1/100 and up). I stick with 1/60 NORMALLY to impart what MOST people EXPECT to see, in terms of motion blur.

Of course, if the subject is a talking head and I'm trying to minimize DOF, I'll push shutter, sometimes as high as 1/250th but I find even talking heads "suffer" any faster than that IF you are looking for a NORMAL looking motion signature.

For 24P, a 1/24th shutter speed would introduce WAY too much motion blur under normal conditions. 1/60 at 60P looks the way I want (and expect) it to look. Your needs and (more importantly) the needs of your clients and/or viewers may differ from mine. Choose accordingly. It's part math, part science, part art and part "diplomacy"!