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fps vs seconds per frame


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#1 Quinten Densak

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Posted 24 January 2007 - 03:11 PM

Could someone please explain the difference please. Also, what are the rules for exposure compensation for high and low speed shooting. I know you need to open up 1 stop shooting at 48 fps. What about 12 fps?

Hopefully and greatfully

Quinten d
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#2 Stephen Williams

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Posted 24 January 2007 - 04:19 PM

Could someone please explain the difference please. Also, what are the rules for exposure compensation for high and low speed shooting. I know you need to open up 1 stop shooting at 48 fps. What about 12 fps?

Hopefully and greatfully

Quinten d
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Hi,

96 fps Open 2 stops
48 fps Open 1 stop
24 fps Normal,
12 fps Close 1 stop
6 fps Close 2 stops
3 fps close 3 stops
1.5 fps close 4 stops
0.75 fps close 5 stops
1.5 SPF close 6 stops
3 SPF close 7 stops
6 SPF close 8 stops
12 SPF close 9 stops

Seconds per frame is an exposure greater than 1 second often used with time lapse. Its where a ND 3.0 (10 stops) comes in useful

Stephen
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#3 Nick Mulder

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Posted 24 January 2007 - 04:22 PM

Could someone please explain the difference please. Also, what are the rules for exposure compensation for high and low speed shooting. I know you need to open up 1 stop shooting at 48 fps. What about 12 fps?

Hopefully and greatfully

Quinten d
Student


one is the reciprocal of the other ...

fps tells you how many frames you shoot per second eg. 24fps = 24 frames per second

seconds per frame is how much time a frame takes up temporally eg. 1/24 = 0.0416666... seconds

If every other factor is kept equal for the time being then yes you will need to adjust something for 1-stop more light when shooting at 48fps and 1-stop less when shooting at 12fps - often it is just an f-stop on the lens, but there are the options of pushing/pulling the film, adjusting shutter angle and pouring more light on the scene

thats the simple answer to your question but in reality (film) cameras only use a fraction of that 0.04166666... seconds
If you want the real deal for film making (ie. super8, 16mm, 35mm etc...) keep on reading:

Due to the mechanical system used a frame cannot be instantaneously in place as soon as the last one has completed its exposure, the system called the 'pull-down' (its usually a mechanical claw) often takes half of the that 1/24th of a second to move the film into place and keep it there so you are only exposing the frame for half of that 1/24th of a second ie. 0.0146666/2 = 0.020833333... 'seconds per frame' (a relative 48fps)

hmmm... - search for 'shutter angle' here in the forums - you'll get a heap of info, without this knowledge your understanding of film cameras and exposure could be a stop or two out ;)
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#4 Quinten Densak

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Posted 04 February 2007 - 12:03 PM


fps tells you how many frames you shoot per second eg. 24fps = 24 frames per second

seconds per frame is how much time a frame takes up temporally eg. 1/24 = 0.0416666... seconds
[b][


Thanks Nick and Steven

If i expose 24 frames per second shooting 24fps, When i expose at 1 frame ps, how many/much of the frame am i using?

Also: does 98 fps capture more information than 24 fps?

Nick, you mentioned the nd 3.0. I read up on that and i now know that nd .3 is 1 stop compensation. so if im reading f 22 on my meter, 9 stops would be f 1 correct?

Thanks you for your help.

I hope you cam answer my other two questions
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#5 David Mullen ASC

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Posted 04 February 2007 - 12:25 PM

Quinten, please change your Display Name to a first and last name, as per the rules of this forum posted when you registered. Click on My Controls to do this.

F-stop compensation is all about halves and doubles -- i.e. doubling the ASA speed is equal to one stop more exposure, doubling the camera speed loses you one stop of exposure, etc. Every f-stop represents a doubling or halving of exposure compared to the next f-stop number.

So 12 fps is half of 24 fps, so you gain one-stop of exposure, 6 fps is half of 12, so that's another stop, 3 fps is half of 6, another stop... since 1.5 is half of 3, you can figure that going from 3 fps to 1 fps is about a stop and a half more exposure.

So that's 4 1/2 stops total more exposure per frame by running the camera at 1 fps instead of 24 fps, assuming the shutter angle is the same (180 degrees.) Since 180 is half of 360, a half-circle spinning in front of the gate, this means that at 24 fps, the per-frame exposure time is 1/48th of a second (i.e. the shutter is closed for half of the 1/24th of a second that the film is in the gate when running at 24 frames per second.) So at 1 fps, the exposure time is 1/2 of a second per frame. That's a lot of blur per frame, hence why intervelometers allow you to alter the per frame exposure time separately from the frame rate.

96 fps captures more temporal information, sure, but since 96 fps is not a projection speed, you get a slow-motion effect when it is later played back at 24 fps.

The f-stop below f/22 is f/16.
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#6 Quinten Densak

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Posted 04 February 2007 - 01:04 PM

[quote name='David Mullen ASC' date='Feb 4 2007, 09:25 AM' post='152578']
Quinten, please change your Display Name to a first and last name, as per the rules of this forum posted when you registered. Click on My Controls to do this. F-stop compensation is all about halves and doubles --
[/b]

Thanks david

I have edited my info as you asked.
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#7 Hugo Alexandre

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Posted 06 October 2008 - 09:16 PM

Hello. I have a question concerning this issue.

If I am shooting a scene at f-stop 4.0, at 24fps, and I decide for some reason to shoot it at 48fps, since the film is moving faster inside the camera isn't each frame receiving less light? So wouldn't the logical thing to do be to open up the lens to have more light, bringing it to an f-stop of 2.8?

I believe my logic is faulty because I was told to change the f-stop to 5.6 if it is at 4.0 and I want to go from 24fps to 48fps, thus closing the lens.

Thank you for your help.
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#8 David Mullen ASC

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Posted 06 October 2008 - 11:28 PM

If I am shooting a scene at f-stop 4.0, at 24fps, and I decide for some reason to shoot it at 48fps, since the film is moving faster inside the camera isn't each frame receiving less light? So wouldn't the logical thing to do be to open up the lens to have more light, bringing it to an f-stop of 2.8?

I believe my logic is faulty because I was told to change the f-stop to 5.6 if it is at 4.0 and I want to go from 24fps to 48fps, thus closing the lens.


No, you were correct, you need to open up one stop when running the camera twice as fast because each frame is being exposed for half as long.
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