Stupid Question on Shutter Speed

[User]

I was shooting in a very dark place. I dropped the shutter speed from 1/48 sec to 1/38 (or 1/30 can't remember exactly).
Now when I'm playing back the footage frame by frame, for every 3 frames, two are the same. Why?

[Benjamin Hilton]

what frame rate were you shooting? It sounds like you were shooting at 48fps and it that were the case the duplicated frames would be because the shutter is opening at a slower pace than your sensor is capturing frames; thus duplication. 

[User]

Interesting Ben, I'm pretty sure I shooting at 23.976. Is this the only possible option?

 

[User]

50 minutes ago, Benjamin Hilton said:

It sounds like you were shooting at 48fps

Actually Ben, come to think for it, I was using the C100 MkII and it doesn't do 48fps. Only 60fps. 

[Benjamin Hilton]

That would explain your two in every three frames then, must have been 60fps.  

[User]

2 hours ago, Benjamin Hilton said:

That would explain your two in every three frames then, must have been 60fps.  

Hi Ben, I checked the original files and they were shot at 23.976 as can be seen in the attachment.

I'm thinking @Andrew Reid could answer this in a flash. Andrew?

[Adam Kuźniar]

Care to post a sample of the recording? 

[User]

2 minutes ago, Adam Kuźniar said:

Care to post a sample of the recording? 

Hi Adam, just imaging you have the file open in a NLE and you are going through the shot frame by frame. In every 3 frames, 2 are the same. These clips look fine but when a subject moves fast there is some small blur due to the slow shutter speed.

[KnightsFan]

Is your timeline frame rate also 23.976?

[User]

1 hour ago, KnightsFan said:

Is your timeline frame rate also 23.976?

You bet amigo. The duplicate frame is there in the original AVCHD material played via Quicktime player.

I'm scouring the net looking for the reason, I haven't found a succinct answer yet but I think it has something to do (like Ben mentioned) with fitting slower refreshed shutter images (1/34) into a 23.976 frame rate... and since the 1/34 images are refreshing slower than the 1/48 images, every second image is 'doubled' so that it can keep up with the frame rate. Yeah... that has to be it.

Looking at these 1/34 shot clips, aside from some occasional motion blur, I wonder if anyone would notice that they were not shot at 1/48? I'm not sure that I would do this again but I really needed the extra light.

Hats off folks... love this place.

[KnightsFan]

You shouldn't be getting duplicated frames unless your shutter speed is actually slower than your frame rate. You should be able to shoot 1/38 shutter with 24p just fine. So that's probably not it.

You didn't transcode the footage at any stage, did you?

If you are 100% sure of the camera settings, 23.976 fps with 1/38 shutter, then my guess is that there is something wrong with your computer playing the files. If you can upload an original clip from the camera, we can verify.

Edit: Is your footage interlaced? Could it be a problem with deinterlacing?

[User]

23 minutes ago, KnightsFan said:

You shouldn't be getting duplicated frames unless your shutter speed is actually slower than your frame rate. You should be able to shoot 1/38 shutter with 24p just fine. So that's probably not it.

You didn't transcode the footage at any stage, did you?

If you are 100% sure of the camera settings, 23.976 fps with 1/38 shutter, then my guess is that there is something wrong with your computer playing the files. If you can upload an original clip from the camera, we can verify.

I mounted the original storage hard drive, and Media Info also indicates that the files are 23.976fps. I posted the attachment.

The C100 menu did not have a shutter speed that I could select that was below 1/24. I'm almost positive I chose 1/34 because the others (1/24, 1/30, 1/40 were causing flicker.  I was shooting in a PAL region but the camera was set to NTSC.

I have other files from the same location that are shot at 23.976 1/48 and are just fine.

[User]

44 minutes ago, KnightsFan said:

You didn't transcode the footage at any stage, did you?

Edit: Is your footage interlaced? Could it be a problem with deinterlacing?

2 of every 3 frames are doubled in both the original material and the transcoded ProRes material.

Material is from the C100 shot progressive.

 

* My edit:

The C100 menu did not have a shutter speed that I could select that was below 1/24. I'm almost positive I chose 1/34 because a couple(?) of the others (1/24, 1/30, 1/40) caused flicker.  I was shooting in a PAL region but the camera was set to NTSC.

 

Can anyone explain why my comment is not viable?
I think it has something to do (like Ben mentioned) with fitting slower refreshed shutter images (1/34) into a 23.976 frame rate... and since the 1/34 images are refreshing slower than the 1/48 images, every second image is 'doubled' so that it can keep up with the frame rate. 

[User]

Alrighty.
I just did some test and my memory just ain't what it use'ta could... too many carbon monoxide w/ benzene sandwiches.
You guys were right on the money in that the only shutter speeds lower that the frame rate will cause the issue. Somehow I forget this.
And somehow the camera was shooting at a shutter speed of 12fps at 23.976fps for these double images. Man.
Thanks again folks.

[User]

I added a PPro blur filter to these these 12fps images to see if I could knock back the motion blur of my subject dancing, but makes obviously makes the whole frame soft. Anyone have an interesting method to treat material that has a fair amount of motion blur so that they don't look like they've had an effect applied?