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Old October 26th, 2013, 10:15 AM   #1
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Frame Rate, 60i, Shutter Speed and, oh, so much more!

Good morning all!

I'm really having a hard time wrapping my head around something. I've researched this topic for the past 3 weeks and have found all sorts of conflicting answers and advice.

Let me give you all the details and then I'll ask my slew of questions.

I have a Canon Vixia HF21. It has multiple frame rate options. One is called PF30. One would assume the P stands for "progressive" and the F stands for "frames." So you think, "Hey! 30fps of progressive video is pretty darned good! I'll shoot at that."

Well, in the owner's manual is one of those little star things indicating tiny text at the bottom of the page, which reads, "Recorded at 60i."

OK, hold the phone. Now I'm confused. This has to be a deception, correct? Video shot at 60i can't possibly be natively 30 progressive frames, correct? That is, the camera is recording 60 FIELDS per second and then must be combining two fields into one "progressive" frame, correct?

If I'm right, and I'm probably not, but if I'm right, the way I imagine this works is that if you set shutter speed to 1/60, every time the shutter opens, you record one FIELD. To help me visualize this, I call it a picture. Yeah, sure, it's only half the picture, but it's still a picture. So at 1/60 you're effectively snapping one interlaced photo every time the shutter opens. The camera (or software, whichever) then smushes two of those photos together and voila! You have a "progressive" frame.

(Which doesn't sound truly progressive to me, as you're combining two images snapped at different moments in time, but that's another issue.)

OK, so assuming all I've said so far, allow me to pose my question. Everything I've read suggests that to capture motion "accurately" a good rule of thumb is to double your frame rate. So if you shoot at 30fps, you should set shutter to 1/60.

So...doesn't that mean that video recorded at 60i should be shot at 1/120?!?

But it seems everyone on the Internet says with conviction that 60i should be shot at 1/60.

Confused.

Any advice, thoughts, discussion on this matter would be very much appreciated.
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Old October 27th, 2013, 12:17 AM   #2
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Re: Frame Rate, 60i, Shutter Speed and, oh, so much more!

I am not familiar with that camera but I think you are overthinking this. Many cameras of that era shot a "pseudo" 24p, meaning it recreated the look of 24p but in a 60i wrapper.

Maybe I am oversimplifying it but 60i is 29.97 frames per second, so the correct shutter speed in most cases is 1/60 second.
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Old October 27th, 2013, 09:42 AM   #3
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Re: Frame Rate, 60i, Shutter Speed and, oh, so much more!

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Originally Posted by Dave Brown View Post
I am not familiar with that camera but I think you are overthinking this. Many cameras of that era shot a "pseudo" 24p, meaning it recreated the look of 24p but in a 60i wrapper.

Maybe I am oversimplifying it but 60i is 29.97 frames per second, so the correct shutter speed in most cases is 1/60 second.
You're likely right, Dave -- I probably am overthinking this...but I wish I could understand the technical workings better. In my mind 1/60 just doesn't jive. It seems to me it should be 1/120.
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Old October 27th, 2013, 10:19 AM   #4
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Re: Frame Rate, 60i, Shutter Speed and, oh, so much more!

The problem is the specification data rate for HD TV was limited by design and it works out that 1920X1080 60i is about the the same data rate as 720 60P. It is relatively easy to store a progressive frame in an interlace stream as long as its frame rate is less than the interlace frame rate. Ie 30 progressive frames can be transmitted within 60i stream. You cannot store 60 progressive frames at 1080 within a 60i 1080 stream. Also some output protocols from devises like HDSDI did not have standards for progressive frames in 1080 so the data was output in 60i even if recorded as progressive. I still like the look of the progressive frames on most modern displays so I wouldn't worry about the how it is transported unless you run into an editing issue. Definitely recommend 1/60 shutter speed for most 30P shooting if you want normal motion. Many cameras default to 1/30th which is pretty blurry on motion.
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Old October 27th, 2013, 05:46 PM   #5
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Re: Frame Rate, 60i, Shutter Speed and, oh, so much more!

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Originally Posted by Norm Kaiser View Post
OK, hold the phone. Now I'm confused. This has to be a deception, correct? Video shot at 60i can't possibly be natively 30 progressive frames, correct? That is, the camera is recording 60 FIELDS per second and then must be combining two fields into one "progressive" frame, correct?
No, not correct.

What you are describing is most properly known as "Progressive, segmented frame", or "psf". The system you describe is known as 1080psf/30.

It captures 30 complete frames every second, and typically with a shutter speed of 1/60 second - so to this extent is identical to 1080p/30, yes?

How it differs is that those frames then get split into two parts - odd lines and even lines - that's a simple process of reordering, so we get 1,3,5,7..... and 2,4,6,8....... instead of 1,2,3,4,5,6......... Hence we get two FIELDS so structurally the signal is a 1080i/30 signal, and the key point is that to an electronic device is the same as a normal 1080i/30 signal.

The difference is that for a true interlace signal, lines 2,4,6,8 etc are captured 1/60 sec later than lines 1,3,5,7.... - with psf they're all captured at the same time. Hence it is true progressive, the information is exactly the same as for 1080p/30 - the difference is solely one of reordering. Consequently, the process is totally and transparently reversible - reorder the lines again and the original true progressive signal is recreated precisely.
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Old December 1st, 2013, 02:30 PM   #6
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Re: Frame Rate, 60i, Shutter Speed and, oh, so much more!

Norm I will reply with the answer I always offer in these situations - try it out for yourself.

Since you own the camera do some testing. Record the same scene at different shutter speeds and compare watching on your TV. Then you can make an informed decision based upon primary information.

Now you are learning instead of just reading internet advice.

Shutter speed in the video world is more about smooth or stacatto motion. I have experimented with 1/90h or 1/120th for 60p shooting and I did not like the look. 1/60th was smoother for my live show application. Now when I watch NFL games, I can see they are above 1/60th which works for their application.

Test, test and test more to build up Your library of primary knowledge. It really is the only thing that matters over time because the people on the internet will not come do your jobs or look your clients in the eye.
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