Is 60i really not 60fps?
 

Ok so I posted a video on youtube of skateboarding in slow motion, and titled it as being shot in 60fps. you can refer to the video here: YouTube - Canon XHA1 Skateboarding in SLOW MOTION 60fps *HIGH DEFINITION*

And I got a comment that was slightly confusing to me.... This person comments as saying:
"sorry Robert, but that cam does not shoot at 60fps."

And I replied that it does in fact shoot at 60fps....

To which he replied:

"What you must understand about the XHA1 is that it shoots "60i" footage, not 60f, or 60p. This means you are shooting "interlaced" frames. which 60/2 would be 30fps. Simple misunderstanding, I almost made the same mistake! I can explain more via PM."

Does he have any truth to his statement? I feel as though he's slightly off.... but also sort of see what he's trying to say. I searched around here and started to read up on things saying 60i isn't really frames per second but actually "Fields" per second or something along those lines, and am not really sure what to think now.

any help would be greatly appreciated.

Thanks!

He's absolutely right.

The XH-A1 shoots 60i, which is 60 interlaced fields per second.

Think of a video picture as a venentian blind.

If you have all the slats, you have one frame.

If you remove every other slat you have one field.
If you put two fields together, you have one frame.

The display device shows one field then the next field at the rate of 60 fields per second.

The XH-A1 has modes to shoot 24F (24 frames per second, professive video) and 30F (30 frames per second, progressive video), but not 60 frames per second, only 60i (interlaced video at 60 fields per second).

Here are a couple of Wikipedia articles:
Interlaced Video:
Interlace - Wikipedia, the free encyclopedia
Progressive Video:
Progressive scan - Wikipedia, the free encyclopedia

Some cameras, such as the JVC HD200 (or the yet unreleased JVC HM100) do shoot 60 frames (progressive) a second as an option, as well as 60i, 24p (24f on Canon), and 30p (30f on Canon).

If you take 1 second of video....

60P (progressive) 60x1=60 frames per second
60i (interlaced) 60/2x1=30 frames per second

Progressive footage is like shooting a still camera at 60 pictures per
second...

Interlaced footage is two fields per frame... each field has alternating
information... when they are played back the rate is fast enough that
most of us just see just one frame.

Most of the time when you slow down your footage to get slo motion its
best to either shoot in 60P or convert your 60i footage to progressive in post ...

The reason why is the software has a much better chance of getting the interpolation
of the footage correctly if its working with full frames versus interlaced fields...

Why? because if you take some footage thats say 10 seconds long and you want it
to play at half speed the software will have to stretch the footage out to 20 seconds.
The software does this by creating twice the amount of frames. It does this by looking
at one frame and comparing it to the next frame, then creates a blended frame from
the two originals... if you try to blend interlaced fields you will get artifacts...

Some of the slo motion software even allow you to set how much blending you want and
how much motion blurr to put into the footage.

Ok I'm beginning to understand what you're saying. So 60i means that it's showing 60 fields per second, 30 odd and 30 even, but how often? every 1/30 of a second? meaning that there are 30 frames in one second = 30fps? the wikipedia page gives the example for PAL and says 50i, 25 odd 25 even every 1/25 of a second meaning 25fps.

is my explanation for the 30fps correct? I'm just confused when you say: "not 60 frames per second, only 60i (interlaced video at 60 frames per second)." you didn't happen to mean "only 60i (interlaced video at 30 frames per second)" did you?

if 60i is really 30fps then why is there a 30f setting along with the 60i setting?.... ah I'm getting confused again haha.

Because at 30F, the camera is capturing 30 TRUE frames per second, and not interlacing them. It looks different. Especially on a display that can do progressive, like you computer screen.

You are right, I typed a mistake (now corrected above): 60i is 60 interlaced fields per second.

Then, putting two matching fields together, you end up with 30 full frames per second. However, as pointed out before, fields are displayed one at a time and human brains (and delay in the picture vanishing) combines the two frames into a single field.

i record rollerblading, shoot in 60i, at a variable of different shutter speeds, (am i correct in saying the higher shutter speed also helps in the quality of slow motion?)

i do de-interlace to either odd or even (FCP)

after all this, does the frame rate become 30p??


~thanks
Janson Williams is online now Report Post Edit/Delete Message

Yes it does become 30p when you deinterlace.

right on, thank you Curt

Yes, yes and yes. But by deinterlacing you're losing half your vertical resolution. If the finished product looks the way you want, then don't worry about it.

30f will eliminate the need to deinterlace and preserve vertical res. However, it makes getting clean slomo more difficult. Some NLEs will use the interlaced fields to create smoother slomo. Can't speak to FCP since I've never used it.

ok Tripp
you have been a lot of help for me,

thanks

The higher shutter speed when you shoot sports (rollerblading) just allows you to get a
sharper picture without too much motion blurr...

A good slomotion software package will allow you to add in motion blurr, you can add
blurr to any footage, but its difficult to remove blurr from footage taken at too slow of
a shutter speed.

when you say to slow of shutter speed, you are referring to below 1/60 (for 60i)?

Hi Janson. Iif you are going to make slo-mo shots of fast action like roller blading, I suggest you use a shutter speed of 1/120s or shorter. Using 1/60s or longer will produce too much motion blur when you slow down the footage. As Ray says, if you need to add motion blur later, you can.

Richard