Viewing film 24fps vs Video 60fps

[Ben Wiens]

Yesterday watched the film Something's Gotta Give starring Diane Keaton and Jack Nicholson. Now I want to paint everything white in my house. I was looking like a hawk for motion jerkiness in this film. I couldn't detect anything at all. And film is shot at 24 frames/second. So why did they feel the need to go to 60 frames/second interlaced with TV and video, with all the problems like inter line twitter and other horrible degrading effects? I hear that film is shot at 24 fps but somehow shown with two flashes per frame for 48 fps. How does that work? Does HDTV progressive flash double frames too, how does that work. It looks smooth. Does 24p video end up looking smooth on a HDTV. It sure doesn't look smooth directly from the camera? Man this stuff is confusing. Any feedback would be appreciated.

[Bret Pritchett]

Film has 24 frames for every one second. Its hard to describe if you've never seen a projector up close, but I'll try...

the part in the projector where the light comes thru does not move fluidly like the rest of the film. The intermittent sprocket controls the frame that is being shown on screen. It pulls the frame down 24 times ever second. That is - its not just a motor pulling at a constant speed. Here is what happens:

*the sprocket pulls down a frame of film - as its pulling it down, the shutter blade (kinda like fan with only 2 blades - it is spinning very fast in a constant motion) blocks the light [coming from the bulb] from hitting the frame - which covers up the movement of the frame changing on the screen
*then the frame is displayed on screen
*the other shutter blade blocks the light as it continues spinning
*the frame is again shown on the screen

and then that repeats

so yes, you are seeing 24 different frames per second - but to make the images on screen look more fluid - someone came up with the idea of flashing the same frame twice on screen.

and here is the mind f#ck... if you're watching a 2 hour movie in the theater - you are watching 1 hour of nothing but black on the screen. Yep, in the 48 flashes you see every second - 24 of those are just a blank screen. Your mind covers up those blank parts with....a term i forget

[Ben Wiens]

Blank, black frames. It makes sense. I did get my 16 mm film projection training years ago. When the film advances, the shutter must cover up the light beam so you won't see the moving film. But how can the picture appear to be moving so smoothly with only 24 real pictures per second. When I tested a Panasonic 24p camera, the output was really jerky.

[Glenn Chan]

Film is nearly always shot with a high (it's high right?) shutter angle which produces lots of motion blur. This makes motion look less strobe-like and smoother.
Sometimes video is shot with a fast shutter so there is less motion blur.

Projected material looks smoother than material on a TV. On a TV the phosphors take time to fade and you have the interlace thing going on. I don't know if it explains things fully.

[Ted Springer]

Bret - the term you are looking for is "persistence of vision".

[Bill Pryor]

To simplify, you're seeing 48 images per second when you watch a film projected. But when you look at something shot at 24 frames a second in the video world, it will be translated via the ol' 3:2 pulldown, which I don't really understand mathematically, but it works, and you end up seeing the strobing, stuttering motion. This is why I end up not being able to use lots of shots, especially pans, done by 16mm cinematographers who were not aware their client was going to video instead of film with the project, or if they were aware they didn't know any better. To me the best looking image for video is 30 fps progressive, as opposed to normal 30 fps which is 60 fields interlaced. Shooting at 24fps is great if it's film or if the video is going to be transferred to film.

[Bret Pritchett]

Ted - thats it! thanks - I couldn't think of it and its been annoying me all day