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80% of your movie like feel comes from the color grading, 20% from the motion artifacts caused by 24 fps frame rate. In a theater when you watch a film print that is being projected the film goes through the gate at 24 frames per second, but to keep from giving you a headache, the projector is equipped with a shutter that opens and closes twice for each frame so you see 48 flashes of light. In other words, your eyes get 48 images per second but only every other one is different. If you're watching a movie and the shot is a wide shot of a vast vista with no motion in the scene you don't automatically start to think that you are no longer watching a movie even though all of the frames you are seeing are the same. This even holds true for close-ups of actors with very little movement.
Let's talk about watching movies on Television. If you are watching broadcast TV all Standard Definition programming is interlaced. The program material may be produced but the broadcast signal is interlaced. In NTSC land you're seeing 60i, in PAL land it's 50i. The only difference is that some programming may have duplicate frames like in a movie theater. If a 24 fps film is transferred to television they do a thing called 3:2 pulldown. Remember that there are actually 60 fields per second but that a field is half a frame, it's every other line. 3:2 pulldown is actually two fields from frame #1 then one field also from frame #1 and one field from frame #2 then two fields from frame #2. This is the magic that makes it possible to play a 24 FPS movie on at device, a projector if you will, that only runs at 30 frames made from 60 fields per second. BTW, just as a side note, all of those cartoons you loved as a kid that you watched on TV, you know, Looney Tunes, were actually only 12 cells (frames) pers second. They just turned the 12 animation cells per second of action into 24 frame per second film by taking two frames of each animation cell.
You basically reverse the process to turn 30 fps footage into 24 fps footage. The easiest, but not best way to do it is to make sure your footage is properly interpreted in the first place. If you're shooting 30p then the info on the footage should say 30p or 29.97 fps in the info at the top of the project panel or 29.97 fps in the interpret footage popup with separate fields turned off. If your footage is 60p then the info should indicate that fields are being separated and the frame rate is 29.97 fps. Your camera is probably not interlacing the footage because that hasn't happened in a camera for a long time, so you have a higher resolution image than a camera that shoots interlaced, and you have more frames available for things like slow motion, but the footage is played back at 30 (29.97) frames per second for normal viewing.
If your footage is properly interpreted then all you have to do to get 24 fps (23.976) footage in a 23..976 fps composition. You'll have exactly what you see when you watch a movie on television. In frame #1 you'll see frame number 1 of your original footage but in frame number 2 you will see a blend of frame number 1 of your original footage and frame number 2 of the original, and so on. The problem is that every third frame that is a blended combination of two different slices of time. It can be a little soft.
The solution that best fixes this slightly soft frame is 3rd party software. Either Twixtor or Magic Bullet work very well at smoothing out the every third frame softness and giving you the motion artifacts kind of experienced in a movie theater but more like the viewing experience of watching a movie on TV. Turning on pixel blending in AE will also help a little.
The true motion artifacts caused by filming at 24 frames per second with a standard motion picture camera can only be achieved by using a motion picture camera because of the way the film is exposed by a horizontal moving shutter. Video and digital cameras scan in lines from the top of the frame to the bottom. Inexpensive cameras take longer to do this scan than expensive ones so you get distortion when things move horizontally in the frame. This is often called 'jello cam' or 'rolling shutter ripple.' You will get less rolling shutter ripple with your camera shooting at 60i but you will also get less motion blur.
The other motion artifact you get with a motion picture camera is caused by a stroboscopic effect caused by the interaction of the frame rate and the retinal retention of the human eye. When you pan a standard motion picture camera across a scene there are critical panning speeds that will create horrible image strobing or judder. Open up any cinematographers manual and you'll find a table with these critical panning speeds listed. Sometimes in a move you'll see a pan that is really uncomfortable to watch because of this problem. It has existed since the first movie was made. To avoid these problems you shoot at a higher frame rate. Anything above 30 frames minimizes this stroboscopic judder. Above 48 fps it's basically gone. Removing these often objectionable motion artifacts is why film makers like Peter Jackson are experimenting with higher frame rates. "The Hobbit" was shot at 48 or more and delivered at 48. Many people, including my wife, didn't like watching the higher frame rate footage in the theater because they were expecting a different "look." More on this later. The same problem exists for a tilt and shows up in poorly timed credit rolls where the titles become unreadable.
Solving the juddering or strobing with motion problem however wipes out one of the other temporal artifacts you get with a standard motion picture camera. Motion Blur. A standard motion picture camera uses a 170º rotating shutter that gives you an exposure time of 1/51 second. This introduces a certain amount of motion blur. The motion blur is closer to 1/60 second exposure time than it is to 1/30 but motion blur is not linear so if you want to approach the same motion blur you'll see in a movie camera when using a video camera, if the shutter speed is adjustable, then shoot at 1/50th or less. On most cameras, that means you should set the shutter to 1/30.
So there's a very long answer to a short question. The short answer is "Yes, you can convert without slowing down, just drag your properly interpreted footage into a 23.976 fps (or 24, or 12 for that matter) composition. For better results first try pixel motion, then try third party solutions like Twixtor or Magic Bullet. To simulate more motion blur in your 60i footage try adding CC Force motion blur."
The long answer is above, but, as I said in the beginning, the best way to make digital footage look the most like film is the properly color grade the footage. Remember my comments on "The Hobbit?" I watched both versions, standard projection and HFR, and the color grading was different on both versions. The standard projection was much warmer and the overall image was a little softer. I don't know if it was the difference in the projectors or if the film was actually given two different color grades in post, but my wife was very happy with the standard frame rate version but walked out of the HFR one and sat in the lobby until I joined her to watch the standard frame rate version. I made a boatload of money from 1975 to about 1985 shooting commercials on 35mm film by running the camera at 29.97 fps and transfering to tape frame for frame instead of using 3:2 pulldown. Some of the biggest add agencies in the country thought I was a genius because my footage looked much better than other DP's when my only 'secret' was getting rid of the 3:2 pull down problem and reducing judder in panning shots by shooting at the higher frame rates.
Thank you very much for this excelent and complete answer! This really helps me out, and I get to know more about filming, it's such an interessing subject!
Thank you again, I really really really appreciated!
Your posting provides a very thorough description of the displeasing nature some of the newer films. My family and i watched the second movie int he "Hunger Games" trilogy ("Catching Fire") this weekend, and the high frame rate of that movie (and other HFR movies) completely "breaks the 4th wall" for me. An amazing feature to have on a TV is the ability to change the frame rate via some sort of interpolation.
Amusingly, my wife was so disengaged from the experience of watching the movie that she became bored and left the room.
Again, great post.
Catching Fire wasn't shot at a HFR. It was shot at the motion picture standard of 24fps. If it had the smoothness of HFR then you likely have smooth motion turned on in your TV. This interpolates frames in between the original 24 to make it look like 60fps. This feature isn't bad for sports but it ruins movies. Turn off smooth motion on your TV. Certain scenes in Catching Fire were filmed on 70mm IMAX film instead of 35mm standard film. These scene will have an added clarity and vibrancy that may seem unnatural compared to the rest of the film, but it was definitely shot at 24fps. Shooting 70mm IMAX film at 48fps would be unbelievably expensive.