This example is a 7 minute 38 second clip with 4 copies of a short 16mm film. The film was originally shot at 16 FPS ( Frames Per Second). In order to digitize the film properly, each frame must be accurately captured but must also conform to normal standards used in the digital world.
With a film shot at 16 FPS and played at 24 FPS, the issue becomes clear when the motion of the subjects in the film move too fast. In order to correct for the proper speed, new frames must be created to bring the film motion speed to normal when played at 24 FPS.
Three methods are used to accomplish this. This video has copies of the same scene:
First is the original digitized film which has motion that is too fast for real life movement. Note the next 3 clips after the original. they are of the same shot. They have had new frames created to allow for normal motion of the film speed when played back.
Method 1. Is creating new frames using a duplication of frames to copy exactly one frame into multiple frames to achieve the needed extra frames to play at 24 FPS. The disadvantage of this, is film motion that seems to hop from one frame to the next. Basically not a smooth motion.
Method 2. Is creating new frames by selecting not only the clip being played but the past and upcoming frames and copying those frames to 50% opacity and then overlaying them on top of each other to blend the picture and improve motion so that it is smoother. The disadvantage of this method is a ghosting effect during more motion movements. Edges seem to have a slight double vision effect during the movement of subjects.
Method 3. Is creating completely new frames never seen before by estimating what the motion of objects may be and adding those frames in between to raise the frame count to 24FPS. This method often has the smoothest and most preferred motion of real life. The disadvantages of this method can be seen when an object is moving rather fast with a background not moving. This disadvantage effect may show up in some areas of the picture as a warping of the picture when an object moves across a still non moving object. The effect is a kind of dragging along of the non moving object with the moving object.
The bottom line is this: All conforming of film to the correct speed has advantages and disadvantages. Which method that eventually is chosen all depends on that particular scene and which disadvantage you are willing to accept in the final output.
This is where paying close attention to each method can greatly assist in the decision of how to conform a film running too fast so that it is corrected to run at the proper real time speed of normal life.