Motion Pictures: Part 4 - Improving Dynamic Resolution

There is definitely such a thing as the film look. Much of it is because of the serious limitations caused by the low frame rate.

Television and motion pictures both involve the illusion of motion, but they turn out completely different in real life. I think the greatest contrast can be found between the motion picture and electronic news gathering.

The motion picture cannot, despite its name, portray motion at all well, but on the other hand the motion picture is a form of story telling in which the technological parts have been drawn in and used to help to tell the story. Every take is pre-planned to a remarkable degree of precision. It is known what the actors will say because they are adhering to a script. The dynamics of the take will be established before any actual filming takes place.

In order to stay within the motion limitations, a tracking shot may be used. This may require rails to be laid along which the camera dolly travels in an agreed way. No one cares how heavy the equipment is. In fact, excess mass may be desirable because it helps to smooth out motion. The actors will be required to move along an agreed path so the camera can track them and so that the focus puller knows what focus setting to use at any point in the take. Lights will carefully be positioned. Nothing is left to chance. Moviemakers sometimes come across as being extremely picky, but that follows from the pursuit of excellence that may even be a form of love.

When the take is actually filmed, it may have been rehearsed many times. The relative movement between the actors and the camera is minimized by the tracking. Background strobing is minimized using a relatively long shutter speed and finally, the use of a large lens and a large sensor allows the depth of focus to be reduced, not just for the purpose of defocusing the sampled background, but also as an integral part of the story telling, because the viewer will automatically look at the parts of the screen that are in focus.

The lack of relative motion achieved by tracking allows the system to work despite the low frame rate. The impression of motion comes from the background whose motion has been smoothed out by two filters. The first one being the smear due to the long shutter speed and the second one being that the background is out of focus.

As the making of a movie does not happen in real time, the opportunity is taken to review each take in case something does not look right. Electronic film production allows immediate viewing of rushes. Further takes can be used until the desired result is achieved.

In contrast, the ENG videographer often has no control whatsoever over the movement or actions of people that are being recorded. Available light may have to be used. There is no script and no story being told and there will have been no rehearsal and probably no chance of a re-take. There is no budget for a focus puller and even if there was, he would not know what to focus on.

That completely different set of requirements reflects in the hardware. The videographer does not want to be weighed down by a ton of equipment. With a higher picture rate of 50 or 60Hz, there is no requirement for shallow depth of focus. In an uncontrolled environment a wide depth of focus may be an advantage. The camera can have smaller lenses and a smaller sensor, which makes it lighter and cheaper.

The videographer has different priorities to the movie maker. Instead of shooting a masterpiece it may be more important to avoid the occasional flying rock.

Even though it all ends up as a bunch of ones and zeros on some storage medium, I think it is clear that the two genres are going to look rather different. It could not be otherwise.

If the motion portrayal of movies is so bad, then why not increase the frame rate? In the days of actual film, that would have been expensive and the consumption of film would increase in proportion. There have been some brave attempts, for example Todd-AO. When Mike Todd proposed upgrading cinema film from 35mm to 70mm, he also proposed raising the frame rate to 30Hz. Although that would have been a technical improvement, it caused practical difficulties because only certain cinemas had 70mm projectors. Release prints for the rest would have to be at 24Hz.

That required all filming to use two cameras, one for each frame rate and it was not practicable. So 30Hz movies died. Now that cinema is electronic, all cinemas can project at any frame rate, so there would be no technical difficulty in using a higher rate.

Although there is no technical difficulty, it was astonishing to find considerable and vociferous resistance to attempts to add higher frame rates into electronic cinema standards, although those attempts eventually succeeded.

It is hard to see why there should have been so much resistance, but then it was seen earlier when efforts were made in television to get rid of interlace. Today no one dreams of using interlace.

Cinema is one of the oldest technologies still in use and it was designed when there was no theoretical basis available to explain it. The choice of 24Hz as a frame rate was not based on any aspect of human vision. Cinematographers had to make the best of movie film without any theoretical guidance. They did remarkably well, learning from experience what worked and what didn’t especially in the area of dealing with motion.

My own suspicion is that in the absence of a theoretical framework, cinematographers learned what to do but not why they were doing it. I suspect that higher frame rates were viewed as a threat because what was currently being done might not work and it might be hard to figure out why without a lot of trial and error. Nor should we forget the growing management technique, which is to avoid making mistakes by not making any decisions.

One of the key truths that modern understanding reveals about film (and television, for that matter) is that static resolution, the performance of the film or the electronic equivalent in the presence of a still picture is meaningless. The only meaningful figure is dynamic resolution; the effective resolution presented to a tracking eye by a moving object. The lower the frame rate, the bigger the difference.

It should have been obvious that raising the frame rate would improve dynamic resolution. The reduction is background strobing no longer required shallow depth of focus, so backgrounds could be seen more clearly. Yet some high frame rate movies made it all the way to the cinema where audiences complained that they could see that the sets were artificial because they had been made good enough for 24Hz. No one had told the set builders they would need to do better, let alone why it was necessary.

The problem could have been avoided by using the same depth of focus that would have been used for a 24Hz movie. In general, there is no reason not to shoot the whole movie under the constraints of 24Hz practice and simply to use the higher frame rate to give a smoother version of the usual film look. 

Fig.1 - At a) a 48Hz movie time axis is shown with a 50% shutter opening period. At b) every other frame has been omitted and the 24Hz movie appears to have been shot with a 25% shutter opening period.

Fig.1 - At a) a 48Hz movie time axis is shown with a 50% shutter opening period. At b) every other frame has been omitted and the 24Hz movie appears to have been shot with a 25% shutter opening period.

This was typically not done and for that and other reasons, high frame rate movies were not necessarily a technical success. Another aspect of the lack of a theoretical framework is that most of what most journalists wrote about HFR was technically complete tosh. The closest some got to the truth was that audiences were so used to 24Hz that they had subconsciously decided it was normal and that, say, 48Hz was wrong. In that case they would accept the high frame rates in time.

Some HFR movies were compromised from the outset. In one case the 48Hz movie had to be shot in such a way that for some markets a 24Hz version could economically be produced. At first sight, it would seem that the requirement was trivial. Surely a 24Hz movie can be made by omitting every other frame from a 48Hz movie? Sadly that doesn’t work.

Fig.1a) shows the timing of a 48Hz movie and the shutter opening time which is about 50% of the frame period. Fig.1b) shows the effect of omitting every other frame. The frame rate is now 24Hz, but the movie appears to have been shot with a camera having a shutter opening time of only 25% of the frame period. It is not possible to have it both ways. Either the shutter time of the 48Hz camera has to be extended to prevent background strobing in the 24Hz version, in which case the dynamic resolution of the 48Hz version will be compromised, or the camera is optimized for 48Hz and the 24Hz version is compromised. Electronic processing of the 24Hz frames to de-focus the strobing is possible but would be an extra expense. 

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