One of the reasons for that is that most frames of moving pictures
are not very sharp anyway, often under 1K actually, regardless the
resolution of the camera. This is because of the use of long exposure
times and motion blur.
Here is an experiment you can do at home: Take a blu-ray from your
collection, and (this is important) browse to your favorite scene
(not a test chart!), and freeze an actual frame of that very killer
scene you like so much with your favorite actor/actress in action! I
bet he/she is probably moving (Be it a little or a lot). Now zoom in
on that frame and assess resolution on the main subject (not the
background). To measure more than 1K resolution on a blu-ray HD 1920
image adjacent pixels must be differentiated on a single row or
column of pixels. If adjacent pixels are the same, or almost the
same, because the subject _or_ the camera moved even a single pixel
during the 1/48th of exposure, then the actual resolution of the
image at hand is 1K (or less). And you will note that this is true
for many, if not most image frames in commercial movies. And 1K
images, shot/projected at 1K, 2K or 4K will be just 1K. The 4K
projector will allow you to see a smoother result (with less annoying
visible projector pixels) but the image (content) resolution is still
the same.
The reason why most cinema images are unsharp is because of a
mandatory countermeasure implemented to counter excessive strobing.
24 fps is well beneath what could be called an adequate frame rate,
from a vision science point of view, and therefore strong measures
are mandatory to avoid excessive strobing/judder.
The main countermeasure applied in cinema is motion blur, created by
using a long exposure time per frame. With 'long' I mean: 1/48th of a
second, which is several orders of magnitude longer than what a still
photographer would use, to get detailed, fine and sharp image of a
moving subject.
But we have no choice because 24 fps presentation simply does not
allow to present sharp images and movement at the same time.
Impossible. It is either one, or the other. If we try to present
both, then it strobes. Images must be either static, or fuzzy. Sharp
images that move, are forbidden in our medium, by rules of physics.
Giving the images a long exposure time is convenient because it will
automatically record sharp images as soon as movement is absent, and
make the images unsharp as soon as anything moves. And it is
proportional too: the more movement, the more unsharp the images get,
conveniently counteracting the strobing.
The downside, however, it that this method renders most cinema image
frames unsharp, as the idea was that the majority of cinema images
would move.
A fixed test chart for evaluating image resolution of a moving
picture system is maybe not actually that relevant after all. When I
shoot a movie then I most often shoot talent. He/she generally moves.
And so do I with my camera. The test chart does not. So the question
can be asked: Is the test with the chart actually relevant to what I
actually intend to use that camera for?
There _is_ a way, to make movies with high resolution. At frame rates
above 48 fps, strobing goes away altogether, and any exposure time,
regardless how short, will work without strobing. Now we can make the
images as sharp as we want, also when they move.
However, there is a catch. If we use this, then that cinema will look
different. It may require some re-thinking of storytelling, and
learning to actually use the new features that will become available
to us. And some long standing rules and methods might not apply in
the same ways any more.
Woow, that's scary! And indeed it is. But it is also exiting and a
way to progress. Progress always has been a bit scary. But also a way
forward. Making images that are in high resolution, also while they
move, is an important change to the medium. We are not used to that
in cinema. It requires not only higher frame rates and shorter
shutter times, it will probably also require some thinking and
research, development and invention in film language and
storytelling. But I think we can do that. I am sure. We already
managed for sound, color, 3D, etc.... :-)