Resolution … Promises I Don’t Intend To Keep

How important is resolution in the grand scheme of things?  This one starts out as follows.  Is it possible for one to see more detail from a well mastered Blu ray disc than on film at the local theater or multiplex?

Yes, it’s a rhetorical question because it would not be asked if the answer was no.

Some of the answers to this lies in a study initiated by SMPTE back in 2004 to determine how much detail people actually could resolve when watching film in a theater at the typical best seat in the house.  Normally speaking, how much we can see depends entirely on where we sit in the movie theater.  Those sitting is row 5 would be expected to see more detail than those sitting in the back of the theater.

SI1280F_Resolution_ChartThe 35 mm film that makes it into a theater is many copies removed from the original camera negative that the film was shot on. Not surprisingly, every time a copy is made from another copy of the film, resolution or detail is lost. Although film is not digital in nature, it is estimated that one frame of 35 mm film as taken from most theaters has a rough resolution of 4000 pixels by 3000 pixels.  The frames are 4:3 aspect ratio after all.  So this gives 12 mega pixels to the film as our starting point.

In the SMPTE test, they projected test patterns on the screen and the participants were able to see roughly 4 mega pixels worth of information from the best seat in the house.  Think of this as the center of the theater.  So at this distance, a 12 mega pixel image has already been reduced to 4 megs worth of data.  And 4 megs is still double the two mega pixels of Blu Ray.

At this point in our story, film still has the disc beat with double the resolution.

Let’s move onto what film projectors do to the film.  In the THX class, when there is time, I would sometimes ask the class what movies are typically shot in, in terms of frames per second.  This one is an easy one.  Everyone shouts out 24 frames per second.  And they would be right.

Then I ask the question of why 24 frames per second?  Why not 20 FPS?  Why not 28 FPS?  Why not 16 FPS?

Now the class does not volunteer an answer so readily.  What is so special about 24 frames per second?

As it turns out, what do films shot at 16 FPS look like?  What do films look like at 20 FPS?  Specifically, what does human motion look like when presented at these off speeds?  The answer is that human motion does not look right.  It looks jerky and unnatural.  The magic of 24 frames per seconds is that this is the minimum number of frames needed for human motion to look normal to us when we watch a film.  So 24 frames per second is the minimum number needed, but why not use 26 FPS or 32 FPS?  Why stick to 24 frames per second?

The answer here lies with when this was actually determined; 1920’s or so.  This was a time when film was the only medium and film stock costs money.  Developing that film stock also costs a lot of money.  If the appearance of human motion is achieved at 24 frames per second, there was no reason to increase the frame count unless a special effect was being looked at.  Figure the 3000+ frames per second needed to capture a bullet being fired from a gun going through a sheet of glass.  So that is the magic of 24 frames per second and it has stuck.

Now the next question.  How many movie theaters actually projected the film at 24 frames per second?  The answer there is few to none.  What is the inherent problem with projecting a film at 24 frames per second?  The issue now is not about human motion looking right, but rather the frame rate itself.  At this speed, the human eye can easily see the spaces between each frame as it is being shown.  This is the flickering of the image as it is being projected.  It is annoying to watch.  Most film projectors actually end up showing the film at 48 frames per second or 72 frames per second or more.  They simply repeat the same frames two to three times or more.  The flicker is still present, but it now occurs at much faster rate that the human eye can no longer see it happening.  The image becomes more solid and watchable.

So back to the original question about resolution and detail of film.  Within a film projector, the individual film frame is moved into a gate mechanism and locked in for a brief split second while the shutter on the projector opens to hit the frame with light and the image gets projected onto the screen in the theater.  Here is were the weak link is found.  The projector.

The locking of the film frame is a violent action that causes the film to bounce up and down.  If I show you a 12 meg image and bounce it up and down, will you still be able to see 12 megs of information?

When the shutter opens and the light hits this bouncing film frame, the light comes from a hot light bulb.  This is a hot gust of air that hits the frame and will cause the frame to move outward away from the light source.  The gate closes and the film cools for a moment and comes back into the original position; then the shutter opens again and the process repeats itself.  A film frame moving closer and farther away from a lens means the image is now defocusing and this causes additional loss in detail on that 12 meg film frame.

What actually makes it to the screen through the projector lens is something in the order of 1 million to 1.5 million pixels of detail.  The mighty 12 meg image has been reduced to this.  So if you are in the theater and you feel the images are not as detailed or crisp as you see at home, you are not imagining things.  It actually might be worse than that Blu ray at home.  The Blu ray material might actually be twice as detailed on a good home system compared to the theater.

Of note here is that the resolution loss is only applicable to film based projectors.  A digital cinema projector be it a 2K unit or 4K will not suffer from this effect as there is no gate mechanism or hot blasts of air that affect film frames.  All 2K worth of detail makes it to the screen in the 2K theater.

 

 

 

 

 

 

Michael Chen

Michael Chen is the only THX Video Systems Instructor in Canada, and beyond these borders, is one of just two THX Video Instructors in the entire world.  He has actively consulted with Spectracal and ChromaPure and has created numerous education videos on the calibration process with still more to come.  His Video Calibration Training Series has quickly become the most comprehensive and simple to understand learning tool on the market today.  He has also taught classes for both the ISF and Spectracal as well and is now spearheading his all new TLVEXP calibration training program. Let Michael teach you Video Calibration and add that additional income stream to your installation and integration business

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