I'm studying the Canon cinema lenses, and they look to be awesome glass. They tout the "4K resolution" for motion pictures, but how does that differ from stills? The RED Epic is 5K at 5120x2700 (16x9), a Canon APS-C is 5184x3456 (3x2). So aren't still lenses resolved out for the full resolution of their stills? Other than f-stop vs t-stop, how are lenses "resolved out" for the different cameras? Thanks for any insight I lack...
For theoretical comparison, you're looking at "perfect case" scenarios concerning how many line pairs per millimeter these camera sensors can resolve (real world results may be considerably lower because of the use of low-pass optical filters and Bayer pattern pixel sampling).
For this you're looking at pixel pitch (5.4 for the Epic as far as I could find, and 4.3 for the 7D) and airy disc size (controlled by the lens aperture). Reading charts widely available online, the theoretical maximum resolution at 50% MTF is ~90 lp/mm for the Epic and 116 lp/mm for the 7D.
Lenses, depending on their aperture size (and airy disc size) will resolve varying amounts of detail. The wider the aperture, the more possible resolution although the lens has to fight distortion and optical aberrations.
The 7D has a diffraction limit at f/6.9 (if you took a "perfect lens") which means the sensor will out-resolve the lens at any aperture that is smaller.
The Epic, on the other hand has a diffraction limit around f/16. A perfect lens is out-resolving the Epic sensor for much of the throw of its iris.
It might mean that Canon is trading off some resolution performance for better overall edge performance, distortion control, better resolution across color wavelengths and so forth... or it could be very close, if not match the performance of their L lenses. There aren't any official MTF readings, test chart samples, lp/mm slit test readings for these lenses.
I'll add to Angelo's excellent and very technical explanation with one that is very down to earth and completely non-technical.
It just comes down to degrees of "good" and "not as good."
For years and years manufacturers, Canon and others, basically made two "grades" of lenses... good ones, and crappy ones. The good ones had very high resolution and were used for cinematographic and still film applications. Note that I said still film. The poorer ones were the ones made for videographic applications.
The ones made for video typically didn't have particularly high resolution... because they didn't have to... they were already resolving greater than the sensor that was capturing the image, so it didn't matter. They only had to be so good.
The better lenses, made for 35mm motion picture film and 35mm (and other formats) of still film photography, had to resolve much higher because their shooting format could. No one has ever really accurately come up with an equivalent "K" designation of what the real resolution of a 35mm frame of real film is... but (depending on the exact filmstock) it's often speculated that the actual resolution is something like 22K, or even higher. Ergo, those lenses really have to resolve absolutely pin sharp. Put that video lens (which looks perfectly fine on a video camera) on a real film camera, and it would likely perform very poorly.
The video lenses were/are quite inexpensive because they don't have to be made to nearly he stringent tolerances that cine lenses do. That's why a set of even good video lenses might be only a few thousand bucks, whereas a new set of Cooke primes is $100,000+. The video lenses are made on an assembly line... whereas high-end cine lenses are hand assembled, tweaked, projector tested, and collimated within an inch of their lives.
The advent of video cameras that can shoot much higher resolutions have forced lens manufacturers to sort of re-brand and re-label their offerings, and some lenses that were previously offered only for cinematographic use and now labeled "digital cinema" or such, indicating they resolve great enough to high-res videographic work.
Those Canon cinema lenses will resolve just fine for 4K, 5K, RED, Alexa, or any high-res format. Including film (if they happen to have the right mount).
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Another good explanation: