4K refers to the resolution of either a display or a camera system. For the general audience and consumers, 4K has become a keyword for quality. This is a result of aggressive marketing by companies who wish consumers to buy more products. The reality is that 4K is important for very different reasons than what is generally thought.
Resolution can be defined as the degree of detail in an image. In case of a display or projected image, it is the amount of pixels which make the image. In case of cameras it is the amount of light-sensitive sensors on a larger image sensor. Resolution should not be discussed in a similar way when it comes to cameras or displays. This is a fact that is often overlooked.
When discussing images, 4K will either mean 3840×2160 pixels for UHD (Ultra High Definition, a broadcast standard) or 4096×2160 pixels for DCI (Digital Cinema, a standard used in theaters). They are different in their aspect ratio (1.78:1 for UHD and 1.89:1 for DCI). 4K Broadcast is in it’s infancy. In fact, a surprisingly large amount of household televisions in the USA are still Standard Definition, 4:3! It is said that we have still yet to transition fully into HD, so it will take years for 4K to become the popular standard. This is also dependent on the ability of cable companies or online providers such as Netflix to fully support 4K, and for the network to be fast enough. We are not yet there.
As far as theaters go, most commercial theaters nowadays have a ‘4K ready projector’ – but this does not mean you are actually watching a 4K image when you go to that theater. In fact, when you go to see a movie at a theater, most chances are that you are watching a 2k image (2048×1080 pixels). This has been the case for quite a few years now, and the integration of 4K into theaters is still slow.
So why bother? 4K is not important because of where we are today, but rather because where we will be tomorrow. Technology is moving fast, and distributors want to be ready for it. The audience will demand to see the movies in the highest and current standard, regardless of when they were made. This is why movies which were originally framed for a 4:3 aspect ratio are re-framed nowadays to fit wider screens.
Distributors are very specific with their delivery requirements, and it is important for a production to involve the Cinematographer and Post-Production Supervisor in this discussion to avoid issues later on. This is where resolution in the camera becomes important. Some distributors go as far as requiring the camera to output an original 4K image, and for the entire post-production workflow to be done in 4K. This will have an effect on the type of camera used and increase the cost for Visual Effects or any other post-production process.