Since the release of UHD, the debate as to whether a given release of a product with the technology is true 4K or not has been rampant on the forums.
It is true that many releases are not true 4K in the sense that the original movie was not made in 4K resolution. This is true especially with movies that are filled with visual effects – they tend to be done in 2K resolution. Since that is the case, why would anybody buy a UHD Blu-ray?
Well, even when the master is in 2K, the upscaling equipment used by the studio to create a UHD master is much better than any upscaler that we use at home. There are exceptions, of course. Several UHD discs, including Tarzan (Warner), look worse than an upscaled regular Blu-ray disc. But this almost never happens.
So does this mean buying a UHD Blu-ray is always a case of buyer-beware? Not necessarily. Even a true 4K master can look like garbage. Take Bourne Identity (Universal), for example. The video portion of the disc looks identical to its Blu-ray counterpart. And even the Blu-ray itself is a bad looking one to begin with. Yet on the other hand, movies like The Magnificent Seven (Sony) and Pacific Rim (Warner) look unbelievably amazing on UHD Blu-ray even though they are both upscaled from their 2K master.
Also, on top of the resolution aspect of UHD, all UHD Blu-ray discs use a better compression algorithm than regular Blu-ray. That alone almost always makes a UHD Blu-ray disc looks clearer than regular Blu-ray, especially in movies with high speed camera pans and fast motion.
As UHD BD also includes 10-bit video for far less colour banding, Wide Colour Gamut upwards to the BT2020 colour space (regular BD is only using the very limited REC709 colour space; all UHD BD use the DCI-P3 colour space, which is 50% wider than 709) and High Dynamic Range of an average of 1,500 nits (standard dynamic range is usually only at 200 nits at best). When used properly (and almost all releases are actually done properly), this enhances the UHD viewing experience even further.
Last but not least, although not exclusive to UHD, with more than 90% of the releases, you get immersive audio, be it in the form of Dolby Atmos or DTS:X. Yes, technically you can get immersive audio through regular Blu-rays, or even a streaming service. But more immersive audio soundtracks tend to be relegated to the UHD releases.
So with all the other aspects of UHD BD and 4K resolution being the least important of them all, what is the problem? Just go out and buy the UHD discs now!