4K Is Not UHD

In a recent post on high-resolution screens for smartphones I used the term 4K for describing the resolution of mobile screens with 3840 x 2160 pixels and was quickly and rightly reprimanded by an observant reader paying attention to detail. Indeed, I had fallen prey to marketing lingo. Strictly speaking 4K is a standard defined by industry consortium DCI (Digital Cinema Initiatives) that defines a resolution standard of 4096 x 2048 pixels used in the production and projection of movies. That’s slightly more than Ultra High Definition’s 3840 x 2160 pixels. UHD also defines double the basic resolution at 7680 x 4320 pixel too. Both share a common 16:9 aspect ratio, and 4K UHD and 8K UHD are terms used to distinguish between the two.

Marketers fromTV manufacturers often substitute 4K UHD with the more punchy 4K, opening terrain for confusion. True 4K has as slightly different aspect ratio of 256:135, which equates to 16:8.44. In other words, watching a 4K movie on UHD TV means either having narrow black bars on the top and bottom sides if you squeeze a full 4K frame onto the UHD screen, or losing small parts of the left and right edges of a frame if the 4K film is to fill the complete UHD screen. Most of us are familiar with this behaviour from previous formats. No big deal.

4K versus UHD

4K	4K UHD	8K UHD
4096 x 2048	3840 x 2160	7680 x 4320
4096 x 2048	3840 x 2160	7680 x 4320
16 : 8.44	16 : 9	16 : 9

Megapixels shots that make UHD sense

A further common fallacy in the multimedia industry is comparing pixels from cameras with those of screens.

Pixels describing a camera’s resolution are counted differently from those specifying a display’s resolution.

In the display world a single pixel consists of three separate RGB (red, green, blue) light sources. In other words, a single pixel on a screen can represent any given colour. Contrast this to the world of cameras, or more accurately image sensors. Each pixel in an image sensor captures one of the three RGB components that represent a colour. But it’s not as simple as dividing the megapixel count of a camera by three to arrive at the best display resolution. In short, cameras use a variation of the RGB model that takes the pecularities of the human eye into consideration and gives green twice as many detectors as red and blue to achieve better luminance resolution. The RGB pixel ratio 1:2:1 of image sensors means you need to divide the camera’s megapixel number by four when figuring out the best screen resolution to display a photo.

For example, take a 4K UHD display resolution with its count of 3840 x 2160 = 8.3 megapixels. Mutiplying that number by four means a 33.2 megapixel camera starts making sense when viewing captured shots on a 4K UHD TV.

In the past camera and smartphone vendors instigated a megapixel race to lure customers into buying their latest high-resolution devices. This marathon seemed nonsensical in a small-screen or High-Definition (HD) world. Ultimately many other parameters govern the overall quality of a snapshot, like the physical size of each “pixel” in an image sensor to name but one.

As more and more 4K UHD TVs are finding their way on to retailer’s shelves, the megapixel madness in smartphones and digital cameras of previous years begins to make sense. Favouring a smartphone with more than 16 camera megapixels is a serious consideration for hi-res aficionados on the way to their next phone replacement spree.