Wex explains the ins and outs of 4K video in the latest of our “What Is … ?” video series.
The term 4K refers to a resolution that has around four thousand pixels across its longest side. There are a handful of these resolutions which are commonly referred to as 4K, although there are two which are commonly used in cameras and displays: UHD 4K and DCI 4K.
UHD stands for Ultra High Definition, and it usually refers to resolution of 3840 x 2160 pixels. It results in exactly four times the pixels as the Full HD resolution, with Full HD measuring just over 2MP and UHD increasing this to over 8.
DCI 4K, meanwhile, stands for Digital Cinema Initiative 4K, as this was a standard defined by the major movie studios. It has a slightly higher resolution of 4096 x 2160 pixels, but with the same pixel count of 2160 across its shorter side, its aspect ratio is slightly different to that of UHD 4K.
Most cameras don’t yet offer 4K video recording, but a handful of major camera manufacturers now have at least one consumer-focused model in their range that does. Cameras that capture UHD 4K and DCI 4K footage also typically offer Full HD recording as an alternative, in the same way that cameras offering Full HD recording will also typically offer the lower resolution 720p options.
Out of the two formats, UHD 4K is the most common resolution across cameras, smartphones and televisions, although higher end cameras typically offer both options. Some cameras also offer related functionality such as 4K image capture, which is simply an 8MP image, and some cameras also offer a 4K resolution as part of their timelapse options, although what’s perhaps most useful is the ability to extract a still image at a UHD resolution from a video, which is something a number of cameras now offer.
Aside from the ability to output high-resolution footage, one thing 4K cameras typically have in common is the ability to capture video at particularly fast and slow frame rate for the purpose of outputting fast- and slow-motion footage. When recording a number of frames quickly for the purpose of creating slow-motion footage, there is usually a trade off with resolution, with footage perhaps recorded at a maximum HD or full HD, although fast motion footage has no reason to impose any limits as images are recorded slowly enough to be captured at their full resolution.
Clearly, recording moving footage at this high resolution means a lot of data. Some cameras manage to write this to memory cards inside the camera although it’s also possible to send this direct to an external recorder of some kind. Those cameras that can manage to do this all in camera require fast memory cards to do so, such as the CFast format whose write speeds are typically much higher than those of standard Compact Flash cards. Otherwise, SDHC and SDXC cards rated to UHS Class 3 are recommended, as these guarantee a minimum write speed of 30MB/s when recording video.