Image resolution refers to the degree of detail an image holds, represented by the number of pixels. Higher resolutions make images sharper, as they increase the number of pixels used to represent images, which adds more detail to them. Screen size can drastically affect the sharpness of an image; if the screen is small enough, even low-resolution images can become nearly identical to far higher-resolution images, as the image is progressively less “pulled apart” as the size decreases and vice versa. Even the highest resolution can become blurry and out of focus when fitted to a large enough screen size.
The title ‘High Definition’ is given to a video format standard with 1080 vertical lines of pixels, usually with 1920 horizontal lines of pixels, as it usually implies an aspect ratio of 16 horizontal lines to every 9 vertical lines, creating a rectangular image. HD is distinct from the lower resolution 720-pixel format in that it is the next big ‘step up’ in resolution from. Videos and images in 2K resolution provide 2048 horizontal × 1080 vertical lines of pixels, and, since the horizontal pixels are the only changes between it and HD, it is simply a slightly wider version of an HD format. The standard known as 4K, however, contains many more pixels than either HD or 2K formats do, approximately quadrupling the amount of pixels of either format. In addition, 4K has two differing standards, one for videos, 4096×2160 pixels also known as Ultra-High-Definition, and one for television and computer monitors, 3840×2160 pixels.
It is important to note that on a VR device, the same image resolution results in a lower degree of detail and sharpness in comparison to a flat screen. There are two reasons for this effect, the first of which is that the spherical surface of a 180° or 360° image representation is much larger than it would be on a flat, rectangular screen, meaning that the same number of vertical and horizontal lines is stretched out over a wider area. The same concept applies to 2D versions of 180°/360° videos, as well. The second reason for the lower level of detail in VR is that in stereoscopic videos two separate images are rendered into a single video file to be displayed side-by-side for both eyes. As a result, the resolution from the perspective of each eye is only half of the actual image resolution.