Above: The general default image format of a smartphone is portrait format with an aspect ratio of 1:2, or 1-wide x 2-high. Graphic: Gradients.iopan/Oscopic.
During the age of the dominance of the 35mm camera--circa 1940-1990--in popular photography, the default format for a photo was landscape--a rectangle wider-than-it-is-tall in the aspect ratio 3:2 (three on the long flat side and two high). We were always told that the 3:2 ratio was supposed to be a rough approximation of the angle-of-view of human eyesight itself. With almost all 35mm cameras, as you take hold of the camera and bring it naturally up to the eye, the view will be landscape. It takes a decision and a slight effort to make a portrait format photo--a rectangle taller-than-it-is-wide. In the age of the dominance of the smartphone--circa 2010-to-the-present-and-beyond; not even ten years yet--the default format for a photo is portrait. And this in a different aspect ratio of 1:2 (the rectangle is about twice as high as it is wide; its structure is roughly two squares, one-atop-the-other). The same effort as before now applies for making a landscape format image: it takes a decision and a slight effort to turn one's phone on its side in order to make a landscape picture.
Digital tech has effected this basic alteration to popular photography: the denigration of the photo-image as landscape, and a human-eyesight approximation, and the elevation of the photo-portrait. Portrait format in the present 1:2 aspect ratio definitely approximates humanness (the human as upright and bipedal is afterall what makes us unique as a species of mammal).
There might well come a time when looking at an image that is landscape format on one's phone will be generally adjudged to be a real pain. It may soon be the case that watching a movie (or playing a video-game) will be the only time a smartphone user is really prepared to make the effort to turn the screen to look at an image in landscape format.
(23 September 2018)