Normal / Standard lens
A normal or a Standard lens is a lens that reproduces an angle of view which looks "natural" to the human eye under normal viewing conditions and is closest to the angle of view of the eye. In comparison the lenses with longer or shorter focal lengths produce an expanded or contracted angle of view which distorts the perspective when viewed from a normal viewing distance. Lenses of shorter focal length are called wide angle lenses, while longer-focal-length lenses are referred to as long focus lenses most common being the telephoto lenses.
For different film /sensor formats different focal lengths are considered as a Normal lens. For a 35mm lens or full frame as it is called these days in the digital era, 50mm is considered as a Normal lens. ( Although the diagonal is 43mm, 50mm is generally known as a normal lens. Any focal lengths between 40 to 55 mm fall under the normal range. For a medium format camera with the film size of 6x7 cm, an 80mm lens is considered as a normal lens. If the focal length of the lens is equivalent to the diagonal of the film/sensor format, then it is considered as Normal for that format. Any Idea what focal length would be Normal for a digital small sensor format ? Certainly not the famous 50mm.
In cinematography a focal length roughly double the sensor /film diagonal is considered as a normal lens.
The following images have been shot with 40mm lens
You will notice that when 40mm is used a bit closer to fill the frame, there is an increased perspective and distortion compared to 50mm. For a smaller sensor, a 40mm lens would have a multiplication factor of 1.5