Focal length is the distance from the optical center of the lens to the camera matrix. Focal length (abbreviated FR) affects the angle of view of the camera as confirmed by the experts https://www.artlook.us/service/elopement-videography/.
FR allows you to understand how much the lens can bring the subject closer or further away. FR is measured in millimeters: the smaller the FR, the greater the viewing angle; the more FR, the more the lens magnifies. FR is an important characteristic of a lens.
FR affects distortion (optical distortion). It also affects the distortion and the distance to the subject. If you make a portrait on the 18 mm FR, you will see strong optical distortions on the face of a person.
This is a clear example of distortion. At 18 mm FR, when shooting a portrait, the distance to the object is very small, since the viewing angle is very wide, and plus, the lens makes distortions at such a focal length.
The optimal FR for shooting a facial portrait is 85-135 mm. Such FRs have minimal distortion, and on such FRs there is a sufficient distance to the subject, in which there is no distortion, and people’s faces turn out to be natural.
But good optics with such FRs is expensive. If there is no lens with such FR, take portraits with the maximum that you have. I got good portraits at 50 mm, on such a FR the distortion will also be minimal.
At FR below 85, there is barrel-shaped distortion, an eye effect is created, but this still depends heavily on the distance to the object, at a large distance this effect will be minimal and almost invisible. On the FR above 135 mm there is also a small pincushion distortion.
FR from 50 to 85 mm is suitable for half-length portraits, from 35 to 50 – for full-length ones. For group portraits, you can use FR below 35 if people do not fit into the frame and there is no way to move further.
Lenses with an FR below 35 are called wide angle lenses. They are used for shooting landscapes, architecture, interiors and for many other tasks. Below is an example of a landscape photo at 18mm.
Lenses with a focal length of 85 or more greatly magnify, they are called telephoto lenses. Such lenses are also used for many tasks: shooting a report from a long distance, shooting wild animals, etc.
There are universal zoom lenses with variable FR, and there are fixed lenses where the FR cannot be changed. Such lenses are called primes, primes are of better quality than zooms.