It is usually believed by novel photographers that focal length (also called zoom at those stages) is just a measure of how close you are to the subject. If you take a picture at 50 mm it is expected to appear the same as if it is made with a 100vmm, as long as you get close enough to make the subject appear the same size in the frame. And those novel photographers usually will pass the following years believing this, as they take thousands of pictures, wondering why some lenses seem to take more appealing pictures of some subjects, why other lenses give best results for another kind of pictures.
But it turns out that when you modify the focal length of the lens, you are not only modifying the apparent size of the subject, but also the relationship between the distances in the picture, and this makes a great impact in the mood of the scene. When taking pictures with a wide angle lens, usually all distances tend to appear exaggerated, especially on the borders. Even the tiniest distance in the frontal axis tends to get amplified. This can be a problem or a feature, depending on how you work it: for a portrait, it tends to exaggerate the facial features, something that can be seen unpleasing when looking for beauty and perfection. But also, this enhancement of the perspective can provide of original compositions when used creatively, or when a more comical approach is desired. This is due to the increase of the field of vision the lens has, capturing more surrounding area and “bending” the light so everything can fit on the frame.
On the contrary, when using a telephoto lens, all distances in the lens axis get compressed. On the real world, a distance of 50 cm and 100 cm are seen as the second one being the double of the first one. But on the final picture this relation doesn’t hold, and the second one only seems slightly larger than the first one. When using a telephoto in portraiture, it usually yields flat faces, with little detail. It is useful, for example, to hide a prominent jaw or nose.
This effect is what it is known as the subject to background compression of focal length. This is the reason that a wide angle lens is usually preferred for landscape photography. It is not only that it allows more field of view, which means capturing more scene. Also it increases the apparent distances, providing the landscape with volume. This is one of the main differences between a good picture and the typical one you get with your phone on a rush… the second one seems flat, boring and artificial. Also, in portraiture, a moderate telephoto is preferred for this reason (usually between 70 and 125 mm). It requires getting further from the model to get the picture, what can be seen as “cold” sometimes, but it helps to avoid giving too much importance to facial defects. Using a shorter length usually produces long noses, big ears or an exaggeration of the distances of the face, making them seem long. Using longer lengths usually provides flat and boring portraits, without personality. Anyway, depending on the face, sometimes getting out of the standard focal lengths can be a good option.
Also, an Instagram video has become popular in the last days, which shows why the camera seems to make us fatter. This is also related with this topic. As it can be seen in the video, as we increase the focal length, the face of the boy seems to get bigger. The fact is that there is no real difference in the size of the face by itself. The posterior parts of the face, which are clearly distinguished with a short focal length, tend to become “close” to the frontal of the face as we increase the zoom, which make us more difficult to differentiate, for example, where the face ends and the neck begins. The effect is also more notable because all the pictures are compared as a succession, giving the impression that the low length picture is more “thinner” and the long length picture is “fatter” than they really are if you look only at one, isolated. The more appealing picture, in fact, would be one of the intermediate ones.
For more information about this topic, and a graphical example of this effect, I recommend reading the article Exploring How Focal Length Affects Images, by Andrew Childress , where a wider and more complete explanation, with example pictures, can be seen. So, the next time you want to take a picture, stop for a second and think: how do I want the background relate to the foreground? How do I want the distances in the lens axis relate with the distance? And once you answer to those questions, choose the right focal length, even if it means having to move a little bit. Remember: you should always move to adapt the picture to the chosen focal length, and never adapt the focal length to the position you have chosen for taking the picture.