Focal length, that is; i.e. size of your lens. For example, when doing portraits.
General rule for headshots: the longer the lens, the better.
But it is not the lens that does the magic. It is your proximity to the subject.
With a short lens, like a 50mm, you need to be close to the subject. That causes some distortion; the closer, the more.
With a 200mm lens, however, you can be far, leading to a much more neutral, less distorted view:
See the difference? And that is viible on real faxes, too:
…and that is why my 70-200 lens is my favourite portrait lens. Provided I have enough space.
And that is where the second advantage comes in: being farther away, you are perceived as less “threatening” by your subjects. Meaning less awkwardness.