So, perspective distortion. Amiright? If you're an expert photographer or cinematographer, you might know and understand what it's all about and how it affects an image, but if you're new to the game or just never thought to bone up on this optical phenomenon in particular, you might be sitting there scratching your head as to how it pertains to you. And because I'm nowhere close to being an expert, I'll let Jesse Agar of This Place explain how the relationship between focal length and distance to a subject affect perspective distortion, as well as the general appearance of your images.
Perspective distortion is when objects and the surrounding area appear distorted compared to what they would look like through a lens with a "normal" focal length (a field of view that appears to be natural to our eye). It's the direct result of where a camera is placed in relation to a subject and makes objects look smaller when they're further away and larger when they're close up.
Even if you don't quite understand what perspective distortion is or how focal length and distance specifically affect images, you probably know what a dolly zoom is. You probably know that faces look weird when you shoot a close up with a wide-angle lens. You probably know that moving closer to a subject changes of your image. These are simple, everyday examples of how these concepts work.
Knowing how focal length and distance affect an image independently is key for understanding how to control what you capture. For instance, moving closer to a subject is not the same thing as zooming into it with a lens, which is a common misconception beginners have ("zoom with your feet"). Distance changes perspective and focal length changes the field of view...and changing both simultaneously results in a kickass dolly zoom.
It's okay if you're still confused about the concepts highlighted in the video. The important thing to take away, at the very least, is that there are many ways to alter the way your image looks, and the focal length of your lens and the distance you put between your subject and your camera are two of them.
Source: This Place