It is often said that a long lens will “compress” a scene, making near and far objects appear to be closer to each other. In actual fact, it is not the longer lens focal length that is compressing the scene. What actually causes this compression is moving the camera further away from the subjects you are photographing. I can demonstrate this with a few images.
This photo was captured at a lens focal length of 100mm:
Standing in the exact same position, I then zoomed in to a focal length of 400mm to capture this photo:
If you examine both of the above images, you can see that the relative sizes of the two key objects (the lamp and a tower of the Brooklyn Bridge) remain the same in both photos. That is because I stood at the exact same position for both photos, only changing the focal length to zoom in. In other words, changing focal length only cropped the scene, not compress the scene.
To help make this concept a little easier to understand, I have created a composite with the 400mm image resized to match the applicable area of the 100mm image, highlighting the area that the 400mm image represents. Here is that image:
Next, I walked closer to my subjects. As I walked, the apparent size of the lamp increased relative to the tower of the bridge in the background. I was reducing the apparent compression of the scene by moving closer to my subjects. Of course, because I didn’t have a ladder and couldn’t hover, the lamp appeared in a higher position relative to the tower of the bridge. But that is an issue of parallax, not compression. You can see the change in relative size for the two objects in this photo, which happened to be captured at a lens focal length of 118mm:
To be sure, when you change your distance to the subjects you are photographing, you may need to change your lens focal length in order to maintain the same framing for the scene. But it is your relative distance to the scene that causes a change in apparent compression of the scene. The lens focal length is merely cropping that scene to achieve the desired framing.
This issue has been the subject of several questions recently in my Ask Tim Grey eNewsletter, and so I hope this blog post helps to provide something of a “final answer” on the subject.