Lens focal length and perspective compression: how to frame the scene at the focal length and how the camera position changes the space
Explain the difference between focal length and field of view, observation position and perspective projection, and explain that the common sense of space compression comes from changes in shooting position; it does not claim to provide a universal golden focal length for portraits or a fixed distortion ratio.
The focal length changes the frame, and the camera position determines the observation position.
Focal length changes the field of view that the lens incorporates into the frame. The perspective relationship is related to the camera's observation position: changing the focal length at the same observation position will change the viewing range, but the focal length number itself will not change the projection relationship between near and distant objects. 〔G-FPC-01〕〔G-FPC-02〕
Why do telephoto images often appear space compressed?
In actual shooting, the photographer may move backward in order to keep the subject at a similar size in the telephoto frame. At this time, the camera's observation position changes, and the relative size relationship between the close-up view and the background will also change, resulting in the often said space compression look. When comparing, record the camera position and framing at the same time, rather than just attribute the results to the focal length. 〔G-FPC-03〕
Close shot and facial proportions
When the camera is close to a person, the part of the face closest to the lens will appear relatively larger in perspective projection. When taking portraits, you can first adjust the distance between the camera and the person, and then choose a focal length that can accommodate the desired frame; there is no unified "golden focal length" or fixed safety distance here. 〔G-FPC-04〕
Make your own reproducible comparisons
Fix the camera in the same position and shoot with different focal lengths. When comparing the same area, you will find that the viewing range is different; then move the camera and adjust the focal length so that the size of the subject is close, and observe the relative relationship between the background and the subject. Note down the camera position, focal length and cropping method to avoid mistaking the changes in the picture in different camera positions as being caused by the focal length alone. 〔G-FPC-02〕〔G-FPC-03〕
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