zoom, prime, and the misundertood concept of "zooming with the feet"

Let’s say we have a lens with two focal lengths (FL) – The angle of view of one is shown by the black arrows and the other with blue arrows. The black arrow FL is the wider of the two. If the scene to be captured was two dimensional (and located along the red vertical line) then when the photographer is positioned at T he would capture CD of this red line with the wider FL and EF with the longer/tele FL. Using feet to zoom, the photographer comes to position S and captures the same EF of the 2D scene with the wider lens. No harms done.

 

However, the world is not two dimensional. It is three dimensional; we only convert it to two dimensional in the photograph. So standing at T the photographer actually captures everything that is there inside the angle CTD (both the arms extended to “infinity”) with the wider FL. Similarly, the tele FL captures everything within angle ETF (again both arms extended to infinity). The wider FL at S may capture the EF width of the scene at the distance of the red line, but beyond the red line the wider FL captures more of the scene (shown by the gray areas) than the tele FL from position T.

 

Now consider the objects A and B. With respect to position T, both A and B lie within the same "light cone" and so whether seen with the wider FL or with the tele FL, A covers B completely. In both the photographs - taken from position T with wide FL and tele FL, B won’t be visible being behind A. However, from position S, B lies only partially with in the light cone that covers A. So, A will only be covering B partially when seen from S. Thus if we take a picture from S with either the wide lens part of B will be visible which is not the case in the photograph with the tele FL from position T. Thus from the point of view of composition and perspective zooming with the feet and zooming a zoom lens are not the same thing.

 

Many primes are fantastic (not all of them), and so are many of the zooms. But a wider focal length cannot substitute a tele focal length because we can just "zoom with the feet" - we cannot "zoom with the feet" and still keep things the same. We just cannot, this is probably the simplest principle of optical physics that goes into the complicated field of photography. The debate here is not between zooms versus prime - as it is often portrayed. But rather it is about wider focal length versus tele focal length - and a wider focal length does not become a tele focal length at a shorter ditance from the subject.

 

Done