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Seeing Layers
The 50mm lens is more front-to-back than side-to-side and compresses things together, perfect for layering foregrounds over backgrounds.
35 is a lonesome focal length
A 35mm views things, captures establishing shots, and pushes backgrounds away because it magnifies foregrounds.
The Rule of Thirds
View something, then around and past it.
Compression and Separation
Lens selection is a creative choice made before stepping outside. A 50mm is how a photographer sees; a 35mm is how one thinks, and a 135mm is how one sees and things while driving.