Why does f/4 let in less light than f/2?

Original 3D entrance-pupil area diagram. At fixed focal length, the f-number is focal length divided by effective opening diameter. In the illustrative 50 mm example, f/2 corresponds to 25 mm and f/4 to 12.5 mm. Halving circular diameter quarters area. With scene, shutter time and lens transmission held fixed, the admitted light is approximately one quarter. This is a simplified on-axis, non-macro explanation. The visible circle represents the effective entrance pupil, not a literal iris-blade mechanism or a measured real lens. No depth-of-field, diffraction, vignetting or photon-trajectory simulation is claimed. All geometry, area calculations and narration are independently authored; AI-generated voices and owned music.
Sources: |
#HowItWorks #Design #Shorts

source