SUMMARY
David Bergman explains how f-stops work by relating focal length to aperture diameter. The f-number is a denominator in a fraction with focal length on top, and dividing them yields the entrance pupil's diameter. This calculation clarifies why a 50mm lens at f/4 has a larger physical opening than an 18mm lens at f/1.8, yet the wider lens is considered faster because it gathers more light. The f-stop sequence itself is based on doubling or halving light with each full stop, related to the area of a circle.
TAKEAWAYS
F-stop is a fraction: focal length divided by the f-number determines the entrance pupil diameter.
A wider maximum aperture (smaller f-number) allows more light, making a lens 'faster'.
Focal length affects framing and working distance, not a lens's 'speed'.
Exposure doesn't need adjustment when changing focal length at a fixed f-number due to balanced light gathering and projection.