Lens speed

Conversely, a smaller maximum aperture (larger minimum f-number) is "slow" because it delivers less light intensity and requires a slower (longer) shutter speed.

Old fast manual focus lenses, such as the Nikkor-S(C) or Nikkor AI-S 50mm f/1.4, or Canon's FD and M39 counterparts, were historically produced abundantly, and are thus sold relatively inexpensively on the used lens market.

This is because lenses with larger maximum apertures require greater care with regard to design, precision of manufacture, special coatings and quality of glass.

[4] Made from various parts found around the factory (the lenses came from a darkroom condenser enlarger), the claimed speed and focal lengths were purely nominal and it wasn't usable for photography.

[4][5] Theoretically, the smallest f-number is 0 (or numerical aperture of 1), corresponding to a lens with an infinite entrance pupil diameter.

In practice, that cannot be reached due to mechanical constraints of the camera system (shutter clearance, mount diameter).

Flange distances are significantly smaller for rangefinder and mirrorless cameras (even below 20 mm), theoretically enabling designs down to something like f/0.7 or even faster.

The chance of seeing such lenses designed for use with 35mm ("full-frame") cameras, digital or film, in practice will be slim, since their cost and weight are likely not competitive with respect to equivalent imaging solutions employing larger sensors.

A fast prime (fixed focal length) lens , the Canon 50mm f / 1.4 (left), and a slower zoom lens, the Canon 18–55mm f / 3.5–5.6 (right); this lens is faster at 18mm than it is at 55mm.
Three 50 mm prime lenses from Minolta with lens speed 3.5 (a macro photography lens, speed of less priority), 1.7 (standard), and 1.2 (large opening and high speed, typically expensive), showing the relation between entry lens diameter and lens speed.
Canon 85mm f / 1.8 and f / 1.2 showing their large entrance pupils
Cosina Voigtländer Super Nokton 29 mm / 0.8